Tracing giants through time: stable isotope insights into the historical ecology of North Atlantic sperm whales (Physeter macrocephalus)

Abstract

Introduction:

Sperm whales (Physeter macrocephalus) are deep-diving apex predators whose offshore ecology limits direct observation. Stable isotope analysis is widely used to infer cetacean diet and habitat use, but its application to archaeological whale remains remains rare despite the regular occurrence of sperm whales in zooarchaeological assemblages. Here, we combine stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analysis with zooarchaeological and palaeontological data to investigate the long-term foraging ecology of sperm whales in the North Atlantic.

Methods:

Bone specimens identified as P. macrocephalus using morphological and palaeoproteomic methods, including Zooarchaeology by Mass Spectrometry (ZooMS), were sampled from Late Holocene contexts across northern and western Europe. Stable isotope values of bone collagen were measured using EA-IRMS. Three known-age specimens were used to calculate an empirically based, eastern North Atlantic sperm-whale-specific marine reservoir correction, which was applied when comparing the Late Holocene specimens with previously published radiocarbon-dated Late Glacial material. Published modern isotope data were corrected for the Suess effect before comparison.

Results:

The three known-age specimens produced a provisional ΔR₂₀ estimate of −192 ± 40 ¹⁴C years. Within the Late Holocene, δ¹³C values showed modest temporal variation, including more depleted values during some earlier periods and relatively enriched values from the Medieval Climate Anomaly onward, whereas δ¹⁵N values showed no directional temporal trend. Late Glacial specimens had significantly higher δ¹⁵N values than Late Holocene specimens, by approximately 2.3‰, while δ¹³C values did not differ significantly between the periods. After Suess correction, modern oceanic North Atlantic sperm whales were isotopically similar to Late Holocene North Atlantic specimens, whereas Mediterranean sperm whales were isotopically distinct regardless of time period.

Discussion:

The results suggest broad long-term stability in carbon isotope baselines or habitat use, alongside substantial reorganisation of nitrogen cycling or deep-pelagic food-web structure between the Late Glacial and Late Holocene. The distinct Mediterranean isotope values are also consistent with the known demographic separation of Mediterranean sperm whales from the wider North Atlantic population. These results provide rare historical isotopic baselines for sperm whales in the eastern North Atlantic and demonstrate that apparent stability in predator trophic ecology can coexist with substantial changes in underlying ocean biogeochemistry. Such baselines are essential for evaluating future responses of this deep-diving predator to anthropogenic pressures and climate-driven reorganisation of deep-ocean food webs.

1 Introduction

Sperm whales (Physeter macrocephalus) are among the largest and most wide-ranging apex predators in the global ocean, occupying a key ecological role in deep-sea food webs (Whitehead, 2018). As specialist foragers on meso- and bathypelagic cephalopods, they link ecological processes across vast spatial and vertical gradients, making them an important component of global marine food webs (Cherel, 2021). However, their offshore distribution and extreme diving behaviour make direct observation difficult, leaving key aspects of their trophic ecology, movement ecology, and long-term variability poorly resolved (Correia et al., 2021; Watwood et al., 2006).

Contemporary sperm whale populations in the eastern North Atlantic are exposed to increasing cumulative human pressures, while climate-driven change is likely to affect habitat suitability and prey availability (Correia et al., 2021; Van Weelden et al., 2021). Establishing pre-industrial ecological baselines is therefore essential for distinguishing natural variability from recent anthropogenic change and for informing conservation strategies.

Stable isotope analysis provides a powerful means of reconstructing diet, trophic position, and habitat use in marine mammals, offering time-integrated ecological signals that complement observational and tagging data (Teixeira et al., 2022). Because baseline carbon isotope signatures vary systematically across marine environments, stable isotope analysis of proteinaceous tissues from marine mammals can provide insight not only into diet, but also into foraging habitat (e.g., Buss et al., 2022, 2025; Newsome et al., 2010). This spatial structuring has been used to identify ontogenetic shifts in dispersal, migratory destinations, and diet across the lifespan of sperm whales through δ¹³C and δ¹⁵N analysis of tooth samples (Mendes et al., 2007). In contrast, isotope data from skin samples of sympatric Mediterranean sperm whales indicate fine-scale sexual segregation, suggesting that males and females exploit partially distinct habitats or prey resources even within overlapping ranges (Pirotta et al., 2020). Together, these findings highlight that interpretation of δ¹³C and δ¹⁵N values requires explicit consideration of species-specific life history, movement ecology, and population structure.

Although widely applied to modern cetaceans, including sperm whales (Pirotta et al., 2020; Wild et al., 2020), its use in archaeological and palaeontological contexts remains comparatively limited (Charpentier et al., 2022). Long-term isotope datasets can therefore provide critical baselines for evaluating ecological stability and change across centennial to millennial timescales (Kowalewski et al., 2023).

Recent zooarchaeological and palaeoproteomic advances have enabled species-level identification of whale remains from European sites, substantially improving taxonomic resolution and showing that sperm whales form part of the recurrently identified large-whale record in some regional assemblages (McGrath et al., 2025; Speller et al., 2016; Van Den Hurk, 2020; Van Den Hurk et al., 2022, 2023a, 2023b; Van Den Hurk and McGrath, 2021). These findings raise important questions about the historical abundance, ecological role, and cultural significance of sperm whales in the eastern North Atlantic.

Interpretation of archaeological sperm whale remains must, however, account for strong zooarchaeological and taphonomic biases. Large cetacean bones are frequently fragmented, weathered, or reworked into artefacts, and morphological species identification is often difficult because diagnostic elements are rare and cetacean skeletons are anatomically conservative (Charpentier et al., 2022; Speller et al., 2016). Whale remains may also enter archaeological sites through multiple pathways, including opportunistic scavenging of stranded carcasses, curation of skeletal parts, and later reuse of worked bone. Consequently, taxonomic presence does not translate straightforwardly into local abundance or direct hunting intensity (Charpentier et al., 2022; McGrath et al., 2025; Van Den Hurk et al., 2023a).

Establishing reliable chronologies for archaeological whale remains is another persistent challenge. Radiocarbon dating of cetacean tissues is complicated by the marine reservoir effect (MRE), whereby marine carbon is depleted in ¹⁴C relative to the atmosphere, causing marine organisms to appear artificially old. Radiocarbon dates obtained from marine organisms must therefore be calibrated using a marine calibration curve and an appropriate regional offset, or ΔR (Van Den Hurk et al., 2025). The Marine20 curve provides the current global baseline for marine radiocarbon calibration (Heaton et al., 2020), but, as a global model, it does not account for regional variability in reservoir ages driven by ocean circulation, ventilation, and related environmental processes (Heaton et al., 2020). Localised ΔR corrections are therefore essential for capturing the characteristics of the water bodies an organism inhabits.

For highly mobile marine taxa such as cetaceans, ΔR variability may also reflect ecological behaviour. Individuals can forage across broad geographic ranges and exploit water masses at different depths, each characterised by distinct carbon reservoirs and turnover rates (Aoki et al., 2012; Key et al., 2004; Levin and Hesshaimer, 2000; McHuron et al., 2016; Watwood et al., 2006). This is particularly relevant for sperm whales, which routinely forage at bathypelagic depths where carbon exchange with the atmosphere is reduced and reservoir ages are typically older (Matsumoto, 2007). Their tissues may therefore incorporate carbon from multiple reservoirs with differing ¹⁴C signatures, complicating the application of uniform regional ΔR corrections. Accordingly, there is increasing recognition of the need for taxon-specific, and where possible population-level, ΔR estimates in zooarchaeological and historical ecology research. In a foundational North Atlantic study, Mangerud et al. (2006) used 19th-century whales, including sperm whales, to estimate marine reservoir ages for the North Atlantic and Norwegian Sea.

Mangerud et al. (2006) provided important reservoir-age estimates for 19th-century whale specimens from the North Atlantic and Norwegian Sea. However, because ΔR can vary through time, by region, and according to ecology, an archaeological bone-collagen framework for sperm whales remains necessary. McGrath et al. (2025) successfully radiocarbon dated sperm whale bones from Late Palaeolithic contexts in the Bay of Biscay, but relied on the general marine reservoir correction implemented in OxCal based on the Marine20 curve rather than a sperm-whale-specific correction. Thus, despite increasing sophistication in cetacean radiocarbon calibration, there is still no well-established sperm-whale-specific ΔR framework for archaeological bone from the eastern North Atlantic.

Sperm whale movement and population structure further complicate interpretation of archaeological radiocarbon and stable isotope records. Females and juveniles form stable social units in lower-latitude waters, whereas adult males commonly disperse widely into higher latitudes and move between feeding and breeding areas (Lyrholm et al., 1999; Whitehead, 2003). Genetic studies show low overall mitochondrial diversity and strong matrilineal structure among ocean basins and social groups, contrasted with weaker nuclear differentiation, consistent with female philopatry alongside long-range male-mediated dispersal (Alexander et al., 2016; Lyrholm et al., 1999). Photo-identification and telemetry studies further demonstrate that individual sperm whales, particularly males, can travel thousands of kilometres over relatively short timescales, moving between high-latitude feeding grounds and lower-latitude breeding areas and, in some cases, across the North Atlantic (Lefort et al., 2022; Lydersen et al., 2025; Mullin et al., 2022). Isotopic variability among archaeological specimens may therefore reflect individual movement histories and ecological trajectories, although caution in interpretation is required as skeletal elements can be transported from different geographic settings, although fewer generally are for larger skeletal elements due to sheer mass (Ryan et al., 2022; Schokkenbroek, 2017).

Unlike coastal baleen whales targeted by pre-industrial whaling, sperm whales are pelagic and would have been largely inaccessible to early hunting technologies. Their presence in coastal archaeological contexts is therefore typically attributed to stranded individuals that were opportunistically exploited (Van Den Hurk et al., 2023a), although see McGrath et al. (2025) for discussion on this topic. In the eastern North Atlantic, this mobility is reflected in the occurrence of males in marginal seas such as the North Sea, which has effectively been described as a trap for this deep-water pelagic species because its southern reaches become progressively shallower and more constricted, with sandbanks, mudflats, and sloping coasts (IJsseldijk et al., 2018; Pierce et al., 2007). Recurrent strandings, often involving bachelor males, have been linked to navigational difficulty and environmental variability, including temperature anomalies that may alter prey distributions (IJsseldijk et al., 2018; Pierce et al., 2007). Whether these patterns reflect long-standing ecological processes or recent environmental change remains unclear.

Direct archaeological evidence for deliberate sperm whale hunting in Europe remains equivocal, and most pre-industrial cetacean exploitation records are dominated by more accessible coastal baleen whales rather than pelagic odontocetes assemblages (Speller et al., 2016; Van Den Hurk et al., 2022, 2023a). Historical sources nevertheless show that sperm whales became an important target of European and Euro-American pelagic whaling from the eighteenth century onward, with later nineteenth- and twentieth-century operations extending far offshore and intensifying removals (Ryan et al., 2022; Schokkenbroek, 2017; Whitehead et al., 2021).

These fisheries had major demographic consequences and contributed to substantial long-term population declines, meaning that archaeological sperm whale finds may predate a markedly different ecological baseline from that observed today (Whitehead and Shin, 2022). Current understanding of sperm whale trophic ecology in the eastern North Atlantic is largely based on modern observations, limiting our ability to assess long-term stability and change. Opportunistic strandings, together with specimens derived from historical whaling catches, therefore provide rare opportunities to access the offshore ecological signals recorded by sperm whales through time.

In this study, we aim to improve chronological and ecological baselines for sperm whales in the eastern North Atlantic. First, we optimise radiocarbon dating of archaeological sperm whale bone by calculating a sperm-whale-specific eastern North Atlantic ΔR correction and applying it to previously dated sperm whale remains reported by McGrath et al. (2025). We then use stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analyses of Late Holocene sperm whale remains from Scandinavia and the southern North Sea area to investigate long-term patterns in trophic ecology and habitat use. By comparing these data with modern North Atlantic isotope records and with the archaeological data reported by McGrath et al. (2025), we assess whether sperm whale trophic ecology has remained stable through time and evaluate how movement ecology and population structure may have shaped isotopic variation. Specifically, we ask: (1) Has sperm whale trophic ecology in the eastern North Atlantic remained stable over millennia? (2) Do archaeological specimens reflect similar foraging habits to modern populations? (3) How do isotope signatures of North Atlantic sperm whales compare with those from other ocean basins? and (4) What do these long-term isotope records reveal about the resilience of this highly mobile predator to environmental change?

2 Materials and methods

2.1 Sample collection, provenance, and taxonomic identification

2.1.1 This study

A total of 41 archaeological and palaeontological sperm whale bone remains were integrated into this study (Figure 1). They primarily derive from two regions: (i) the southern North Sea [with samples from the Netherlands (n = 32) and Germany (n = 2)] and (ii) Scandinavia [samples originating from Norway, (n = 4) and Sweden (n = 3)]. These specimens were obtained and taxonomically identified by Van Den Hurk et al. (2022, 2023a) using Zooarchaeology by Mass-Spectrometry (ZooMS). One sample was identified as sperm whale based on aDNA analysis (Sigfusdottir, 2024). 28 out of the 41 specimens are currently dated based on the archaeological contexts from which they originated. These primarily date to the past c. 2000 years, though a handful of older specimens have been included (see Supplementary Table S1 for details). A single specimen with known chronology from its archaeological context was dated using radiocarbon dating as part of this study. For the remaining, mostly paleontological, specimens (n = 13), precise dating information was not available and the possibility of radiocarbon dating these specimens was beyond the scope of this study, so these were classified as “undated”.

Figure 1

McGrath et al. (2025); (B) Archaeological and palaeontological sperm whale specimens previously analysed by Van Den Hurk et al. (2022, 2023a) of which most are analysed as part of this study.

2.1.2 Data collation from the literature

To contextualise our results, we compiled published stable isotope data (δ¹³C, δ¹⁵N) for sperm whales from the eastern North Atlantic to assess regional temporal variability. We further incorporated data from other ocean basins to evaluate whether individuals originating beyond the North Atlantic exhibit distinct isotopic baselines, and are therefore potentially distinguishable within the archaeological record (Borrell et al., 2013; Courto, 2023; Jackson, 2016; MacKenzie et al., 2022; McGrath et al., 2025; Mendes et al., 2007; Peters et al., 2022; Pirotta et al., 2020; Wild et al., 2020).

The resulting dataset integrates values derived from multiple tissues and tissue-derived substrates, including blubber, muscle, skin, dentine and bone, and draws on published studies, associated archived datasets and the present study. Because available isotope datasets for sperm whales are geographically and temporally uneven, and because different tissues integrate dietary information over different timescales, this compilation is used to situate our archaeological bone collagen values within the known isotopic range of the species, rather than to assume equivalence across tissue types or study systems.

We further explored the dataset published in McGrath et al. (2025), which comprises archaeological sperm whale remains from Spain and southern France and dates back to the Late Glacial period. This dataset provided a rare opportunity to explore deep-time stable-isotope trends in sperm whales over many millennia, during a period of cooler sea-surface temperatures when their distributions were likely shifted farther south.

2.2 Collagen extractions

Sampling and collagen extraction was undertaken as described in Van Den Hurk et al. (2023a). Samples were taken using a Dremel® rotary tool removing a small piece of bone weighing up to ca 500 mg. Each rotary disc was used once, while the tool was cleaned using ethanol in between sampling. Collagen was extracted using a modified Longin (1971) method as detailed in Seiler et al. (2019), with the addition of a lipid extraction step and the use of a higher acid concentration, at the National Laboratory for Age Determination (NLD), Norwegian University of Science and Technology (NTNU), Norway. Initially, the samples were crushed to small pieces, placed into test-tubes, and cleaned in an ultrasonic bath with 18.2 MΩ cm ultrapure water (type 1) (three times 5 min). The samples were then ultrasonicated for 15 min in dichloromethane and methanol (2:1). This step was repeated three or more times, until the solution was clear. Following this, the material was demineralized overnight using 2.44 M HCl (50 ml of solution per 100 mg of bone) in glass tubes within a vacuum desiccator kept at room temperature. The HCl was removed from the samples and ca 8 ml of ultrapure water was repeatedly added to the tube and removed until a pH of 3–4 was reached. Subsequently, 4 ml of 0.5% NaOH was added for 2–4 h at room temperature to dissolve humic acids. The NaOH was removed from the samples and ca 8 ml of ultrapure water was repeatedly added to the tube and removed until pH < 10. Afterwards, 5 ml of 1.22 M HCl was added for 1–2 h to remove atmospheric CO₂. The sample was then washed repeatedly with ultrapure water until pH = 3 ± 0.2 and hydrolysed to gelatin at 70°C overnight. Finally, the gelatin was filtered through a prebaked (900°C) quartz filter (Merck Millipore, AQFA04700, 99.998% capture for 0.3 µm particles) and the filtrate freeze-dried. The final collagen was subsequently used for stable isotope analysis, alongside radiocarbon dating of one specimen (DAG342, see 2.3).

2.3 Radiocarbon dating and Delta R (ΔR)

One specimen from the site Schagen - ‘t Noord-Marees in the province of North Holland, The Netherlands, with an archaeological date of 1200-1299 CE (Diederik, 2003), was selected for radiocarbon dating and optimization of sperm whale specific ΔR20. This sample was selected due to its tightly constrained archaeological date (c.100 years). 3.4 mg of collagen for this sample was weighed, placed into a tin capsule and AMS dated following the protocol detailed in Seiler et al. (2019). Specifically, the sample was first reduced to produce homogenous graphite material in the automated reduction systems TOR (Trondheim Oxidation and Reduction system). The system performs iron-catalyzed hydrogen reduction of CO₂ (Vogel et al., 1984), freezing out the residual water with Peltier coolers (Wacker et al., 2010). ¹⁴C of the resulting sample was then measured on the NLD 1 MV AMS system made by High Voltage Engineering Europa B.V. The specimen, together with two other sperm whale bones previously analysed by Mangerud et al. (2006), was subsequently calibrated using the Marine20 curve (Heaton et al., 2020). A species-specific ΔR20 for eastern Atlantic sperm whales was subsequently estimated through Bayesian modelling in OxCal (Bronk Ramsey, 2009; Bronk Ramsey et al., 2012; Macario et al., 2023). We adopted a normally distributed prior with mean of 0 and standard deviation of 500 years to accommodate diversity within potential eastern North Atlantic ΔR20 values as tabulated in http://calib.org/marine/ (Reimer and Reimer, 2001). A broad normal prior of 0 ± 500 years was used as a weakly informative prior to avoid constraining the ΔR20 estimate toward known regional values, while still allowing for the possibility that sperm whales, as wide-ranging offshore predators, may incorporate reservoir effects that differ substantially from local coastal marine baselines. Sensitivity analysis using an alternative uniform prior of −800–1,000 years (cf. Macario et al., 2023) yields an identical ΔR20 rounded to the year. The OxCal code is provided in Supplementary File 1 and the resulting output in Supplementary Figure S1 and Table S2.

The calculated ΔR20 for eastern Atlantic sperm whales was subsequently used to recalibrate the sperm whale specimens originally identified and analysed by McGrath et al. (2025), providing up-to-date chronological information for the associated stable isotope analysis following current best-practice.

2.4 Stable isotope analysis (EA-IRMS)

For stable isotope analysis, samples that yielded <0.5% collagen were excluded (leaving us with n = 39). For all sample exclusion steps see Table 1. Carbon and nitrogen stable isotope ratios of sperm whale bone collagen for all new samples were measured using an elemental analyser (Thermo Fisher Scientific) coupled to an isotope ratio mass spectrometer (IRMS; Delta-Q, Thermo Fisher Scientific). Approximately 0.75 mg of collagen was weighed into tin capsules, and the samples were analysed in triplicate on the EA-IRMS. Isotope ratios are reported in δ-notation relative to international standards, Vienna Pee Dee Belemnite (VPDB) for carbon and AIR for nitrogen. Calibration was performed using internationally recognised and in-house reference materials (see Supplementary Table S3), with a two-point normalisation applied. Instrumental drift was monitored through repeated analysis of reference material (IAEA-600) interspersed throughout each run and corrected when necessary. Analytical precision, based on repeated measurements of standards, was less than ±0.2‰ for δ¹³C and ±0.3‰ for δ¹⁵N. Collagen quality was assessed using atomic C:N ratios, which fell between 2.9 and 3.6. Samples with a standard deviation greater than 0.5‰ for δ¹³C or δ¹⁵N across triplicate measurements were excluded from further analysis (n = 1, δ¹⁵N only).

The Suess effect refers to the anthropogenic decline in oceanic δ¹³C values caused by the uptake of isotopically light carbon from fossil-fuel CO₂ emissions, a signal that has penetrated the global ocean unevenly according to regional ventilation, circulation, and water-mass structure (Eide et al., 2017; Kwon et al., 2022). To standardize δ¹³C values of contemporaneous samples that will have been influenced by the anthropogenic Suess effect (Eide et al., 2017), all samples dated to post-1850 CE were corrected using the SuessR package in R (v. 0.1.6) (Clark et al., 2021). This included stable isotope data from the literature only. The correction was calculated from each specimen's year of collection and assigned oceanographic region using the package's built-in regional models; in the present dataset, specimens were assigned to the Subpolar North Atlantic and, where applicable, Gulf of Alaska regional calibrations (North Pacific data only).

The North Atlantic is a region of active deep-water formation and increased ventilation, such that δ¹³C declines associated with the Suess effect are somewhat detectable, even at depth. Therefore, as sperm whales forage in deep waters [∼600–1,000 m; (Watwood et al., 2006)], a water depth counter-correction of 80% was applied to post-1850 samples for the North Atlantic. This value was chosen based on the mean penetration depth of the ¹³C Suess effect (‰) for the North Atlantic calculated by [see Figure 4 in Eide et al. (2017): 0.2‰–0.05‰ for depths 500–1,000 m]. This depth-based scaling does not assume that meso- and bathypelagic prey are isolated from surface production; rather, it uses the depth-resolved estimates of Eide et al. (2017) to account for the attenuated penetration of the Suess signal into the water masses supporting deep-pelagic prey fields.

For Mediterranean samples, as overturning is high and anthropogenic CO₂ has been shown to penetrate throughout the water column (Schneider et al., 2010), no counter-correction was applied.

In contrast, due to the weak ventilation and long water-mass residence times of water masses in the Indian Ocean and South Pacific, especially in deep waters (where sperm whales forage), any Suess-effect signal in these regions is expected to be negligible [<0.005‰ year⁻¹; (Eide et al., 2017)]. Accordingly, specimens originating from the Indian Ocean or South Pacific were not corrected for the Suess effect. All pre- and post-Suess corrected data are available in Supplementary Table S5.

2.5 Statistical analysis

All statistical analyses were performed in R (version 4.5.1; R Core Team, 2025) (Supplementary File 2). For our dataset, mean and standard deviation (SD) of stable isotope data (δ¹³C, δ¹⁵N) were calculated and the resulting dataset screened for outliers. For each dataset, values lying outside a two-sided threshold of ±2.95 SD were removed (n = 2) (see Table 1). This threshold closely approximates the conventional ±3 SD criterion, which captures approximately 99.7% of normally distributed observations, while applying a marginally more conservative boundary for identifying extreme values in relatively small datasets. All subsequent analyses were conducted on the outlier-removed datasets.

Additionally, as it was not possible during sub-sampling to determine whether four bone fragments from the Netherlands derived from a single individual (DAG samples 158, 159, 161, and 162; all from context V552, K55 Oostermiddenmeerweg, southeast of Wieringerwerf), only one sample (DAG158) was retained for downstream analysis. This sample was selected based on collagen quality (Atomic C:N ratio = 3.23), resulting in a final dataset of 34 sperm whales (of which, 13 had known associated chronological ranges).

First, temporal changes in sperm whale trophic ecology during the late Holocene were assessed using δ¹³C and δ¹⁵N values of the samples collected as part of this study (specimens included in this analysis are shown in Supplementary Table S1). For δ¹³C or δ¹⁵N plotting and regression analyses, each specimen was assigned a single central age estimate, using the median of the associated chronological range or, for radiocarbon-dated specimens, the median of the calibrated posterior distribution. This follows common practice when a point estimate is required for exploratory temporal analyses, while recognising that full calibrated probability distributions better represent chronological uncertainty (Schulting et al., 2010). No samples were dated post-1850 CE, thus δ¹³C values were not corrected for the Suess effect. Regression coefficients, standard errors, and 95% confidence intervals were extracted using the R package broom (Robinson et al., 2025). Model assumptions were evaluated through inspection of residual distributions and standard diagnostic plots. Regression results were visualised using scatter plots with fitted regression lines and associated 95% confidence intervals.

Second, to identify any temporal changes in sperm whale trophic ecology during the Late Glacial transition, these analyses were repeated for samples from both the Oldest Dryas [also known as Heinrich Stadial 1 (HS1)] and the Bølling–Allerød interstadial (Barker et al., 2009; Rasmussen et al., 2014) using the dataset from McGrath et al., 2025 but with updated chronological ranges following radiocarbon corrections (see 2.3; Supplementary Figure S1). Fixed dates for each sample were chosen based on median values of the posterior distributions estimated by OxCal, spanning 16.5–12.9 ka BP.

Third, to evaluate variation in sperm whale trophic ecology across millennia, δ¹³C and δ¹⁵N values were compared among groups assigned by climatic phase. Broad climatic phases were defined as the Late Glacial transition period (including samples dating between 16,264 and 13,938 cal BCE, representing the Oldest Dryas and Bølling–Allerød interstadial periods) or the Late Holocene (including samples dating between 3490 BCE and 1800 CE). Temporal groups were modelled as factors. Given normality and heterogeneity assumptions were met across groups [assessed using Shapiro–Wilk and Levene's tests; following (Hosken et al., 2018)], differences between broad climatic groups (Late Glacial vs. Late Holocene) were tested using Welch's t-test (Welch, 1947).

Additionally, to infer temporal change at a finer scale, isotopic variability across the same datasets assigned to finer-resolution time bins as follows: the Oldest Dryas, here treated as ∼16.6–14.7 ka BP; the Bølling–Allerød interstadial, ∼14.7–13.7 ka BP; the Medieval Climate Anomaly, conventionally c. 950–1250 CE, although early Medieval samples were also included in this broader Medieval grouping; the Little Ice Age, c. 1300–1850 CE; and the Industrial period, here defined as c. 1850 CE to the present. All time periods and sample sizes are available in Supplementary Table S4. Normality (Shapiro–Wilk) and homogeneity of variance (Levene's test) were assessed for δ¹³C and δ¹⁵N. Climatic periods with fewer than three observations were excluded from statistical analyses. Given that normality assumptions were not consistently met across groups, differences between finer-scale climatic groups were assessed using Kruskal–Wallis, with pairwise post hoc comparisons performed using Dunn's test with Bonferroni corrections. Results were visualised using boxplots, created using the R package ggplot2 (Wickham, 2016).

3 Results

3.1 Collagen extractions

For the 41 archaeological sperm whale samples (Figure 1), collagen yield ranged from 0.2% to 18.7%. Two samples provided <0.5% collagen yield results and were excluded (see Table 1 for all exclusion steps). An additional two samples fell outside of a 2.95 standard deviation check implemented here and were excluded as well. Four samples did provide >0.5% collagen yields, but due to relatively low starting material were of inadequate size to be run on the IRMS in triplicate. This resulted in an updated dataset of 37 zooarchaeological sperm whale samples from Scandinavia (n = 7) and the southern North Sea (n = 30). Three of the North Sea samples did not have atomic C:N ratios indicative of “pure” collagen and were excluded. This resulted in a final total dataset of 34 zooarchaeological samples.

3.2 Updating the radiocarbon ΔR20 for north Atlantic sperm whales

By radiocarbon dating one well-stratified Medieval archaeological specimen from Schagen - ‘t Noord Marees and combining the results with the two well-dated historical sperm whale specimens dated by Mangerud et al. (2006), an updated ΔR20 of −192 ± 40 ¹⁴C years was estimated for eastern North Atlantic sperm whales. The OxCal model agreement index (A_model) and overall individual agreement index (A_overall) are 148.6 and 148.0 respectively (Supplementary Figure S1), above the recommended threshold values of 60.0 (Bronk Ramsey, 2009). Pending additional known-age specimens providing broader spatiotemporal coverage, −192 ± 40 should be regarded as a provisional but empirically based best estimate ΔR20 for eastern North Atlantic sperm whale specimens until additional datasets become available. Using this calibration, the radiocarbon date for the single archaeological sperm whale specimen (Schagen - ‘t Noord Marees specimen) was assigned an age estimation of 1046–1327 cal CE (median: 1203 cal CE) in comparison to the 1200–1299 CE (median: 1250 CE) archaeologically inferred age.

Additionally, the recalibrations of the sperm whale radiocarbon dates from McGrath et al. (2025), resulted in slightly younger chronological ages with c.200–350 cal year adjustment across all samples (Supplementary Figure S1). Given the original dates from McGrath et al., 2025 had date ranges of millennia, this produced very little relative temporal change between specimens.

3.3 Stable isotope results

Stable isotope values of nitrogen (δ¹⁵N) and carbon (δ¹³C) were quantified across the 34 zooarchaeological sperm whale samples that met quality-control criteria (see methods 2.5 for details) (Table 1; Figure 2A). Across the full QC-filtered dataset (n = 34), C:N ratios ranged from 2.95 to 3.56 (mean = 3.30 ± 0.13, median = 3.28, IQR = 0.12), consistent with archaeological collagen (Van Klinken, 1999). δ¹³C values ranged from −14.55‰ to −11.00‰ (mean = −12.97 ± 0.88‰, median = −13.02‰, IQR = 1.00‰), while δ¹⁵N values ranged from 13.14‰ to 16.87‰ (mean = 14.94 ± 1.09‰, median = 14.80‰, IQR = 1.71‰). This isotope dataset in conjunction with isotope data from the literature were used to address the four primary research aims.

Figure 2

Following recalibrated radiocarbon date corrections of the Late Glacial isotope sperm whale samples from the North Atlantic (McGrath et al., 2025), the updated dates of these specimens spanned the Late Glacial transition (∼16.6–13.7 ka BP), comprising five samples from the Oldest Dryas (∼16.6–14.7 ka BP) and six samples from the Bølling–Allerød interstadial (∼14.7–13.7 ka BP).

In contrast to the Late Holocene, ordinary least squares regression of the Late Glacial samples revealed no significant temporal trends for either δ¹³C (p = 0.11) or δ¹⁵N (p = 0.73), although δ¹³C exhibited a weak non-significant negative slope (Figure 2D). The negative but not statistically significant relationship between δ¹³C and sample age (β = −4.13 × 10⁻⁴ year⁻¹; 95% CI = −9.36 × 10⁻⁴ to 1.10 × 10⁻⁴), potentially indicated a weak decline in δ¹³C values through time. The model explained a small proportion of the variance (R² = 0.26; adjusted R² = 0.18). Similarly, δ¹⁵N showed no significant temporal trend (β = 8.69 × 10⁻⁵ year⁻¹; 95% CI = −4.63 × 10⁻⁴ to 6.37 × 10⁻⁴) and explained negligible variation in the data (R² = 0.01; adjusted R² = −0.10) (Figure 2E).

δ¹³C, δ¹⁵N, and C:N ratio values did not deviate from normality in either dataset (this study; McGrath et al., 2025) (Shapiro–Wilk, all p > 0.05), and variances were comparable between groups (Levene's test, all p > 0.05). When comparing isotopic values between time periods, Welch's t-tests revealed no significant difference in δ¹³C between the Late Holocene (this study: −13.09‰) and the Late Glacial transition (McGrath et al., 2025: −13.12‰; t = −0.09, p = 0.93) (Figure 2F). In contrast, δ¹⁵N values were significantly higher during the Late Glacial transition (16.82‰) than in the dated Late Holocene (14.55‰; Supplementary Table S6), with a mean difference of 2.27‰ (t = 7.49, p < 0.001; 95% CI = 1.63–2.91). We also note that C:N ratios did not differ significantly between datasets (3.29 vs. 3.26; t = 0.84, p = 0.41). Summary statistics are available in Supplementary Table S6.

Within the finer climatic resolution dataset, C:N atomic ratios remain tightly clustered across all periods (∼3.2–3.4) with minimal variability, demonstrating consistency in collagen quality regardless of sample age (Figure 2G).

δ¹³C values varied among periods. δ¹³C values range from more depleted values in the Roman Warm Period (mean −14.15‰; one sample) and Dark Age Cold Period (−14.11‰; one sample) to relatively enriched signatures during the Medieval Climate Anomaly (−12.47‰; sd 0.4), Little Ice Age (−12.99; sd 0.42), and Industrial Revolution (−12.67‰; one sample) (Figure 2G; Supplementary Table S6).

Figure 3

Late Holocene North Atlantic specimens (n = 34) exhibited relatively enriched δ¹³C values (mean = −12.97‰, SD = 0.88; range = −14.55‰ to −11.00‰) compared with both modern Mediterranean (n = 34; mean = −16.33‰, SD = 1.33; range = −17.32‰ to −11.43‰) and modern oceanic North Atlantic samples (n = 19; mean = −13.61‰, SD = 2.02; range = −18.17‰ to −11.71‰). Patterns in δ¹⁵N were similar. Late Holocene North Atlantic samples showed higher δ¹⁵N values (mean = 14.94‰, SD = 1.09; range = 13.14‰–16.87‰) compared with modern Mediterranean (mean = 11.75‰, SD = 0.70; range = 10.60‰–13.46‰) and modern oceanic North Atlantic samples (mean = 14.25‰, SD = 0.89; range = 12.80‰–16.10‰).

Significant group-level differences were observed for both δ¹³C and δ¹⁵N. For δ¹³C, Kruskal–Wallis tests showed significant differences among groups (χ² = 39.74, d.f. = 2, p = 2.35 × 10⁻⁹). post-hoc comparisons indicated significant differences were between the Late Holocene North Atlantic and modern Mediterranean sperm whales (adjusted p = 1.26 × 10⁻⁸), as well as between modern Mediterranean and modern oceanic North Atlantic samples (adjusted p = 1.99 × 10⁻⁵), but not between Late Holocene North Atlantic and modern oceanic North Atlantic samples (adjusted p = 1.000). For δ¹⁵N, significant differences were also observed among groups (χ² = 63.37, d.f. = 2, p = 1.74 × 10⁻¹⁴). Post hoc comparisons indicated significant differences between Late Holocene North Atlantic and modern Mediterranean sperm whales (adjusted p = 4.78 × 10⁻¹⁴), as well as between modern Mediterranean and modern oceanic North Atlantic samples (adjusted p = 1.01 × 10⁻⁶), but not between Late Holocene North Atlantic and modern oceanic North Atlantic samples (adjusted p = 0.502). Pairwise results are provided in Supplementary Table S9B.

These results indicate that, after Suess correction, modern oceanic North Atlantic sperm whales broadly overlap isotopically with Late Holocene North Atlantic specimens, and are distinct from Mediterranean samples regardless of time period.

Scandinavian samples were significantly enriched in δ¹³C (mean = −12.12‰ ± 0.99‰) relative to the southern North Sea (mean = −13.2‰0 ± 0.71‰; Welch's t-test: t = 2.72, df = 7.68, p = 0.028). δ¹⁵N values were also higher in Scandinavia but not significantly so (15.56‰ ± 1.17‰ vs. 14.63‰ ± 1.05‰; Welch's t-test: t = 1.59, df = 8.54, p = 0.15). C:N ratios were comparable (t = −0.69, df = 30.3, p = 0.50).

Scandinavian samples showed a larger interquartile range in δ¹³C (IQR = 1.62‰) than southern North Sea samples (IQR = 0.87‰), although the total δ¹³C range was larger in the southern North Sea (3.14‰) than in Scandinavia (2.51‰). This is most likely driven by the three highly enriched δ¹³C Bohuslän outliers from the Scandinavian dataset. δ¹⁵N variability was comparable between regions (Supplementary Table S7).

Figure 4

Using available global sperm whale datasets, clear variability in isotopic composition was observed across ocean basins (Figure 5A). Kruskal–Wallis tests confirmed significant basin-level differences for both δ¹³C (χ² = 134.023, d.f. = 4, p < 2.2 × 10⁻¹⁶) and δ¹⁵N (χ² = 161.067, d.f. = 4, p < 2.2 × 10⁻¹⁶).

Figure 5

North Atlantic pelagic samples were enriched in δ¹³C values (mean = −13.21‰, median = −12.95‰, SD = 1.33‰, range = −18.17‰ to −11.00‰, n = 65), while North Atlantic Mediterranean samples were more depleted in δ¹³C (mean = −16.33‰, median = −16.75‰, SD = 1.33‰, range = −17.32‰ to −11.43‰, n = 35). Indian Ocean samples showed intermediate δ¹³C values (mean = −16.61‰, median = −16.90‰, SD = 1.51‰, range = −18.10‰ to −13.40‰, n = 7), whereas North Pacific and South Pacific samples were the most depleted (North Pacific: mean = −17.40‰, median = −17.34‰, SD = 0.63‰, range = −18.86‰ to −16.27‰, n = 100; South Pacific: mean = −17.80‰, median = −17.38‰, SD = 1.28‰, range = −20.15‰ to −16.45‰, n = 14).

For δ¹⁵N, values were highest in the North Pacific (mean = 16.80‰, median = 16.76‰, SD = 0.75‰, range = 15.09‰–18.25‰), followed by North Atlantic pelagic samples (mean = 14.88‰, median = 14.63‰, SD = 1.18‰, range = 12.80‰–18.70‰), South Pacific samples (mean = 14.47‰, median = 14.47‰, SD = 0.63‰, range = 13.82‰–16.32‰), and Indian Ocean samples (mean = 14.40‰, median = 14.80‰, SD = 1.09‰, range = 12.90‰–15.60‰). North Atlantic Mediterranean samples had the lowest δ¹⁵N values (mean = 11.75‰, median = 11.65‰, SD = 0.70‰, range = 10.60‰–13.46‰).

For δ¹³C, post hoc comparisons showed that pelagic North Atlantic samples differed significantly from the Indian Ocean (adjusted p = 0.0114), North Atlantic Mediterranean (adjusted p = 1.150 × 10⁻⁴), North Pacific (adjusted p = 8.069 × 10⁻²⁸), and South Pacific samples (adjusted p = 1.700 × 10⁻⁹). North Atlantic Mediterranean samples also differed significantly from the North Pacific (adjusted p = 1.167 × 10⁻⁴) and South Pacific (adjusted p = 0.0235), but not from the Indian Ocean (adjusted p = 1.000). No significant δ¹³C differences were observed between the Indian Ocean and North Pacific (adjusted p = 1.000), Indian Ocean and South Pacific (adjusted p = 1.000), or North Pacific and South Pacific samples (adjusted p = 1.000).

For δ¹⁵N, post hoc comparisons showed that Mediterranean and pelagic North Atlantic samples differed significantly from each other (adjusted p = 1.395 × 10⁻⁶). Both North Atlantic groups also differed significantly from the North Pacific, including Mediterranean vs. North Pacific (adjusted p = 1.025 × 10⁻³⁰) and pelagic North Atlantic vs. North Pacific samples (adjusted p = 5.668 × 10⁻¹³). The North Pacific also differed significantly from the Indian Ocean (adjusted p = 0.00226) and South Pacific samples (adjusted p = 4.964 × 10⁻⁶). No significant δ¹⁵N differences were observed between the Indian Ocean and North Atlantic Mediterranean (adjusted p = 0.379), Indian Ocean and pelagic North Atlantic (adjusted p = 1.000), Indian Ocean and South Pacific (adjusted p = 1.000), North Atlantic Mediterranean and South Pacific (adjusted p = 0.0613), or pelagic North Atlantic and South Pacific samples (adjusted p = 1.000). Full pairwise results are provided in Supplementary Tables S9, S10.

Bivariate isotopic niche analyses further separated sperm whales among ocean-basins (Figure 5B). The pelagic North Atlantic, Mediterranean, North Pacific, and South Pacific, were all distinct, with Indian Ocean overlapping with the South Pacific. This suggests that broad-scale oceanographic and biogeochemical processes, including ocean circulation and nutrient cycling, shape sperm whale isotopic signatures and may reflect population structuring among basins, consistent with genetic evidence for inter-basin structuring in sperm whales (Alexander et al., 2016).

Because tissue identity is confounded with time period, the δ¹³C differences cannot be assigned confidently to either tissue-specific discrimination or temporal variation. However, the absence of corresponding δ¹⁵N differences suggests that any effect is unlikely to reflect a major change in trophic position or nitrogen assimilation. Further work quantifying isotope fractionation and routing among sperm whale bone, skin, and dentine collagen would help determine the precise magnitude of tissue-related variation. However, because large inter-tissue offsets generally require either substantial physiological fractionation or tissues integrating isotopically distinct dietary inputs, we expect tissue effects in sperm whales to contribute mainly to minor within-group variability rather than to drive the broader isotopic patterns observed here; this is discussed further in Section 4.2.

4 Discussion

Here, we combine improved radiocarbon calibration with stable isotope analyses to show that sperm whale trophic ecology in the North Atlantic has remained broadly consistent over millennial timescales. We identify three main patterns: (i) a significant reorganisation in nitrogen cycling between the Late Glacial and the Late Holocene; (ii) consistent isotopic differentiation between the Mediterranean and the wider North Atlantic; and (iii) basin-scale oceanographic structuring of sperm whale isotope values globally.

4.1 Basin-scale ecological structuring and implications for ocean change and management

The elevated δ¹⁵N values observed in Late Glacial samples relative to the Late Holocene most likely reflect changes in North Atlantic nitrogen cycling and deep-pelagic food web structure, rather than a major shift in sperm whale trophic level. This is because the last deglaciation was characterised by substantial reorganisation of North Atlantic circulation, including marked reductions in Atlantic Meridional Overturning Circulation during Heinrich Stadial 1 and the Younger Dryas, with consequences for nutrient delivery and productivity (Lynch-Stieglitz et al., 2024; McManus et al., 2004). Because nitrate supply, nitrate utilisation, and nitrogen fixation influence baseline δ¹⁵N values, such changes can propagate through pelagic food webs and ultimately affect predator tissues (Altabet and Francois, 1994; Ren et al., 2009; Straub et al., 2013). Alternatively, enrichment may have resulted from restructuring of mesopelagic communities, such as shifts in food-chain length, or changes in the trophic position of key prey taxa, as δ¹⁵N values of mesopelagic fishes and cephalopods vary with body size, feeding mode, and trophic position (Annasawmy et al., 2020; Cherel et al., 2009; Parzanini et al., 2019).

Although in principle, higher δ¹⁵N values could reflect increased reliance on higher-trophic-level or demersal prey, available North Atlantic dietary evidence indicates persistent sperm whale foraging on oceanic cephalopods, including meso- and bathypelagic squid (Clarke et al., 1993; Santos et al., 1999, 2002). Although sperm whales can incorporate demersal fish in some regional systems (Guerra et al., 2023), and recent fishery interactions and depredation have been documented in parts of the North Atlantic and elsewhere (Hamer et al., 2012; Oyarbide et al., 2023), our specimens pre-date modern fisheries, and fishery-related depredation therefore cannot explain the archaeological isotope pattern. There is currently no clear evidence that a shift to alternative trophic levels provides a better explanation. We therefore interpret the elevated Late Glacial δ¹⁵N signal primarily as reflecting altered nitrogen biogeochemistry and reorganisation of deep-pelagic ecosystems, consistent with temporal variability observed in foraminifera-bound nitrogen isotope records and historical squid-beak isotope archives (Golikov et al., 2024; Ren et al., 2009; Straub et al., 2013).

Although genetic differentiation is evident among major ocean basins, evidence for discrete population structuring within the oceanic North Atlantic remains limited. Instead, sperm whales show pronounced sexual and social structuring, with strong matrilineal organisation and female philopatry contrasted by long-range male-mediated dispersal (Alexander et al., 2016; Lyrholm et al., 1999). Isotopic differences within the oceanic North Atlantic are therefore more likely to reflect ecological and oceanographic variation than discrete population boundaries. Consistent with this, our results identify a single isotopic putative foraging deme across the oceanic North Atlantic, irrespective of time period (Figure 4B, cluster 1), whereas Mediterranean samples are clearly distinct (Figure 4). This separation is consistent with genetic evidence for differentiation between these two regions and therefore our isotopic data supports treating the Mediterranean population separately in ecological and management contexts (Drouot et al., 2004; Notarbartolo di Sciara, 2014; Violi et al., 2023). Importantly, the Bonferroni-adjusted comparisons indicate that Suess-corrected modern oceanic North Atlantic sperm whales do not differ significantly from Late Holocene North Atlantic specimens in either δ¹³C or δ¹⁵N. This strengthens the interpretation that oceanic North Atlantic sperm whale isotope values have remained broadly stable from the Late Holocene to the modern period, while Mediterranean animals occupy a distinct isotopic space relative to the wider oceanic North Atlantic.

Although Scandinavian and southern North Sea samples differed significantly in δ¹³C, their shared assignment to the same GMM cluster indicates that this regional difference was minor compared with the larger ocean-basin-scale separation between oceanic North Atlantic and Mediterranean sperm whales. However, because bone collagen reflects a multi-year isotopic average, the regional δ¹³C difference between Scandinavia and the southern North Sea may still be ecologically meaningful at a finer scale, potentially reflecting mild differences in habitat use, prey availability, or regional baseline conditions. Alternatively, this difference may partly reflect chronological heterogeneity rather than distinct putative foraging demes. Some Bohuslän specimens may be older than the Medieval Trondheim samples, and several whale and seal bones from Bohuslän have been dated to the Late Glacial or early–mid Holocene, with calibrated dates mostly between c. 6,000 and 13,000 cal BP (Fredén, 1975). These ages overlap with periods of changing Skagerrak hydrography, including relatively warm early–mid Holocene sea surface temperatures and strong North Atlantic water influence, followed by cooling in the northeastern Skagerrak between ∼6,300 and 5,400 cal BP and broader Neoglacial cooling after ∼4,350 cal BP (Krossa et al., 2017). Thus, the contrast between Bohuslän and Medieval Trondheim samples may partly reflect temporal shifts in regional baseline isotope conditions, rather than persistent spatial structuring alone.

Taken together, these patterns suggest that, excluding the Mediterranean, oceanic North Atlantic sperm whales function isotopically as a basin-scale ecological unit that integrates variability in prey fields, nutrient regimes, and deep-pelagic food webs across large spatial scales. Consequently, temporal shifts in isotopic composition are more likely to reflect broad oceanographic and ecosystem-level change than localised population responses. In this context, we hypothesize that sperm whales may act as useful sentinels of modern ocean change: because they integrate ecological signals across broad spatial and depth ranges, changes in their isotopic niche may provide early evidence of reorganisation in deep-ocean ecosystems under ongoing climate change.

A small number of Bohuslän specimens nevertheless remain unusual in isotopic space and should be treated cautiously until directly dated (Supplementary Figure S2). Their position could reflect older baseline conditions, or less likely, either human transport (unlikely due to radiocarbon dates of other whale bone specimens in the region being relatively old) or occasional use of isotopic niches similar to those of Mediterranean sperm whales.

We additionally note that, although the North Atlantic sperm whale samples used in this study likely span a broad portion of the species' geographical range, including individuals from Spain, Denmark, Portugal (Azores), Iceland, Norway (Svalbard), and the Mediterranean, no samples were available from Canada or the United States. This represents a key data gap in the western North Atlantic, a region where sperm whales are widely distributed along the continental shelf edge and slope, with pronounced latitudinal structuring in habitat use: females and juveniles primarily occupy lower-latitude waters, while adult males range into higher-latitude foraging grounds (Hayes et al., 2024). Historical whaling logbook data further underscore the importance of this region; Townsend's Whaling Charts indicate particularly high encounter densities for sperm whales in the subtropical western Atlantic, including waters off Florida and along the Gulf Stream (Townsend, 1935). Although these records reflect patterns of whaling effort as well as whale distribution, they nevertheless highlight the longstanding ecological significance of the region. Additional sampling from the western North Atlantic would therefore help resolve potential longitudinal structure in population structuring and improve our understanding of regional variability in trophic ecology for sperm whales ocean-basin wide.

4.2 Challenges of isotopic inferences from archaeological and historical specimens

Archaeological and palaeontological whale bones provide one of the few routes to deep-time insight into past whale ecology, but their interpretation is constrained by several limitations. First, ontogenetic age is difficult to assess. Most specimens analysed here are highly fragmented and cannot be assigned confidently to skeletal element or age class. This matters because nursing mammals can show elevated δ¹⁵N values relative to older, fully weaned individuals, reflecting trophic enrichment between mother and offspring (Borrell et al., 2016; Fuller et al., 2006; Newsome et al., 2006). Inclusion of unweaned juveniles could therefore inflate apparent trophic-level estimates. However, nursing offsets typically generate structured enrichment, often on the order of ∼1–3‰ in δ¹⁵N depending on tissue and taxon, and may produce distinct mother–offspring clusters (Fuller et al., 2006; Newsome et al., 2006).

We note that several samples have enriched δ¹⁵N in comparison to the full dataset, including DAG49 (16.87‰), DAG239 (16.68‰), DAG 527 (16.72‰), and DAG529 (16.65‰). These specimens are highly fragmented, not allowing for an age estimate based on morphology. However, the available osteological evidence taken together argues against a major influence from actively nursing individuals. Most specimens are undiagnostic fragments, although a small number of ribs and vertebrae could be identified. Ribs provide limited age information, whereas vertebral epiphyseal fusion allows cautious assessment. The vertebrae analysed here include both fused and unfused examples, indicating that some individuals were skeletally immature. However, skeletal immaturity should not be equated with active nursing. The timing of vertebral and appendicular epiphyseal fusion remains poorly understood in sperm whales, and modern North Sea strandings are dominated by bachelor males rather than females with dependent calves. In the 2016 North Sea mass-stranding event, for example, necropsied individuals were all young males, mostly between 10 and 15 years old, indicating that immature but clearly post-weaning individuals may be represented in such events (IJsseldijk et al., 2018; Schnitzler et al., 2018). As the North Sea is shallow and thus does not represent key sperm whale habitat, it is possible that European sperm whale collections in the past also resemble similar mass-stranding events, and because nursing in sperm whales is generally completed within the first few years of life (Gero and Whitehead, 2007; Shirihai and Jarrett, 2006), few, if any, of the analysed specimens are likely to derive from actively nursing individuals. Instead, the enriched δ¹⁵N values observed in these four specimens may reflect individual dietary differences, such as foraging on larger or higher-trophic-level prey, or temporal and regional variation in nitrogen baseline conditions. The latter possibility is consistent with the observation that the Bohuslän specimens, which may derive from older regional contexts, and the Late Palaeolithic samples analysed by McGrath et al. (2025) also show elevated δ¹⁵N values (>16.0‰). Because the four specimens highlighted here are undated, it remains unclear whether their enriched δ¹⁵N values reflect older baseline conditions, individual foraging differences, or a combination of both.

Additionally, some variability may reflect differences among tissues and skeletal elements. This is an important limitation of this study because bone, skin, and dentine collagen do not necessarily represent identical temporal windows of dietary intake. However, the likely magnitude of this effect depends on whether tissue differences reflect physiological routing/fractionation alone, or whether they also reflect tissues integrating isotopically distinct diets, habitats, or life-history phases. Where skin and bone collagen are ultimately synthesized from the same broadly consistent year-round dietary protein pool, any residual offset between these tissues should primarily reflect intrinsic physiological fractionation, amino-acid routing, or collagen composition, rather than seasonal differences in prey, habitat use, or turnover time. In such cases, offsets are expected to be smaller than those reported for species whose tissues integrate isotopically distinct foraging periods, migratory phases, or ontogenetic habitats.

Although tissue differences therefore preclude direct equivalence among all datasets used here, published marine mammal and cetacean studies suggest that many tissue-specific offsets are relatively small, commonly on the order of tenths of a per mil to ∼1‰–2‰ for δ¹³C and δ¹⁵N, although values vary among tissues, species, diet, and turnover rates (Borrell et al., 2012; Browning et al., 2014; Hobson et al., 1996; Teixeira et al., 2022). Larger inter-tissue differences can occur where tissue turnover causes different tissues to record different phases of an individual's life history or migration, particularly in species that move among isotopically distinct freshwater, estuarine, and marine habitats (Hesslein et al., 1993; Kline et al., 1998; Suring and Wing, 2009; Tieszen et al., 1983). This mechanism is unlikely to apply strongly to the sperm whales analysed here, because sperm whales rely predominantly on meso- and bathypelagic cephalopods and are not expected to undergo repeated seasonal shifts between isotopically distinct feeding systems comparable to anadromous, estuarine, or coastal-offshore migratory taxa (Clarke et al., 1993; Santos et al., 1999, 2002).

Nevertheless, inter-tissue and intraskeletal isotopic offsets are documented in cetaceans and ideally require species-specific correction factors, which remain limited for most taxa (Borrell et al., 2012; Smith et al., 2020; Teixeira et al., 2022). We therefore treat mixed-tissue comparisons cautiously and do not assume strict equivalence among bone, skin, and dentine collagen. However, given the broadly consistent prey base and foraging ecology of sperm whales, tissue-specific effects are unlikely to drive the major isotopic patterns identified in this study (although they may still contribute to smaller differences among groups/ocean basins).

Another limitation concerns potential individual duplication within fragmented assemblages. Four samples from Oostermiddenmeerweg, Wieringerwerf, the Netherlands (DAG158, DAG159, DAG161 and DAG162), may derive from a single individual represented by several large cranial fragments. Attempts to refit these fragments were unsuccessful, so their association remains uncertain. DAG158, DAG159 and DAG161 yielded highly similar stable isotope values, consistent with possible common origin, whereas DAG162 differed isotopically and had a higher C/N ratio (>3.4), reducing analytical confidence relative to the other fragments (Ambrose, 1990; Van Klinken, 1999). To minimise pseudoreplication during analyses, only DAG158, which produced the strongest collagen quality indicators (C/N = 3.23), was retained for interpretation. More broadly, this highlights a common challenge in zooarchaeological assemblages: highly fragmented remains can make it difficult to distinguish unique individuals from repeated sampling of the same animal. Conservative single-specimen sampling or DNA-based individualisation currently provide the most direct ways to address this problem; this approach has been used previously to distinguish historical bone specimens among baleen whales (Buss et al., 2025).

A further complication is temporal variability in marine baseline δ¹³C and δ¹⁵N values. The Suess effect is well recognised as a key driver of baseline δ¹³C since the start of the industrial revolution, reflecting anthropogenic inputs of isotopically light carbon into marine systems altering oceanic biogeochemistry, especially of surface waters (Eide et al., 2017). Therefore, regionally resolved correction frameworks are increasingly available for modern and recent historical datasets (Clark et al., 2021). Over longer timescales, however, natural changes in ocean circulation, productivity and nutrient cycling can also shift baseline δ¹³C and δ¹⁵N values independently of predator diet. For prehistoric datasets such as those analysed here, baseline conditions remain poorly constrained, and robust corrections are not yet possible. Long-term isotopic archives from foraminifera, squid beaks and fish otoliths are beginning to resolve such variability at regional and temporal scales (Golikov et al., 2024; Grønkjær et al., 2013; Li et al., 2025; Ren et al., 2009; Straub et al., 2013). As these baseline reconstructions improve, it should become increasingly feasible to disentangle isotopic shifts driven by ocean biogeochemistry from those reflecting ecological change in marine predator diets.

Chronological uncertainty is closely related to this problem. The inclusion of undated specimens is an important source of uncertainty in our Late Holocene dataset. While most undated material is likely to derive from the Late Holocene based on provenance, direct chronological information was unavailable for these specimens. Our sensitivity analysis indicates that their inclusion does not substantially affect the isotopic distribution of the Late Holocene dataset, as δ¹³C and δ¹⁵N values did not differ significantly between dated and undated specimens, including when the Swedish specimens were excluded. The Bohuslän specimens from Sweden should be treated with particular caution because radiocarbon dating results of other whale and seal taxa from the same area suggests they may be older than the rest of the undated material here (Fredén, 1975). However, excluding these specimens did not materially alter the observed isotopic patterns, suggesting that the main conclusions are not dependent on their inclusion. Future radiocarbon dating of undated specimens, particularly those from Bohuslän, would help refine the temporal resolution of the dataset and further test their placement within the Late Holocene comparison group.

Dates based on archaeological context alone should be treated cautiously for large whale bones, particularly where curation, reuse or transport of skeletal elements is possible. Direct radiocarbon dating is preferable, but cetacean tissues are affected by the marine reservoir effect, and ΔR can vary spatially, temporally, and due to ecologically according to habitat use, foraging depth and regional oceanography (Van Den Hurk et al., 2025). Species feeding in offshore or deeper waters may incorporate older carbon into their tissues than taxa using shallow coastal food webs. This is directly relevant to sperm whales, which commonly forage at several hundred metres and may exceed 1,000 m during deep foraging dives (Aoki et al., 2012; Watwood et al., 2006). To account for this as far as possible, we calculated an eastern North Atlantic sperm-whale-specific ΔR20 of −192 ± 40 ¹⁴C years. We therefore suggest that this provisional ΔR20 estimate is currently the most appropriate empirical correction for eastern North Atlantic sperm whale remains, while emphasising that future known-age specimens may refine or revise it. Provenance adds another layer of uncertainty. Most archaeological specimens analysed here show traces of working, indicating that sperm whale bone was used as a raw material for artefacts and tools. Worked whale bone need not derive from locally stranded animals, because whole bones, partially processed blanks or finished artefacts may have been transported over considerable distances. This issue also applies to the specimens analysed by McGrath et al. (2025), several of which were recovered far inland. Stable isotope values therefore cannot always be assumed to reflect the local ecology of the find location.

Moreover, the concentration of archaeological and palaeontological sperm whale finds around the southern North Sea (Van Den Hurk et al., 2022, 2023a) is consistent with this region functioning as a long-term zone of recurrent sperm whale encounter, probably linked to the tendency of young males to strand in the shallow and geomorphologically complex southern North Sea (IJsseldijk et al., 2018; Schnitzler et al., 2018). Thus, we assumed here that our samples reflect localized signals rather than long-distance transportation from elsewhere. However, the archaeological record alone cannot distinguish repeated local strandings from selective human transport and curation. Nevertheless, the isotopic distinction between our samples and existing Mediterranean and extra-North Atlantic samples provides support that the analysed specimens represent an oceanic North Atlantic, rather than Mediterranean or extra-basin, isotopic signal. However, the present bulk isotope and GMM results cannot distinguish local southern North Sea strandings from material transported from elsewhere within the oceanic North Atlantic, because such specimens would likely fall within the same broad isotopic cluster. The concentration of archaeological and palaeontological sperm whale finds around the southern North Sea is therefore best interpreted as evidence for a likely encounter or trapping zone, rather than as direct evidence for local population density or exclusive local procurement.

4.3 Looking towards the future

The environmental changes associated with the Late Glacial Transition provide a useful, although not direct, analogue for ongoing transformations in the modern ocean. This interval was marked by rapid reorganisation of North Atlantic circulation, changes in ventilation and nutrient supply, shifts in productivity, and restructuring of deep-sea ecosystems (McManus et al., 2004; O’Neill et al., 2021; Skinner et al., 2021; Yasuhara et al., 2014). Comparable processes are now being observed or projected under anthropogenic climate forcing, including ocean warming, increased stratification, acidification, deoxygenation, altered nutrient cycling, and changes in primary productivity and food-web structure (Bindoff et al., 2019; Doney et al., 2012; Petrik et al., 2020). Late Glacial isotopic baselines therefore offer a rare opportunity to assess how large, wide-ranging marine predators responded to abrupt ecosystem change in the absence of direct industrial exploitation. During the Late Glacial, a reduction in the strength of the Atlantic Meridional Overturning Circulation (AMOC) is thought to have enhanced upper-ocean stratification and altered nutrient supply, reorganizing, and in some regions, reducing, primary productivity in the North Atlantic (McManus et al., 2004). Such conditions would favour more complete nitrate utilisation, leading to elevated baseline δ¹⁵N values. This pattern is consistent with the elevated δ¹⁵N observed in Late Glacial sperm whale samples (McGrath et al., 2025), indicating that these large-scale oceanographic changes were transmitted through pelagic food webs to deep-diving predators. The persistence of distinct nitrogen isotope structure between the Late Glacial and Late Holocene samples (Figure 2) further suggests that sperm whales may be effective candidate sentinels of ocean change going forward, as shifts in their isotopic signatures, are likely to provide early evidence of changing productivity, trophic pathways, prey availability, or nitrogen baseline conditions in deep-pelagic ecosystems.

Although biopsy sampling of living sperm whales is feasible, strandings in the North Sea provide valuable opportunistic access to tissues that could support long-term monitoring of isotopic change in the eastern North Atlantic. Coordinated sampling of stranded individuals, ideally combining bulk isotopes, compound-specific isotope analysis, genetic data, and life-history information, would provide a powerful framework for tracking temporal variation in nitrogen baselines and deep-ocean food-web structure. Such monitoring could contribute to early-warning indicators of climate-driven change in offshore and bathypelagic ecosystems.

Recent methodological developments offer several ways to strengthen this approach. Compound-specific isotope analysis of amino acids can help separate baseline from trophic effects by comparing “source” amino acids, which largely retain baseline isotope information, with “trophic” amino acids, which become progressively enriched with trophic transfer (Chikaraishi et al., 2009; Nash et al., 2024; Whiteman et al., 2019). Here, “source” amino acids refer to compounds whose nitrogen isotope values mainly reflect the isotopic baseline at the base of the food web, whereas “trophic” amino acids are those that become enriched in δ¹⁵N with each step up the food chain. This approach has already been applied to sperm whale tissues to reconstruct temporal shifts in ocean biogeochemistry and trophic structure (Ruiz-Cooley et al., 2014). It therefore offers a promising route for distinguishing changes in sperm whale trophic position from shifts in nitrogen baselines, both in modern monitoring and in well-preserved archaeological material.

Future work should also integrate isotope data from potential prey taxa, particularly meso- and bathypelagic cephalopods and fishes, to assess whether changes in sperm whale δ¹⁵N values reflect shifts in prey-community structure or prey trophic position. In parallel, independent baseline archives such as foraminifera-bound nitrogen isotopes, fish otoliths, and cephalopod beaks could help constrain temporal changes in nitrogen cycling and separate oceanographic baseline shifts from predator-level dietary change. Additional isotope systems may provide complementary information. Sulphur isotopes (δ³⁴S) can help distinguish marine, coastal, freshwater, and terrestrial sources and may improve interpretation of habitat use and provenance when combined with δ¹³C and δ¹⁵N (Nehlich, 2015; Raoult et al., 2024; Richards et al., 2003). Zinc isotopes (δ⁶⁶Zn), measured in skeletal mineral, are emerging as a useful trophic proxy in archaeological and palaeontological marine mammals and may be less affected by some of the baseline problems that complicate bulk nitrogen isotope interpretation (McCormack et al., 2021). Because these proxies can preserve in skeletal tissues over millennia, they offer strong potential for reconstructing dietary and biogeochemical change across recent geological and archaeological timescales.

Once sufficiently large spatial isotope datasets become available, these same methods may also support provenance studies of sperm whale bone and teeth. Worked sperm whale remains are unlikely to represent local strandings, because whale bone could have circulated as raw material, blanks, finished artefacts, or curated objects. A spatial baseline combining bulk δ¹³C/δ¹⁵N, δ³⁴S, and δ⁶⁶Zn would make it possible to test whether archaeological sperm whale products found inland or outside known stranding zones were locally acquired or transported through exchange networks. Such an approach could help reconstruct the movement of sperm whale products through ancient Europe and clarify the relative importance of local scavenging, opportunistic collection, long-distance trade, and artefact curation (Charpentier et al., 2022; Lefebvre et al., 2021; McGrath et al., 2025; Van Den Hurk et al., 2023a).

5 Conclusion

By combining improved radiocarbon calibration with stable isotope analysis, this study provides a millennial-scale perspective on sperm whale ecology from a large span of the North Atlantic. Additional samples from the far western North Atlantic are required to fill data gaps. Our results suggest that sperm whale trophic ecology has remained broadly stable over time, despite substantial reorganisation of nitrogen cycling since the Late Glacial. As wide-ranging, deep-diving predators, sperm whales integrate ecological signals across large spatial, vertical, and temporal scales. Their consistent isotopic structure across the wider North Atlantic likely reflects this broad-scale foraging ecology, supported by extensive movement and male-mediated gene flow rather than strong regional specialisation.

At the same time, persistent differences between Mediterranean and oceanic North Atlantic sperm whales, together with isotope-specific basin-scale patterning globally, highlight the importance of large-scale oceanographic and biogeochemical processes in shaping sperm whale isotope values. These findings provide a rare long-term isotopic baseline for sperm whales in the eastern North Atlantic and demonstrate that apparent ecological stability at the predator level can coexist despite major changes in underlying ocean systems.

This long-term perspective is increasingly important as sperm whales face multiple anthropogenic pressures, including acoustic disturbance, fisheries interactions, pollution, and climate-driven shifts in prey availability. Although sperm whales appear to have maintained broadly consistent trophic strategies over millennial timescales, their ecology remains closely tied to deep-ocean food webs and large-scale ocean processes. Consequently, future changes in their isotopic niche may provide valuable early indicators of reorganisation in deep-pelagic ecosystems under ongoing anthropogenic climate change.

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