ORIGINAL RESEARCH article

Front. Ecol. Evol., 26 August 2022

Sec. Behavioral and Evolutionary Ecology

Volume 10 - 2022 | https://doi.org/10.3389/fevo.2022.919093

Movement and olfactory signals: Sexually dimorphic antennae and female flightlessness in moths

  • 1. School of Biosciences, The University of Melbourne, Melbourne, VIC, Australia

  • 2. Centre for Integrative Ecology, School of Life and Environmental Sciences, Deakin University, Burwood, VIC, Australia

Abstract

Darwin argued a role for sexual selection in the evolution of male sensory structures, including insect antennae, the strength of which will depend upon the importance of early arrival at receptive females. There is remarkable variation in the nature and degree of sexual dimorphism in moth antennae, with males of some species having spectacular, feathery antennae. Although it is widely assumed that these elaborate structures provide greater sensitivity to chemical signals (sex pheromones), the factors underlying the interspecific diversity in male antennal structure and size are poorly understood. Because male antennal morphology may be affected by several female life–history traits, including flight ability, we conducted a phylogenetic comparative analysis to test how these traits are linked, using data from 93 species of moths across 11 superfamilies. Our results reveal that elaborate antennae in males have evolved more frequently in species where females are monandrous. Further, female loss of flight ability evolved more frequently in species where males have elaborate antennae. These results suggest that elaborate antennae have evolved in response to more intense male competition, arising from female monandry, and that the evolution of elaborate antennae in males has, in turn, shaped the evolution of female flightlessness.

Introduction

Signalling is a crucial component of reproduction for mobile diecious species, playing a role in both bringing mates together and in facilitating mechanisms of pre–mating sexual selection (e.g., ; ; ). The latter is responsible for the evolution of an extraordinary diversity of conspicuous, sexually selected signals, across the range of sensory modalities, and has attracted very extensive research interest. In many moths, females use sex pheromones (olfactory signals) to advertise their location to mate searching males. Although long–distance location–revealing sex pheromones are typically not regarded as being subject to sexual selection [see ], they may cause sexual selection to act on male receptor organs (; ).

suggested that sexual selection will favour improvements in “organs of sense” if that improves the likelihood of male mating success in a competitive environment. Darwin did not explicitly mention antennae as “organs of sense,” because pheromones and the odour receptors located on antennal sensilla (e.g., ) were not known at that time (). Nevertheless, his perspective suggests that selection will favour males with antennal features that improve the speed of detection of sex pheromones if that allows males to locate females more quickly than rival males. The taxonomically widespread sexual dimorphism in insect antennal morphology (e.g., ; ), together with several lines of empirical evidence, are consistent with this perspective (). Field experiments on a sexually dimorphic moth, the gum-leaf skeletonizer Uraba lugens, demonstrated that males with longer antennae were more likely to detect lower amounts of pheromone (), and laboratory experiments with the same species revealed that larvae developing in higher densities (thereby anticipating greater reproductive competition as adults) resulted in males with larger antennae and testes (), and in females releasing more attractive sex pheromone ().

Sexual dimorphism in some species of moths is remarkably striking, with the elaborate, feathery, pennate antennae of males contrasting with the simple threadlike or filiform antennae of females (; Young, 1997). Phylogenetic comparative analyses indicate that while elaborate antennae are linked with larger body size, suggesting a functional cost to these structures, this pattern is not necessarily consistent with a greater capacity to detect chemical signals, because larger females might be expected to release larger quantities of pheromone (). On the other hand, in species where males have elaborate antennae, there is a negative correlation between male abundance and male antennal length. This result suggests that lower population densities with concomitantly lower concentrations of pheromone, may select for larger antennae in males, at least in species where males have elaborate antennae (). Regardless, males of most moth species possess relatively simple filiform antennae, whilst females still emit long–distance sex pheromones. This suggests that the strength of selection on male antennal morphology is linked to other factors that determine the importance of quickly locating a female, and thus rapidly detecting her sex pheromone. A previous comparative analysis of male elaborate antennae in geometrid moths () found that they were more likely to be found in species where females were capital breeders (i.e., that eclose with greater body reserves already available for breeding). suggested that such a strategy may be associated with traits that make them more difficult for males to locate quickly, such as reduced mobility.

There is emerging interest in the effects of movement on the production and detection of signals and cues, although research is largely confined to visual and auditory sensory modalities (). Nevertheless, movement may be consequential for chemical signalling: female moths may adjust the detectability of their sex pheromones by selecting different kinds of locations where they call (emit pheromones). For example, pheromones released in closed habitats may be less easily detected, and females may compensate by moving to a more open location (). Clearly this option is not available to less mobile, flightless female moths. Females are flightless in roughly 1% of lepidopteran species, although it is taxonomically widespread, occurring in 25 families (). Female wing loss in these species varies from a complete loss of wings (aptery) to retaining full sized wings but with a loss of function (Tweedie, 1976; ). Flightlessness is associated with winter–active adults and spring feeding, as well as high host breadth (). In most flightless species, the females are unable to move far and remain on or near their pupation site throughout their typically short adult life (). noted that these locations may not necessarily be optimal for calling and suggested that this selects for greater sensitivity to the pheromone in males. The converse may also be possible: that loss of flight evolved in part due to the speed with which highly sensitive males can locate females.

Here we use a phylogenetic comparative approach to examine several potential selection pressures favouring male antennal complexity in moths, including flightlessness and other life–history traits. Specifically, we ask whether certain female traits that potentially reduce their reproductive window (i.e., monandry, short lifespan, and the stage of egg development) are associated with the evolution of elaborate male antennae. Our hypothetical framework is that rapid location of a female would be favoured under female monandry, or when females eclose with a full complement of eggs (proovigeny): slower males may arrive after a female has mated and is no longer receptive, or after she has commenced ovipositing, and thus their sperm will fertilise fewer eggs (; ). We hypothesise that these female life–history traits would likely increase the level of competition among males and increase the strength of selection favouring elaborate antennae, because such antennae should increase the male’s likelihood of locating the female more quickly.

Subsequently, we explore the links between male antennal morphology, female life-history and the evolution of female flightlessness. We specifically test the hypothesis proposed by that female flightlessness will be associated with the need for greater male sensitivity to sex pheromone, and hence with male elaborate antennae.

Materials and methods

Data collection

We collected information on female mating strategy, male antenna type, female flight ability, egg development, oviposition behaviour, lifespan, and fecundity for 93 species of moths from 11 superfamilies. The species were selected based on the availability of these data, which were collated from various sources including published literature, field guides and online lepidopterist resources (see Table 1).

TABLE 1

SpeciesASFAMSEDOBLSFWSReferences
Acentria ephemerellaSFB1180123, 18, 28, 125
Achroia grisellaSMM2018.512, 27, 66, 107, 229
Adoxophyes oranaSMPM30019.525, 156, 215, 222
Alsophila pometariaSFMPB10027.537, 150, 171, 188
Anarsia lineatellaSMM2412013.555, 87, 152, 194
Auchmophila kordofensisEFB2130133
Austromusotima camptozonaleSMMM5.724, 241
Autographa gammaSMS122104037, 85, 138, 230
Biston betulariaEMPM67052.537, 106, 171
Bupalus piniariaEMM101503516, 44
Cadra cautellaSMPS2001712, 106, 124, 142, 143, 229
Callimorpha (Panaxia) dominulaSMMS6.252504850, 51, 71, 230
Chilo suppressalisSMM102502254, 108, 159, 160, 206
Chloridea (Heliothis) virescensSMPSM80032140, 164, 175, 201, 211, 215, 229
Choristoneura fumiferanaSMPSM9422.557, 184, 215, 225, 229
Choristoneura rosaceanaSMPS75029, 57, 136
“Cnephasia” jactatanaSMPS137109, 110, 111, 161
Corcyra cephalonicaSMMSS8.21601768, 163, 189
Cornutiplusia circumflexaSM129, 138, 185
Cryptoblabes gnidellaSMMSM12.7105159, 100, 151, 238
Cydia pomonellaSMPPS1720018.526, 96, 120, 229
Dahlica lichenellaFMB10701564
Dasystoma salicellaSFM1044018.548, 112, 174
Desmia funeralisSMS92002519
Diparopsis castaneaMPSS11.41523045, 106, 137
Earias insulanaSMPS730016.58, 115, 116, 201, 203, 229
Elcysma westwoodiEMMM121, 122
Ephestia elutellaSMPM211751710, 124, 189
Ephestia kuehniellaSMPSS92642.257, 106, 114, 124, 189, 208, 239
Epiphyas postvittanaSMPSM40018.525, 72, 73, 82, 110, 126, 215
Epirrita autumnataSMMPS10150355, 89, 197, 205, 207
Etiella hobsoniSMM6563, 157
Etiella zinckenellaSMMS101662563, 88, 135, 229
Euproctis chrysorrhoeaEMM20037.537, 74, 80, 118, 139
Euxoa messoriaEMMSS14.21,3033429, 43, 65, 106
Grapholita molestaSMPS3424512.511, 26, 33, 62, 96, 167
Helicoverpa (Heliothis) armigeraSMPSS9.7538035.552, 104, 158, 201, 229, 243
Helicoverpa (Heliothis) punctigeraSMPSS1,400201, 243
Helicoverpa zeaSPSS1560038.52, 49, 69, 119, 201, 229
Heterogynis penellaEFB123, 220
Homoeosoma electellumMMS143371958, 144, 183
Hyalophora cecropiaEMMP13538, 182, 216, 229, 233
Lambdina fiscellariaEMMS22.520091, 92, 153, 197, 236
Leucoma salicisEMPM9.46504575, 80, 221, 226, 229
Leucoptera coffeellaSMMSM14756.583, 132, 147, 148
Lobesia botranaSMMSS1001226, 100, 213, 215
Lymantria dispar*EFMPB88005034, 42, 80, 181, 226, 229
Lymantria fumidaEM99, 226
Lymantria mathuraEMM3754556, 226
Lymantria monachaEMMPM1904537, 80, 106, 192, 226
Lymantria obfuscataEFMM7.628531.860, 84, 209, 226
Lymantria xylinaEMB80040, 195, 226
Mamestra configurataMPSM2,1504097, 232
Manduca sextaSMSS25010061, 117, 169, 229, 240
Metisa planaEFMPB7155177, 179
Mnesampela privataSMMSM11.830041196, 197, 201, 223, 224
Mythimna separataMPS6.58503594, 231
Mythimna unipunctaMPSM141,5004032, 199
Oiketicus kirbyiEFMB6,40017, 177, 178
Oncopera fasciculatusSMMM1,50047, 131
Operophtera bruceataSFS27.575, 90, 170
Operophtera brumataSFPPM8.810026.530, 90, 106, 218, 219, 229
Operophtera fagataSF31.590, 193, 229
Orgyia antiquaEFMPB71752538, 106, 180, 204, 226
Orgyia leucostigmaEFPB16732.538, 79, 204, 210
Orgyia pseudotsugataEFMB1803467, 98, 200, 226
Ostrinia nubilalisSMPSM157503070, 106, 186, 229
Paleacrita vernataEFM25129, 149, 171
Panolis flammeaSMSM1110036106, 127, 128, 201
Paralobesia viteanaSMS18.5331039, 100, 113
Phigalia titeaEFM1103531, 53, 170, 171
Phthorimaea operculellaSMPSM18.31001446, 134, 155, 229
Platynota stultanaSMMM30712.522, 78, 81, 233, 234
Plodia interpunctellaSMPS8015.57, 50, 76, 106, 124, 215, 229
Plutella xylostellaSMMSS18.810013106, 202, 227, 229
Siederia listerellaFMB10501564
Sitotroga cerealellaMPM11.512511.24, 189
Sparganothis sulfureanaSMM2059, 78, 168,
Spodoptera exiguaSMPPM13.91,00027.51, 37, 212, 214, 245
Spodoptera littoralisSMPSM161,800386, 77, 115, 187, 229
Spodoptera lituraSPSM1,84930106, 141, 190, 245
Sthenopis argenteomaculatusSM8037
Synanthedon pictipesMMSS9.214024.5106, 176, 237
Teia anartoidesEFMPB4002541, 86, 198, Pers. Obs.
Thyrocopa kikaelekeaSF145
Trichoplusia niSMPSS181,50035.5191, 228, 229, 242, 244, 245
Utetheisa ornatrixSMPSM1735037.523, 37, 101, 102, 103, 130
Vitacea polistiformisEMMPS735021, 162, 166, 235
Yponomeuta cagnagellaSMPSM502313, 95, 146, 165, 217, 229
Yponomeuta evonymellaSMM13105, 146, 217
Yponomeuta padellaSMPM3113, 93, 165, 173, 217
Zeiraphera canadensisMMSM287913.535, 36, 154, 172
Zeiraphera dinianaSMMS301501914, 15, 20, 26

Classification of species for traits examined.

Male antennal structure (AS) = simple (S) or elaborate (E); Female flight ability (FA) = macropterous (M) or flightless (F); Female mating strategy (MS) = monandrous (M) or polyandrous (P); Egg development strategy (ED) = synovigenic (S) or proovigenic (P); Oviposition behaviour (OB) = single (S), multiple batches (M), or one batch (B); Lifespan of adult females (LS) = number of days; Fecundity (F) = number of eggs; Male wingspan (WS) = mm.

*The subspecies of Lymantria dispar used for this study was the European or North American spongy moth, which is flightless in nature and has the most available data.

1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12 (moths.csiro.au); 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65 (eol.org); 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95; 96; 97; 98; 99 (jpmoth.org); 100; 101; 102; 103; 104; 105; 106; 107; 108; 109; 110; 111; 112; 113; 114; 115; 116; 117; 118; 119; 120; 121; 122; 123; 124. Manaaki Whenua (www.landcareresearch.co.nz); 125; 126; 127; 128; 129. (lepiforum.org) 130; 131; 132; 133; 134; 135; 136; 137; 138; 139; 140; 141; 142; 143; 144; 145; 146; 147; 148; 149; 150; 151; 152; 153. Canadian Biodiversity Information Facility (2013–2017); 154; 155; 156; 157; 158; 159; 160; 161; 162; 163; 164; 165; 166; 167; 168; 169; 170; 171; 172; 173; 174; 175; 176; 177; 178; 179; 180; 181; 182; 183; 184; 185; 186; 187; 188; 189; 190; 191; 192; 193; 194; 195; 196; 197; 198; 199; 200; 201; 202; 203; 204; 205; 206; 207; 208; 209Thakur et al. (2016); 210Thurston and MacGregor (2003); 211Tingle and Mitchell (1991); 212Tisdale and Sappington (2001); 213Torres–Vila et al. (2002); 214Torres–Vila et al. (2001); 215Torres–Vila et al. (2004); 216Tuskes et al. (1996); 217Van der Pers et al. (1980); 218Van Dongen et al. (1998); 219Van Dongen et al. (1999); 220Vegliante (2005); 221Wagner and Leonard (1979); 222Walker (2005); 223Walker and Allen (2010); 224Walker and Allen (2011); 225Wallace et al. (2004); 226Wallner (1989); 227Wang et al. (2005); 228Ward and Landolt (1995); 229Watson et al. (1975); 230Watson and Dallwitz (2003–2022); 231Watson and Hill (1985); 232WCCP (1995); 233Webster and Cardé (1984); 234Webster and Cardé (1982); 235Weihman (2005); 236West and Bowers (1994); 237Wong et al. (1969); 238Wysoki et al. (1993); 239Xu et al. (2008); 240Yamamoto and Sota (2007); 241Yen et al. (2004); 242Zacharuk (1985); 243Zalucki et al. (1986); 244Zhu et al. (1997); 245Ziaee (2012).

We characterised male antennae as either simple or elaborate: the former is filiform or ciliate, whereas elaborate antennae have one or more side branches (pectinate, bipectinate, quadripectinate). Information on male antennae was obtained from published descriptions or images from various resources (see Table 1 for a full list). Females were designated as capable of flying (macropterous) or flightless: flightless females may still have wings, but do not fly (e.g., Lymantria dispar) or may be brachypterous (small remnant wings) or apterous (wingless).

Mating strategies of females were classified as either monandrous or polyandrous, using information on female remating frequencies (mating frequency of males was not taken into account, as less information is available on this). Monandrous species are those with a remating frequency of 30% or less. This is a conservative value compared with rates previously used to categorise monandry [for example, <50% in , and <40% in Torres–Vila et al. (2002)]. Where detailed data on remating frequency were not available, we followed published qualitative descriptions of species as being either monandrous (including mostly monandrous) or polyandrous.

Females were classified as either synovigenic (continuing to form eggs during their adult life) or proovigenic (eclosing as an adult with a full complement of mature eggs). Species where females eclose with some mature eggs but produce more as an adult were classified as synovigenic. We also distinguished between females that oviposit eggs singly, in multiple small clutches of eggs, or in a single clutch. We then made this classification binary by combining the data for species in the latter two categories who did not lay eggs singly, as this was necessary for statistical analysis (see below). The fecundity and lifespan of the females were obtained from published papers, and we included the midpoint if a range was given. Fecundity was the total reported number of eggs laid over the lifespan. Female lifespan was the average number of days as an adult, as observed in natural field populations where possible. Finally, as body size may affect aspects of mating behaviour () and male antennal morphology (), we also obtained measures of the maximum wingspan to use as a proxy for body size (, ). We included male wingspan if the wingspan of both sexes were reported, and the midpoint of the range if a range was reported.

Phylogenetic comparative analysis

We used phylogenetic comparative methods to control for common ancestry () in our analysis of evolutionary associations between traits. There is no single phylogeny that incorporates all the species in our sample, and genetic data covering these species is sufficiently poor to make phylogenetic estimation unreliable. Following recommendations by , we compiled a composite phylogeny from the published phylogenies of the families included in our sample. The full tree, along with character mapping for binary traits, is presented in Figure 1.

FIGURE 1

The main framework for the phylogeny was the superfamilial tree constructed by . The superfamilies Hepialoidea, Sesoidea, and Zygaenoidea each contained two species only, and so further resolution was not necessary. Any species from the same genus were also grouped together. We used the phylogeny from to resolve relationships within the Pyraloidea. No further details were available for the Phycitinae family, leaving this group of eight species as a polytomy except for those species from the same genus, which were grouped together. The Tortricoidea were resolved to species level where possible following . Within the Tortricoidea, Paralobesia viteana was absent from all published phylogenies and therefore its position was left unresolved as a basal polytomy within this clade. The Archipini was also lacking detail on species from our analysis leaving those four species unresolved. was used to fully resolve the Yponomeutoidea phylogeny, whereas was used to resolve the three species in the genus Yponomeuta. The Bombycoidea phylogeny was taken from . also provided further resolution to the Psychidae, although two species were missing rendering this group not fully resolved. Yamamoto and Sota (2007) and were used to resolve the Geometroidea to subfamily, but no further details were available for all species present in our phylogeny leaving the six species of the Ennominae unresolved as well as the three species in the genus Operophtera. The Noctuidae were resolved using Zahiri et al. (2013), and the Lymantria phylogeny was taken from . The superfamily Gracillariidae was fully resolved using the phylogeny from . Relationships within the Gelechiidae were taken from and .

In the absence of branch length information, all branches were assigned equal length (=1). The exception to this rule was cases where there remained uncertainty over the branching pattern; any polytomies were arbitrarily resolved but the resolved branches were assigned minimal branch lengths of 0.00001, giving them negligible weight in the analyses. This resolution was necessary because the phylogenetic comparative analysis approach requires fully dichotomously resolved phylogenies.

Following , we ran multiple tests to improve the strength of our inferences. These tests included generalised linear mixed models with Bayesian estimation (MCMCglmm) () and the concentrated changes test (). The MCMCglmm models were conducted using the MCMCglmm package in R (; ). To determine which traits are linked to our dependent variables when controlling for phylogenetic relatedness, all independent variables of interest were tested using MCMCglmm in separate models where wingspan was included as the covariate to control for body size. Specificially we performed one set of tests with antennal morphology as the dependent variable, coded as simple (1) and elaborate (2) antennae. In these tests the independent variables were female mating system (monandrous/polyandrous), egg development (proovigenic/synovigenic), and lifespan. In the second set of tests the dependent variable was female wing type coded as macropterous (1) or flightless (2). In these tests the independent predictors were female mating system, lifespan, antenna type, egg development, oviposition strategy (single eggs laid/multiple eggs clutches), and fecundity.

These models cannot incorporate missing data; therefore, the data sets were reduced to include only those species where all data were available for the variables being tested in each analysis. These tests require a prior to be set, and when the response variable is categorical in nature the prior for the residual variance should be fixed. For our analyses, we set the residual variance to 1 and used a χ2 distribution with 1 degree of freedom (v = 1, nu = 1000, alpha.mu = 0, alpha.V = 1) for the random effects variance, as suggested by Villemereuil et al. (2013). We used the categorical family response, and used the slice function to improve mixing. To obtain adequate mixing with low autocorrelation, 5 × 107 iterations were used with a 10,000 burnin period, followed by sampling every 2,500 iterations (thinning). The R code used in this procedure is provided in the Supplementary material.

The Concentrate Changes Test (CCT) was used to investigate the co–evolution of two binary discrete characters across a phylogeny, using the computer program MacClade 4 (). CCTs examine changes from one state to another in a character of interest (the dependent variable) and whether these are more concentrated in lineages that have evolved a particular state of interest in another character (the independent variable) than would be expected by chance (determined by comparison with 1,000 permutations of the dependent variable on the phylogeny). When characters are mapped onto the phylogeny, the most parsimonious reconstruction of the character is applied for all species, including those with missing data, allowing us to leave all 93 species in each analysis. CCTs require a fully resolved phylogeny and we used MacClade to randomly resolve any polytomies 20 times in our composite phylogeny, performing the analysis on each tree individually, thereby yielding a mean p-value ± SEM. For each pair of traits, the tests were run in both directions, with the independent and dependent traits switched, to determine the order of evolution. We also applied the test only to changes where the parsimonious resolution of the evolution of the dependent was unequivocal. These tests lack the statistical power of MCMCglmm, and cannot control for body size, but unlike the MCMCglmm, they do provide information about the direction of evolution (i.e., which correlated trait preceded the evolution of the other), and possible patterns of causality.

Results

We found a significant association between antennal type and female mating system (MCMCglmm analysis: Table 2), with the concentrated changes test indicating weakly (p = 0.062) that elaborate male antennae evolve more frequently in species where females are monandrous (CCT analysis: Table 3). Indeed, most species in our analysis with elaborate male antennae have monandrous females, whereas most species with simple male antennae have polyandrous females (Figure 2A). Similarly, male antenna type is also associated with egg development pattern, with elaborate male antennae being associated with proovigenic females (Table 2). Indeed, within our sample of species with elaborate male antennae, only one is synovigenic, producing eggs as an adult (Figure 2B). However, the CCT analysis suggests (again weakly: p = 0.081) that proovigeny has evolved more frequently in species where males have elaborate antennae (Table 3). The type of male antennae was not associated with female lifespan (Table 2).

TABLE 2

Dependent variableIndependent variableNPosterior mean95% CIP-value
Male antennae—Simple (1),Female mating system—Monandry (1), Polyandry (2)45Mating–6.17–13.69, 0.430.043
Elaborate (2)WSpan0.22–0.01, 0.500.017
Egg development—Synovigenic (1), Proovigenic (2)39EggDevel7.591.02, 15.570.005
WSpan0.10–0.02, 0.240.051
Female lifespan40Lifespan–0.15–0.53, 0.200.410
WSpan0.16–0.05, 0.390.112
Female wing type—MacropterousFemale mating system—Monandry (1), Polyandry (2)53Mating–3.39–8.78, 1.360.155
(1), Flightless (2)WSpan–0.16–0.41, 0.040.115
Female lifespan48Lifespan–0.64–1.29, –0.060.005
WSpan–0.002–0.23, 0.230.979
Male antennae—Simple (1), Elaborate (2)62Antennae5.210.02, 10.920.028
WSpan–0.20–0.43, –0.010.014
Oviposition—Single eggs (1), Egg batches (2)60Oviposition4.47–0.47, 10.30.056
WSpan–0.17–0.40, 0.040.08
Fecundity64Fecundity–0.002–0.01, 0.010.556
WSpan–0.16–0.39, 0.040.113

Results from the MCMCglmm analysis comparing the dependent variable (male antennal type, female wing type) to the independent variable, controlling for body size (male wingspan).

Each categorical variable was given a value (1 or 2) according to the state (e.g., polyandry = 2 and monandry = 1). N values for each model are listed. The model of wing type vs. egg development is not included due to lack of transitions with which to adequately test the relationship. Bold values indicate statistically significant relationships.

TABLE 3

Character 1Character 2p-value ± SE (Character 1 = dependent)p-value ± SE (Character 2 = dependent)
Male antennae—ElaborateFemale mating system—Monandry0.062 ± 0.0040.317 ± 0.008
Egg development—Proovigenic0.159 ± 0.0110.081 ± 0.002
Female wing type—MacropterousFemale mating system—Monandry0.093 ± 0.0040.957 ± 0.002
Male antennae—Elaborate0.052 ± 0.0030.676 ± 0.027
Oviposition—Egg batches0.076 ± 0.0050.983 ± 0.001

Results from the concentrated changes test investigating evolutionary associations between discrete characters with the state of interest listed.

Results are presented where characters are assigned as dependent or independent, and then vice versa. The p-value ± SE corresponds to the mean p-value obtained from the analysis of the 20 randomly resolved trees. N = 93 in all tests.

FIGURE 2

We found a link between female flight ability and male antenna type (Table 2), with the CCT analysis suggesting that female flightlessness is more likely to have evolved in species where males have elaborate antennae, rather than vice versa (Table 3 and Figure 3C). Female flightlessness is also associated with a shorter female lifespan (Table 2), and the association between flight ability and oviposition strategy (single eggs vs. batches) approached significance in the MCMCglmm analysis (p = 0.056, Table 2 and Figure 3D). The CCT results suggest that flightlessness has evolved more often in species where females lay eggs in clutches (Table 3). While all the flightless species in our sample are proovigenic (Figure 3B), the lack of transitions from synovigenic to proovigenic severely limits the power for the MCMCglmm analysis, and these analyses did not converge, so we do not include these tests in our analysis. Although most flightless species have monoandrous females (Figure 3A), the phylogenetic comparative analyses found no significant association between female flight ability and mating system, or between flight ability and fecundity (Tables 2, 3).

FIGURE 3

Discussion

Prevailing wisdom states that elaborate antennae in males evolved to increase their ability to detect odours (specifically the sex pheromone of females). However, this hypothesis is contradicted by the considerable number of moth species where males do not possess elaborate antennae, despite most species utilising long-distance female sex pheromones. Our results suggest a more nuanced version of the hypothesis where ability to detect and locate females quickly is advantageous in some species: elaborate male antennae being more common in species where females are monandrous, proovigenic, or flightless. Concentrated changes tests indicate that it is more likely that the evolution of elaborate male antennae is concentrated lineages where females are monoandrous rather than vice versa (suggesting that the mating system evolved prior to the antennal morphology). Similarly, that the CCTs suggest that female flightlessness is more likely to have evolved in lineages where elaborate male antennae had already evolved rather than the opposite, which argues against the hypothesis that female flightlessness selects for males with more sensitive antennae. These trends provide some insights into the conditions leading to the evolution of both male elaborate antennae and female flightlessness, and more generally highlight how olfactory signal perception can be linked with movement.

conjecture that sexual selection favours male sensory receptor traits that improve the ability to detect and locate females more quickly is supported by the strong association between complex male antennal structure and female monandry. Female monandry places a premium on males being able to rapidly locate virgin females, whose numbers may decline during the male’s adult lifespan, and larger or more elaborate male antennae apparently facilitate this process [see also ]. This interpretation implicitly assumes that elaborate antennae bestow greater sensory capabilities, and this has been widely assumed in previous analyses of antennal morphology in moths (; ). However, while elaborate antennae might facilitate the trapping of air flow and hence chemical compounds (), the direct evidence that they have higher sensitivity is surprisingly rare. In this context exceptions to the patterns outlined above can be informative. For example, selection may still favour elaborate male antennae in the polyandrous satin moth Leucoma salicis because females initially oviposit a large clutch of eggs, and subsequently lay smaller clutches (Wagner and Leonard, 1979), which places a premium on males locating virgin females. In this context, it is interesting that there is a relationship between egg development strategy and male antennal morphology, with proovigeny (where females eclose with their full complement of eggs) being associated with elaborate male antennae.

Although the large majority (>90%) of species with elaborate male antennae have monandrous females, males have filiform antennae in 17 of the 30 monandrous species in our sample. In these species, perhaps males with filiform antennae utilise other means of improving sensitivity in detecting female pheromones. For example, detectability may be improved by increasing the number and/or density of sensilla, changing the distribution of the sensilla (; ), or adjusting the angle of antennal scales (Wang et al., 2018). Additionally, features of the adult population, such as high population density () or male flight speed, may relax selection on rapid detection. Finally, male mating system may also affect the evolution of antennal morphology, although the nature of the relationship between typical male mating frequency and the strength of selection for greater detection capacity seems unlikely to be independent of other factors, including female mating strategy. Consequently it may be that it is overall mating strategy (monogamy vs. polygamy) that is the stronger determinant of selection on male antennal morphology.

Female flightlessness is thought to allow females to invest more resources in egg production, thereby yielding higher fecundity (Tweedie, 1976; ; ). Our analyses revealed that female flightlessness is associated with a shorter adult lifespan and confining oviposition to a single clutch of eggs, an unsurprising pattern because laying eggs singly or across multiple clutches would require an ability to disperse to different oviposition sites (). However, there was no significant correlation between female flightlessness and fecundity after controlling for phylogeny and body size. reported a positive correlation between female flightlessness and fecundity, but this pattern was phylogenetically constrained, and the correlation was no longer significant after taking account of phylogeny. Our analysis supports that finding. Nevertheless, we cannot rule out the possibility that females can improve reproductive success by increasing egg size rather than egg number. Indeed, the flight ability of female tussock moth Orgyia thyellina varies seasonally: individuals eclosing in autumn have reduced wings whilst those emerging in summer have fully functional wings (). The flightless females produce much larger eggs than females that emerge in summer, a strategy that may improve the survival of eggs that diapause over winter. Males of O. thyellina have elaborate, bipectinate antennae (Table 1), and it would be interesting to know if there is a similar seasonal pattern in antennal size.

While our data broadly support the idea that the presence of elaborate antennae in males favoured the evolution of female flightlessness, there are seven species within our data set where females are flightless, and yet males have simple, filiform antennae (Figure 3). Interestingly, females in five of these species are still mobile, either by walking or hopping (; ; ), and females of the other two species have unusual biological features. Females of Acentria ephemerella are aquatic and the loss of flight is likely to be an adaptation that supports this lifestyle (), highlighting that the evolution of female flightlessness is not linked exclusively with issues associated with mate search. The second species indicates that the strength of selection through mate search can override the relationship between elaborate antennae and flightlessness: around 80% of Alsophila pometaria females are pseudogamous asexuals (), who produce asexual offspring after mating, resulting in a strongly female–biased population. So, while the loss of flight has likely evolved due to similar pressures affecting other species in our data set, the pseudogamous nature of this species may relax selection favouring elaborate male antennae.

To conclude, our findings suggest that the communication and mating systems of moths are inherently associated, with elaborate male antennae being associated with lineages where females are monandrous, suggesting that necessity to detect females quickly has selected for more sensitive males. In turn male elaborate antennae may have subsequently driven the evolution of female life-history and flightlessness where having sensitive males promoted selection for complete development of eggs at the expense of movement capability. A key aspect of this narrative is that elaborate antennae in males is associated with greater sensitivity, an assumption that still needs to be more thoroughly tested in moths.

Statements

Data availability statement

The original contributions presented in this study are included in the article/Supplementary material, further inquiries can be directed to the corresponding authors.

Author contributions

TJ assembled the comparative data set. TJ and MS conducted the comparative statistical analyses. All authors conceived and designed the research, interpreted the data, prepared and edited the manuscript, and approved the submitted version.

Funding

We thank the Australian Research Council (DP0987360) for financial support.

Acknowledgments

We thank Nina Wedell and the late Matthew Gage for their helpful comments on an earlier version of this manuscript.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fevo.2022.919093/full#supplementary-material

References

Summary

Keywords

sex pheromone, sexual selection, mating system, antennal morphology, flightless moth, mate location

Citation

Johnson TL, Elgar MA and Symonds MRE (2022) Movement and olfactory signals: Sexually dimorphic antennae and female flightlessness in moths. Front. Ecol. Evol. 10:919093. doi: 10.3389/fevo.2022.919093

Received

13 April 2022

Accepted

03 August 2022

Published

26 August 2022

Volume

10 - 2022

Edited by

Jennifer M. Gleason, University of Kansas, United States

Reviewed by

Gerhard Johannes Gries, Simon Fraser University, Canada; Astrid T. Groot, University of Amsterdam, Netherlands

Updates

Copyright

*Correspondence: Mark A. Elgar, Matthew R. E. Symonds,

This article was submitted to Behavioral and Evolutionary Ecology, a section of the journal Frontiers in Ecology and Evolution

Disclaimer

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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