Who Cares? Parenting in Invertebrate Animals

AGE: 8 Many of us find it hard to relate to spiders or other types of invertebrates, including insects, snails, clams, jellyfish, and squid. With over a million species, invertebrates (which are animals that do not have spines) make up most of Earth’s animals, and their behaviors are quite diverse. It might be surprising to hear that some invertebrates take care of their o spring by feeding, protecting, and raising them. The type of care given to o spring, including which parent is involved, generally depends on the environment these animals live in. Invertebrates often have parenting strategies that allow them to produce the greatest number of surviving o spring over the course of their lives. Since their time and energy is limited, some invertebrate species prioritize making lots of o spring over providing care, while others do the opposite.We also describe special examples of parental care, to illustrate the fascinating sacrifices some invertebrate parents make. These might not seem so di erent from what our own parents are willing to do for us!


INTRODUCTION
Most of us grow up relying a lot on our parents. Our moms and dads feed us, clothe us, teach us, and keep us safe. If you think about what parenting looks like in animals, you might think about mama bears, known for their fierce protectiveness of their cubs. Or you might think of birds who take turns feeding their helpless chicks, or kangaroos carrying joeys in their pouches.
But what about other animals, such as invertebrates? Invertebrates

WHY DO SOME INVERTEBRATES CARE FOR THEIR OFFSPRING, WHILE OTHERS DO NOT?
There are a lot of di erent and complex ways that invertebrates care for their young, which makes it hard to say how this behavior evolved in the first place. However, there are a few ideas about factors that influence parental care in di erent species. One important factor is the environment the invertebrates raise their young in. First, parental care is really important for animals that live in harsh environments, like in a desert, where eggs or young cannot easily survive without the help of their parents [ ]. Second, when food is scarce and only around for a short time, there can be lots of competition for it. In this case, young animals need their parents' help to get enough food. Last, parents may need to guard their o spring from predators that want to eat them.
Another important factor influencing parental car is the number of o spring that survive when parents are around vs. when parents are not. The number of surviving young influences how many copies of the parents' genes are passed on to the next generation. In fact, the total number of surviving o spring over a parent's life is much more important than the success of a single batch of o spring (called a brood) [ , ]. Depending on the circumstances, parents might benefit BROOD A group of young animals, often hatched at the same time.
the most from either staying and caring for their young, or from leaving and letting their young fend for themselves [ ]. For instance, if parents have only one brood of o spring in their lifetimes, it makes the most sense for them to stay and help as many young as possible survive to adulthood. Parents are also more likely to stay and care if they have already spent a lot of time and energy on their o spring, such as guarding their eggs for several weeks, or if the brood is very large. Otherwise, if parents can have several broods of children, it might be a better strategy to use their energy making lots of o spring and letting those o spring try and survive by themselves.

SPECIAL EXAMPLES OF PARENTAL CARE IN INVERTEBRATES
Feeding o spring Some invertebrate parents help feed their o spring. This is possible even in species in which the female dies before her eggs hatch. These females lay what are called trophic eggs, which are an important first

TROPHIC EGGS
Unfertilized eggs laid by a mother for her o spring to eat. meal for the young after they hatch, feeding them until they are old enough to look for food themselves ( Figure A). Having trophic eggs to eat also reduces cannibalism among hungry siblings!

CANNIBALISM
The act of eating one's own species.
Although mammals are known for mothers producing milk for their babies, a few invertebrates do something similar. Most insects produce many o spring, but female tsetse flies only produce one larva at a time. This larva stays in the female's uterus, where it feeds on a milk-like liquid [ ]. In the jumping spider Toxeus magnus, the mother also produces a nutritious, milk-like liquid that her o spring drink from her body, much like a cow and her calf [ ].
As an extreme example of a mother's sacrifice, velvet spiders literally allow their young to eat them alive ( Figure B)! A velvet spider mother originally feeds her children a nutritious liquid through regurgitation,

REGURGITATION
The act of bringing swallowed food up from the stomach to the mouth again, like vomiting. like some birds do. Eventually, the mother spider allows her kids to eat her alive. The spiderlings do not leave the nest until all traces of their mother have been consumed [ ]. While this behavior might seem cruel and unfair to the mother, the spiderlings end up much larger and better able to catch prey and survive because of their mother's sacrifice.

Brooding
A few invertebrate parents spend a large amount of time physically carrying or protecting their o spring. Male water bugs are model fathers, since they can carry more than eggs on their backs until they hatch ( Figure C). This behavior keeps the eggs from being gobbled up by predators. Unsurprisingly, brooding so many eggs decreases how well the fathers swim and can make catching prey harder as well [ ]. Not only do these water bug fathers carry their unhatched children everywhere, but they spend a lot of time cleaning the eggs with their hind legs. Eggs that stay on their father's backs have a good chance of hatching, while those that fall o never hatch [ ].

Nurseries
Other invertebrate parents create nests, burrows, or nurseries for their o spring to grow up in. Jamaican bromeliad crabs, Metopaulias depressus, are unlike many crabs because they live their entire lives on plants called bromeliads, instead of in the ocean or on the beach. Bromeliads are tropical plants whose leaves overlap at the bottom and form a cup that collects water. These pools of water might seem small, but they are ideal nurseries for young crabs of this species to grow up in. Mother crabs make sure these pools have the right balance of oxygen, carbon dioxide, and pH. They do this by removing rotting leaves that might make the water too acidic [ ]. Mother crabs also increase the calcium content of the water by allowing calcium-rich snail shells to dissolve in it; without this extra calcium, the crab larvae cannot successfully molt and develop to their next stage.

WHO CARES? MOM OR DAD?
Single motherhood and fatherhood are common among invertebrates, although in some species, both parents stay around to care for o spring. Once again, the type of parenting a species performs depends on how they can produce the greatest numbers of surviving o spring over their lifetimes.
Mothers are most likely to take care of the young alone if they are limited by time or resources. For example, there are not many males available for her to mate multiple times with and produce many di erent sets of o spring with. Or if it takes a long time to make another batch of eggs, it might make sense for the mother to take care of her young and try to make sure that as many of those o spring survive as possible. Species that only mate once might care more for their o spring, because they only have one chance to make sure some of them survive-some octopodes, for example, guard their eggs for months or even years at a time without leaving to hunt. Because it takes so long for their eggs to hatch, the mothers spend all of their remaining energy guarding a single batch of eggs (Figure ).
Childcare might fall on the fathers if the mother is busy producing another set of eggs [ , ]. Since the father is spending his time and energy caring for the o spring, the mother can use her energy in making bigger and healthier eggs, which increases the chance of the eggs surviving. For some animals, a father that guards his o spring is very attractive to other females, because he is actively proving what a good father he is. Other females may choose to mate with him as well and leave their eggs in his territory. This increases the number of o spring he ends up having. Fathers that carry their eggs with them spend a lot of energy caring for o spring, and likely do so only because there are no places to leave them that are safe from predators.
Both parents will partner up to take care of their o spring when lots of work is required and when tasks can be divided between the parents [ ]. For example, dung beetle parents work together to claim, build, and bury dung balls into specially made nurseries for their o spring (Figure ). One parent cannot both build a nest and guard it from intruders, so both parents need to contribute. Both parents are also more likely to raise the young together if the o spring are all full siblings and the female did not mate with several males [ , ]. Having two parents around is also like an insurance for their young, in case one parent dies before the o spring are independent.

AUTHORS
ANGELA CHUANG I am currently a Ph.D. candidate at The University of Tennessee, where I study the role of animal behavior in global change, especially biological invasions. At age , I finally got over my lifelong fear of spiders, and am now fascinated by them. Spider behaviors are alternately relatable, alien, and taboo to me, and I am excited to learn how these behaviors will prepare them to survive in an increasingly human-dominated world. *angelachuang@tennessee.edu ORLANDO SCHWERY I grew up on a little Swiss farm on the edge of town and have always enjoyed getting lost in nature, observing its many forms and wondering how they came to be. I am currently a Ph.D. student at The University of Tennessee, studying diversification-how and why we have many species of some kinds of organisms, but very few of others. I do this using computer simulations and models, but also by looking at real organisms-plants in the past and dung beetles now. Asides from my research, I enjoy hiking, board games, cooking, reading, and martial arts.