Sex on Six Legs: Lessons on Life, Love, and Language from the Insect World - Marlene Zuk (2011)
Chapter 7. Parenting and the Rotten Corpse
I HAVE never understood why nature shows on animal families are always filled with images of doting monkeys nursing their infants, or diligent songbirds delivering a beak full of worms to the nest, when much more tender sacrifice takes place under leaf litter in the garden. If you want an ideal example of a good animal mother, for my money you can't do better than an earwig. Now there's a devoted parent for you. After they lay their eggs, earwig mothers stand protectively over the clutch, scrubbing them clean of fungus and other nasty contaminants and keeping predators at bay. Once the eggs hatch into minuscule copies of their parent, mama earwig goes resolutely out into the world to catch prey in the form of aphids and other tiny invertebrates for her brood. In some species, the female digests the food first and then regurgitates it to her begging offspring, as if offering a squalling infant a bottle. If the young earwigs signal their distress, she responds to the solicitation with eager defensiveness. Oh, and that business about them climbing onto people's heads and into their ears? Utter nonsense. According to entomologist James Costa, the name was probably originally ear-wing, after the resemblance of the insect's hind wing to a human ear (honestly, I don't see it, but that's an urban legend for you). How that got transmogrified into an auricular horror story is anyone's guess.
I happen to have an admittedly unpopular fondness for earwigs, but there are a lot of other good insect parents out there. The real champions, of course, are the social insects such as bees and ants, in which the mother goes into the hive or nest after a brief mating flight, never to emerge again. Day in and day out for months, sometimes years, the queen mother produces egg after glistening egg, like chocolates on an assembly line, forswearing any other activity. After the first batch, of course, the maternal care itself—the feeding, the cleaning, the guarding—is foisted off onto the queen's other offspring, but she is still at it, using the sperm she has saved from a dimly recalled frolic in the open air. She can never wait "until the children are grown" to do something for herself; her nest will never be an empty one, and her sacrifice is lifelong. I suppose it's true that not a lot of alternative careers are out there for older female ants and bees, but then many of the human mothers who dream that if only it weren't for the children they would be the CEO of a Fortune 500 company are not exactly being realistic either.
Comedian Milton Berle said, "If evolution really works, how come mothers only have two hands?" The answer is that at least some of the time, they don't—they have six. The social wasps, ants, and bees are highly specialized in their parenting, but many other kinds of insects, from true bugs to ladybird beetles, tend their young to varying degrees. Giant water bug females glue their eggs to the backs of the males, who carry the eggs around until they hatch. Assassin bug parents stand guard over their eggs and hatchlings. Ladybird beetles (a more entomologically correct name than ladybugs, since they lack the soda straw-like mouth parts of true bugs) produce infertile eggs alongside the more conventional ones, apparently solely to feed the voracious young beetle larvae when they hatch. And in some species of cockroaches, insects even more reviled than the earwigs, parents will remain together after mating, and females feed the young with special secretions, almost like milk. Sometimes females do all the parenting, sometimes males do, and sometimes both parents cooperate to raise their young. Finally, in most insects, no one cares for the young at all—the eggs are plopped unceremoniously in their place to fend for themselves after hatching.
All of this variation means that insects are an ideal place to look for insight into the evolution of family life. Why did parental care evolve in some groups and not others? Why does the amount of care, from a brief cleansing swipe over the eggs to putting the kids through college, differ so much among different kinds of animals? Why does mother do most of the work in many species, both parents in a few, and the father alone in fewer still?
Humans and other social mammals are of little help in answering these questions, because we always invest an enormous amount of energy into raising our offspring; we cannot compare situations where care is and is not given. Scientists often use birds to study the evolution of parenting, but these too are rather invariant in their behavior, at least when viewed next to insects. True, some birds, for example, ducks, are ready to waddle at hatching and need only judicious direction to the nearest body of water, while others, for example, robins, spend days or weeks of hard labor ferrying food to the gaping mouths in the nest. But that difference pales in comparison with the contrast between a butterfly who lays her eggs on a plant stem and flits away, and a pair of beetles who collaborate to prepare a ball of carrion for their larvae and then respond to the begging motions of their offspring by feeding them a liquefied meal.
People have dearly held opinions about parenting, of course, which is one reason it is an interesting and worthwhile behavior to understand; for example, we want to know if it is "natural" for fathers to take less of an interest in their daughters than their sons, or in being an attentive parent at all. But beyond questions such as these, understanding how the family evolved can help us understand the evolution of social life itself. At the core of society is the oldest bond between individuals that exists: the bond between a mother and her offspring. Once a female stays with her young, the stage is set for siblings to collude and squabble, for alliances to form and dissolve. That in turn makes more complex social interactions possible. In the wink of an eye—well, maybe in a few million years—presto, you have a society, with insurance companies and a movie industry and supermarket tabloids. Families become labor unions, political parties, and royal dynasties. And it all started with a female bug standing guard over her egg. Looking at insects can help us see how we got there. The true social insects, which include the ants, termites, and many of the bees and wasps, are a situation unto themselves and, as I discuss elsewhere, share unique genetic relationships that make the costs and benefits of their behaviors different than those in other organisms. I will therefore confine myself here to the "other" social groups, where only the first timid steps toward a complex network of interaction have been taken.
Why Should You Care?
SO IF most insects, and indeed most animals, get by with recklessly flinging eggs to the four winds, metaphorically speaking, why has more elaborate parental care evolved where it has? People often assume that human children require as much care as they do because of two characteristics of our species: high intelligence, sometimes seen as a reliance on learning rather than instinct, and being born at a comparatively early and helpless stage of development. Our intelligence, and the accompanying complexity of our lives, supposedly means that parents need to spend a great deal of time teaching their offspring the ins and outs of life in society. If we were simple little automatons, the conventional wisdom goes, we too could dump our babies into the world and let them fend for themselves. But faced with an unopened cereal box and a carton of milk, much less a gazelle or a yam, a child needs someone else to open, kill, or cook the meal, and hopefully that someone will make sure that eventually the child will be able to do the same thing unassisted.
The problem with that argument is obvious once you think about insects. Although they do learn more than had previously been believed, as I discussed earlier, not even I am going to champion their qualifications for Mensa. And yet many groups still show elaborate parental care. Furthermore, the species that have doting parents don't seem any more or less intelligent than those lacking them.
What about the idea that we are stuck with helping our children because they are born at such a nascent state? Our big brains mean that the female pelvis can't accommodate a baby born any later, according to this notion, so it is related to that "we are so smart we have to toddle around for years before we can manage to leave our mothers" idea. But here too the insects make that suggestion look dubious, since egg size doesn't have much to do with the size of the brain of the bug that is hatched from it. Earwig mothers, doting though they are, never labor to bring forth their six-legged progeny.
What is essential is the guiding principle behind the evolution of every trait, whether that trait is a behavior, such as offspring care, or the shape of a body part, such as the length of a tail. Doing it has to increase the bearer's fitness, the likelihood of passing on its genes, more than not doing it. When it comes to parental care, that means that even if tending the young takes away valuable time and energy that could theoretically be used to have more offspring, it's worth it if you leave more copies of your genes that way than you would if you deserted your young and went off to have more children. Having offspring is valuable, certainly. But it will win you the evolutionary jackpot only if those offspring survive. Rampant fecundity for its own sake goes unrewarded.
For insects, and maybe for the rest of us, the threat that made it worth giving up future offspring to focus on the current batch seems to be predation. It's a beetle-eat-beetle world out there, so to speak, and eggs are about as vulnerable a stage to be exposed to it as can be imagined. In a small, unassuming black bug called a burrower bug, females guard the mass of eggs they produce in the leaf litter on the forest floor. Once the young hatch, the mother feeds her babies with nutlets, products of mintlike plants that grow nearby. When Japanese researchers Taichi Nakahira and Shin-ichi Kudo removed a female from her eggs on the grounds of Hokkaido University, the eggs succumbed to predators or were attacked by fungus and virtually never survived to hatching. Female shield-backed bugs in the harsh deserts of Baja California crouch protectively over their tiny brood on the stems of croton plants, and if the mother is removed, her young are almost immediately eaten by ants and other insects. If they happen to fall to the desert floor without their mother's watchful presence, they shrivel and die. It looks like love. But it's just a smart investment.
The earwigs demonstrate this trade-off even more clearly, because unlike the burrower bugs, in some species of earwigs an individual female may or may not tend her young. Some broods get more care, others less. In a paper titled "Benefits and Costs of Earwig Family Life" (with just a little bit of jazzing up, can't you see this as a sitcom?), Mathias Kolliker at the University of Basel in Switzerland pointed out that female earwigs sometimes lay two batches of eggs in a season, but if they invested in the work of protecting and provisioning the first set of young, they were less likely to produce a second. Even when they did lay another clutch, they did so later in the season than mothers who had abandoned the first clutch. That delay can be crucial when the cold weather threatens, which means that the benefit of nurturing the first batch of offspring has to be weighed against the cost of having the second batch die in the first storms of autumn.
Similarly, females of a kind of treehopper found in eastern North America will guard their eggs for varying lengths of time, but sometimes desert them right after they are laid. Andrew Zink painstakingly followed 370 female treehoppers over the course of an entire season, dabbing their bodies with colored paints so that he could identify individuals day after day. In terms of the benefits and costs of staying versus going, it turned out to be six of one and half a dozen of the other, or (in the case of the treehoppers) perhaps a hundred of one and five score of the other. Females that stayed longer had more eggs that hatched than the females that left after they laid their eggs, but the protecting females then had fewer and smaller broods later on. The bottom line is that taking care of children has to be selfish in evolutionary terms. A mother who died in the process of keeping one youngster alive under difficult circumstances would leave fewer copies of her genes than one who cut her losses but lived to reproduce again and again.
Sometimes it does pay to go for broke on the first batch of young, if life is so uncertain that survival to produce another brood is unlikely. In such instances, mothers sometimes offer up the ultimate sacrifice, as is detailed in a paper on a social spider by Ted Evans and his colleagues in Australia, delightfully titled "Making a Meal of Mother." Spiders are not insects, of course, but I include this one because it is such a wonderful example of how evolution can produce apparently self-destructive behavior. As any Charlotte's Web aficionado knows, all spiders show some kind of maternal care, but in this one, the young spiderlings slowly suck blood from the leg joints of their mother while she is still alive. Gradually they consume more and more of her body, until, in Evans's words, "After several weeks, she is decrepit, unable to move, and the offspring eat her entirely." When the scientists weighed the young spiders, they had gained virtually exactly the amount their mother had lost. And being fatter at the outset meant you had more to offer; scrawnier mothers were consumed sooner than their more zaftig counterparts, which then meant that the young spiders were less likely to turn from their parent to an arguably even more unsavory occupation: eating each other. Having your children feed on your still-quivering flesh to keep them from cannibalizing their siblings sounds like something out of a parody of a Philip Roth novel. But for the spiders, it's sensible, since each brother or sister eaten is a genetic investment lost to the parent.
Under what circumstances do we expect these and other less dramatic efforts by parents to show up? In his classic book Sociobiology: The New Synthesis, Harvard biologist (and ant lover) E. O. Wilson lists the "prime movers" that make the evolution of parental care more likely. First is the threat of predators that I mentioned earlier. Other contributors include the kind of environment in which a species finds itself. If food is unpredictable or the climate harsh, offspring will benefit from a parent's buffering them from the vagaries of the world.
Doug Tallamy and Thomas Wood from the University of Delaware took this one step further and pointed out that in addition, the species that evolve parental care have to have several preconditions. They have to reproduce only during a narrow window of time and during a brief period of the year, simply to make parenting cost-effective. They have to survive long enough to provide enough care to be meaningful, a tall order for animals as short-lived as most insects. And because insect eggs don't last well unprotected, particularly during long cold periods, the adults can't spend much of the summer or other favorable time of year growing up themselves. Instead, they have to be mature, or nearly so, at the start of clement weather so that the young ones get the full benefit of their care over the nicer times of the year. And last of all, some behavior patterns that could be adapted to parental care have to be present already in the species; Tallamy has seen the same kind of aggressive display used by mother lace bugs in defense of their brood when the females are discouraging overly ardent males during the mating season.
These kinds of traits are sometimes called preadaptations, characteristics that happen to be present for some other reason and that can then be co-opted for a different use under different selective pressures. Natural selection can't use an empty palette; the raw material for parenting, or flight, or digesting a new food, or any other evolutionary innovation, has to be there already. Preadaptations aren't to be confused with evolutionary premonitions—you don't live in a climate with fleeting resources or bursts of cold because nature decided you should be a parent someday and thought those things would nudge you in that direction. They are more like the bits and pieces that happen to be lying around in your garage; if you want to build a bookshelf, and Home Depot doesn't exist, you are stuck using what you have available. So parenting used the lifestyles already out there and built upon them to make the patterns of care that we see today.
Your Turn, or I Got Up Last Time
Mothers are fonder than fathers of their children because they are more certain they are their own.
A MALE friend of mine says that when he used to take his son to day care, the other parents—all mothers—would stare at him like he was a child molester. Social progress of the last few decades notwithstanding, women in virtually all parts of the world do much more of the child care than men. It's also much more common for mothers than fathers to take care of the young in animals, insects included. Why is that?
As with many basic truths, Aristotle was onto something when it came to parenting. The concept he alludes to above is what behavioral biologists call confidence of paternity. It implies nothing about a conscious awareness of one's likelihood of having been cuckolded. A male bird, say, that fetches mouthful after mouthful of painstakingly collected caterpillars to the chicks whose beaks gape insistently in his nest will not pass on his genes if it turns out that the female mated with another male during her fertile period several days or weeks earlier. Even worse, he has wasted time he could have spent trying to mate with other females himself. The female, on the other hand, either laid those eggs or didn't, and she was there at the time (aside from exceptions such as brood parasitism, which I'll discuss later). In mammals, where the events of mating and birth are even more widely separated, the problem of knowing which babies are sired by which male is even worse. There, of course, only the female can supply the offspring with milk, but males can perform other fatherly acts, for example, protecting everyone from predators or getting food for the mother.
This disparity between the likely payoff to each sex that accrues from devoting oneself to one's offspring is often cited as the sole reason for males rarely being the sex that takes care of the young. It's evolutionary good sense to refuse to take on someone else's genetic investment, and males are generally thought to benefit more by competing for mates than by sticking with the offspring. But while certainty of paternity probably plays some role in the evolution of different reproductive behaviors, it is now emerging that it can't be the whole explanation. It's all very well and good to say that a male "should" go off and seek other females to mate with rather than stick around and care for the ones his current mate produces. But what are his chances of succeeding? Everyone likes to think he could have been a contender, but in reality, it's tough out there. Females may be scarce, they may be unwilling to mate with every philandering male that passes by, and the risk of being eaten by a predator before one is found may be high. Tip the balance of any one of these variables, and the parental care patter can skew toward mom, dad, or both. And insects are perfect test cases for ideas about "innate" parenting roles, because once the eggs are produced, either sex can easily protect or bring food to the babies, unlike mammals, where females have all the milk-producing apparatus.
Single fathers are rare among insects, as they are elsewhere, but when they occur they do a bang-up job. Giant water bugs are true bugs, meaning they have strawlike mouthparts that they use to suck up their food. Some bugs, for example, aphids, merely sip sap, but the giant water bugs are fierce predators that ambush prey such as other invertebrates, fish, salamanders, and frogs. Once they grasp their victim, the bugs inject enzymes that liquefy the contents so that the interior can be extracted. About the size of an almond with the shell on, the bugs patrol lakes and streams around the world.
When it is time to breed, males attract females by suggestively rippling the water's surface. Unlike most insects, after mating, instead of the female taking off with her store of sperm, giant water bug females from most species proceed to lay eggs on the back of the male, where they sit in neat pearly rows until it is time for them to hatch. Although the conspicuous bugs were noticed by early naturalists, the individuals bearing the eggs were assumed to be females. Even when the sex of the brooding individuals was known, scientists as late as 1935 declared that the females must be forcing the males to carry the eggs. The father, however, always mates with the female before allowing her to deposit her eggs, and the pair may go through several rounds of mating followed by egg laying, apparently at the male's behest. This probably ensures that at least a majority of the eggs carried are indeed the male's own. He then solicitously ensures the eggs are supplied with oxygen by periodically raising his back above the surface. Although more than one female can deposit her eggs on a given male, the space on his back may become limited, and the males do what they can to position females so that they lay eggs to fill in any gaps in the row. Carrying around the eggs is no easy task; their combined weight can be twice as heavy as the male himself. In one group of giant water bugs, the females lay egg masses that are attached to vegetation at the water's edge, and the males then guard the eggs while the female departs, perhaps to lay another batch fertilized and protected by another male. The male stays to guard the young bugs even after they have hatched, preventing both predation and cannibalism from occurring; a photograph of one such species shows the tiny striped nymphs clinging to a plant stem, as if to a pool toy, while their father bobs beneficently nearby.
Biparental care, in which both parents cooperate to take care of the offspring, is at least as rare as male-only care among animals. It is seen in a few insects, however, including some that have the unfortunate attribute of being able to clear a room better than any other species. When I was doing my doctoral research at the University of Michigan Biological Station, the state-of-the-art Alfred H. Stockard Lakeside Laboratory had recently been completed. It was a lovely building, with excellent facilities for studying all kinds of local flora and fauna, from algae to woodpeckers. I was lucky enough to have a room to myself on the second floor, where I housed my crickets and recording equipment. Most days I worked happily at the microscope or with my experimental subjects, chatting with the other students and faculty, and generally having a grand time. But sometimes one of us would see David Sloan Wilson or one of his helpers walking toward the building with a white bucket, and we all knew to scatter. A multitalented evolutionary biologist, at the time Wilson was working on burying beetles, and the stench was enough to make paint peel.
As their name suggests, burying beetles locate recently dead animals by smell. If a male arrives at a carcass first, he sends out a chemical signal to attract a female. The pair then prepares the carcass as a nursery by stripping off any fur or feathers and shaping it into a ball. They cover their prize with specialized secretions that deter mold from growing (though this does nothing to deodorize the body, as we discovered to our regret). They then dig in the soil underneath the carcass, allowing it to sink into the forest floor. Once the carrion ball has been safely sequestered underground, the pair mates. The female then lays her eggs in the soil surrounding the carcass, and when the eggs hatch, the larvae beg for food from their parents by turning their heads toward mom or dad and waving their tiny arms. They are fed with either bits of carrion carved off the carcass like slices from a roast, or regurgitated meat that has been partially digested by a parent.
Although this lifestyle has much to recommend it (a steady and nourishing food supply, protection from predators once the carcass is buried), burying beetles face a daunting problem: finding a suitable dead animal, and once it is found, defending it against rivals. Even avid hikers and wilderness lovers rarely come across a dead animal at all, much less one that is both small enough to handle and still fresh enough to provide the right environment for raising young. The beetles have extraordinarily sensitive odor detectors on their antennae, and can sniff out a carcass from miles away, but even so it is an extremely difficult undertaking. Recently acquired carcasses are therefore highly sought after, and if a burying beetle comes across a carcass that has already been colonized by another individual, pitched battles may result. Both parents participate in defense of their property, and if the male happens to be away from the carcass when a male intruder threatens, the invading male will kill the offspring and mate with the female. Female intruders are not as successful in taking over a carcass. If both parents are present, they can usually fend off the invaders and hang onto their prize. It is thought that this advantage of pair defense is what led to the evolution of parental care by both the mother and the father in this insect group. If the carcass is large enough, multiple females may stay and lay their eggs, although one of them is generally dominant over the others.
Perhaps because of this difficulty in finding carcasses, one species of burying beetle has abandoned the role of "nature's undertakers," as one website refers to them, entirely. The substitute for dead birds and mammals that they chose, however, has to give one pause. They didn't switch to eating live insects, say, or any kind of vegetable matter. Instead, they are found in snake nests, eating the eggs and feeding them to the offspring. The beetles do not bury the eggs, since the snake takes care of that herself, and then leaves, so that the beetles do not have to face an irate mother snake. It seems to me that this was not the most sensible choice to have made—quick, which would you find easier to locate in a forest, a dead mouse or a snake egg?—but presumably it allows the beetles to exploit a resource that is less likely to be taken over by competitors.
What Happy Families?
I HAVE a Six Chix cartoon depicting two generic-looking insects sitting in armchairs; one is obviously bloated and looks a bit guilty, and is being reassured by the other, "It's natural to eat your young, Marilyn ... especially when they start running around the house like that—sometimes you just lose it."
Insect mothers do indeed sometimes eat their young, but it's not annoyance that elicits the behavior. Infanticide and subsequent cannibalism are yet another manifestation of the rule that parenting is worthwhile only if it furthers the parent's interests. If investing in offspring now means losing out on future opportunities to reproduce, natural selection will not favor it. But a problem can arise if the world looked a certain way when a mother first produced her offspring and then changed once they were a little older. Because insects live their lives so quickly, often relying on transitory sources of food and shelter, and because they can replace a batch of eggs with relatively little trouble, simply offing one generation of young and starting from scratch is a more reasonable proposition for them than for at least some vertebrates, who need a lot of time to gin up another generation. This makes them excellent subjects for studying the circumstances under which infanticide is favored.
Say a mother beetle lays a batch of eggs when the environment is benign and food abundant. She will benefit most by depleting her fat stores and turning them into eggs, since she can replenish those stores with the food around her. The eggs will survive best if she guards them on the plant, since predators would eat them if she were not around. But unexpectedly, the food supply dries up—maybe the gardener stops watering her food plant, or a cold snap makes it hard for her to move around and eat. What should she do? If she could read, she would be advised to take a look at an insightful paper by Hope Klug and Michael Bonsall pithily titled, "When to Care for, Abandon, or Eat Your Offspring." In it, the scientists outline the circumstances that favor each option. Cannibalism of offspring is particularly likely to evolve if parents can be selective about which young they eat, focusing on the lower-quality ones. Eating eggs is also expected to be common if doing so increases the parent's reproductive rate later on.
These principles are nicely illustrated in a group of insects with the delightful name of assassin bugs. These often brightly colored bugs ambush prey from their perches on vegetation, and in many species one or another parent guards the eggs. One African species is unusual because the father, rather than the mother, is the caregiver. A male will guard the eggs laid by several females, so he does not lose future mating opportunities by running the bug equivalent of a day care center. Males frequently eat a portion of the eggs but tend to focus on those at the edges of the cluster. Interestingly, Lisa Thomas and Andrea Manica from the University of Cambridge in the United Kingdom found that those peripheral eggs were the most likely to have been parasitized by a tiny wasp and, hence, were going to yield baby wasps rather than miniature assassins. It doesn't appear that the males can tell which eggs are parasitized, because in the wasp-free laboratory they still eat eggs from the same position in the cluster as they do in the wild. In stead, natural selection presumably favored fathers that removed fewer of their future offspring from the gene pool. The eggs are an important food source; cannibalistic males didn't lose any weight while they were guarding, even though they couldn't go out and hunt.
Infanticide and subsequent consumption of young was frequently observed in laboratory animals such as rats, and for many years the behavior was interpreted as abnormal and pathological, an artifact of captivity. Its documented occurrence in insects somehow didn't seem relevant to people, perhaps because we don't automatically see ourselves mirrored in their behavior. But then it began to be seen in wild animals such as lions as well, and now it is clear that at least some of the time it is probably adaptive in nature, because rearing young when life is harsh, or at the expense of the parent's well-being, may be too big a gamble for it to be continued. (Several kinds of animals, including lions, also commit infanticide without cannibalism, and of others' rather than their own offspring, for different but equally adaptive reasons.) If the going gets tough, the tough—and the smart—stop taking care of their children.
Some insects even go so far as to produce infertile eggs, called trophic eggs, that they either eat themselves or use as extra provisions for the young that do hatch. Although in some cases this behavior may be opportunistic, with some eggs not developing because they are defective, in others the trophic eggs seem to have evolved as a food source. Ladybird beetles are particularly well known for the production of such extra meals and will lay more trophic eggs when prey are scarce, fewer if they are well fed. Starved female ladybird beetles may lay an egg and then immediately turn around and eat it, a much tidier solution to hunger than, say, gnawing off a limb. The trophic eggs often look different from those that develop normally. Putting extra provisions into additional eggs rather than simply making larger eggs that hatch into more robust offspring with the surplus may have evolved because mothers cannot manufacture larger eggs with more yolk reserves than they already do. Scientists suspect that trophic eggs may be cheaper to produce than the usual variety, although the details of this cost difference are still not clear. The trophic eggs can also deter earlier-hatching babies from eating the eggs of their tardier siblings, which means that the mother gets more offspring surviving to maturity.
In addition to being excellent subjects for examining cannibalism, insects are perfect for exploring another stark reality of family life: parent-offspring conflict. In extremely influential work published in 1974, Bob Trivers, the same biologist who worked out some of the niceties of sex ratio theory discussed in an earlier chapter, pointed out that while parents and children have half their genes in common, they don't necessarily both benefit from the same things. Imagine that a mother beetle has a brood of twelve offspring. All else being equal, natural selection will favor her giving equal amounts of food to each of her children, because they are each equally related to her and equally likely to pass on her genes. But from an individual offspring's perspective, getting more care for itself at the expense of its siblings will also be favored, since it is 100 percent related to itself and only 50 percent related to its siblings. Thus there is a difference of opinion, evolutionarily speaking, in where the attention should go. Trivers called that difference of opinion parent-offspring conflict, and it is now thought to occur in a wide variety of animals and even plants. Trivers's theory explains many seemingly paradoxical family behaviors, including the grimmest: infanticide, discussed above, and its cousin, siblicide, the killing of one's siblings.
At some level, everyone with siblings understands the urge to murder them. Parent-offspring conflict theory suggests that such desires are not necessarily maladaptive. For the ladybirds, cannibalism is a particularly potent risk because the beetles' bright colors indicate the presence of toxins in the body, and a noxious taste to go with them. Your average bird learns to avoid ladybirds, but the beetles themselves are not affected by the chemicals, and so a ladybird is its own worst enemy. Eating just a single egg, whether trophic or not, significantly boosts the growth rate of a ladybird larva, putting a premium on hatching early. In a study of European ladybird beetles, less than half of noncannibals made it to adulthood, but more than 80 percent of the larvae that had eaten a sibling matured successfully.
Earlier laid means earlier out of the egg, with a chance to prey on one's less precocious siblings. But the day that an egg hatches relative to its nest mates is largely under the control of the mother, since she, after all, is the one that puts them all there. This fact brings up an even more sinister aspect of siblicide: frequently the parent encourages it, or at least does nothing to keep it from happening.
Why should parents tolerate such shocking behavior? Once again, think of the offspring not as individuals in their own right, but as units that pass on the parents' genes. If a mother lays many eggs, and they all hatch, but then there isn't enough food for them all, they all starve, and she has lost, big time. If she lays only a few, and it turns out that there would have been enough food for more, she has still lost, albeit not as much. But if she goes ahead and overproduces, and then lets sibling rivalry take its course, she can achieve the golden mean, since exactly as many offspring as the available resources can support will have survived. So a mother that lays her eggs over several days, creating a situation in which they hatch at different times, may be hedging her bets; if food is abundant, everyone gets enough after hatching and all is well. But if not, the earlier-hatching individuals can eat their siblings and ensure that at least some of the offspring make it to adulthood.
Even if they do not actually consume their brothers and sisters, competing for limited food supplied by a parent, and shoving a less insistent sibling away, can achieve the same effect: more food for the one that shouts the loudest. Encouraging the competition, or at least turning a blind eye to it, will benefit the parent more than trying to break up the squabbles and ensure that food is divided equally among the members of the brood. Actual siblicide, rather than garden-variety making one's brothers' and sisters' lives miserable, is expected only under extreme circumstances, since of course the siblings have half their genes in common, and so eliminating them entirely has its costs as well.
Here too is where the parental and offspring interests diverge. No offspring is expected to willingly sacrifice itself, and indeed each one should attempt to get more food and attention for itself than its siblings. This greed is even expected to extend to the hypothetical future siblings the parent could produce; from an offspring's perspective, getting as much as it can right now will benefit it more than allowing its parent to keep some energy in reserve to invest in future offspring. But the parent should only give as much as necessary for the offspring to become independent and able to fend for itself, because the parent will be served best by saving some resources to produce offspring later. The resulting conflict, according to Trivers, leads to weaning tantrums in mammals and many other kinds of behavior in which young animals compete fiercely with their siblings and try to get more than their parent is willing to offer. Social insects, because they share different proportions of their genes with their siblings than their parents or offspring, are particularly prone to these kinds of disagreements.
If all of this sounds uneasily familiar from your own life, or perhaps the lives of some of your friends, you are not alone. One of the foremost theorists in the field of parent-offspring conflict, H. Charles J. Godfray, notes, "There are clear dangers in overinterpreting such behaviors (especially as they are observed through the prism of one's own family experiences)." This is where the insects come in handy; it is harder to anthropomorphize gleaming white grubs wriggling near a mouse corpse than, well, virtually any other baby animal I can think of. Although much of the early research on the evolution of parent-offspring relationships in nonhumans used birds as subjects, scientists are increasingly realizing that the begging behavior of burying beetle young is no different than that of a robin squawking in its nest, and it is much easier to use in experiments.
Not My Problem
RATHER than deal with quarreling offspring or hungry mouths, some insects whose young require care after the eggs are laid have abdicated at least some of that care entirely. We've long known that cuckoos, cowbirds, and a few other bird species are brood parasites, which means that females lay their eggs in the nest of another species, the host. Some ants do much the same thing by using a different species of ant to rear their young, either by capturing eggs of the foreigners and bringing them back to the nest or by killing a queen and replacing her with one of their own kind.
More recently, a more subtle but no less effective means of getting someone else to do the work of child rearing has been recognized in both birds and insects. Rather inelegantly but descriptively called egg dumping, it means exactly that: depositing eggs into the nest of another member of the same species. Often a female that practices egg dumping still cares for some of her own eggs, but the farmed-out offspring serve as a kind of bonus rainy day account, allowing her to literally not put all her eggs in the same potentially vulnerable basket. In other cases, skipping out on maternal care means that a mother can keep churning out batch after batch of eggs at a rate that a mother spending time and energy on demanding offspring could not equal.
Lace bugs, delicate insects with filigreed wings that live on a variety of garden plants, face a trade-off between protecting their young from predators and losing future reproductive opportunities by doing so. Doug Tallamy has studied maternal care in several kinds of lace bugs, and found that they will keep laying eggs in the egg masses of other females and abandon them to the care of the female that first started the egg mass. They keep doing so until they cannot find another suitable host, at which time they proceed to guard their own eggs and offspring, presumably along with those that other females have foisted onto them.
The burying beetles will also engage in a little stealth egg laying if their carcass has been taken over by another pair. The defeated female will stay near the dead animal and sneak into her former nursery to feed on the carcass herself and surreptitiously lay some eggs. Sometimes, if the carcass is large enough, she is even tolerated by the female in possession of the corpse, and both remain to rear their young in the same underground chamber.
As with infanticide, egg dumping used to be dismissed by biologists as an aberrant behavior that occurred because the female was too stupid to figure out how to breed properly; an early researcher who discovered egg dumping in ducks rather pejoratively called it "careless laying," and "degenerative." Presumably the idea that a mother would callously abandon her own offspring hit a little too close to home, as Godfray suggests above. Perhaps be cause no one expects insects to be smart in the first place, or identifies with a bug on a leaf, these biases have not gotten in the way of scientists developing hypotheses about the evolution of egg dumping in insects, another illustration of how using insects rather than vertebrates as models can make it easier to understand behavior.
One's immediate thought is that the dumpee, or host, is a sad patsy here, but unlike the case for brood parasites such as cowbirds or cuckoos, where the host's own offspring virtually always suffer as a consequence of the interloper's demands, egg dumping can actually benefit everyone concerned. Predators often do not gulp down the entire egg mass or group of babies. Instead, they usually nibble off a few eggs here and there, or they pluck the most vulnerable larva from a cluster. In such cases, it pays to be part of a teeming horde, because one's chances of being the unlucky victim go down the more options the predator has; if a wasp or spider snatches one egg, and ten are present, you have a one in ten chance of being eaten. But if one hundred eggs are in the mass, your chances go down to one in a hundred, much better odds. At the same time, guarding a mass of a hundred eggs isn't much more work, if any, than guarding ten. In a North American treehopper that lives on goldenrod leaves, the hatching success of broods that were supplemented with dumped eggs was 25 percent higher than that of broods reared with only their own brothers and sisters.
Given this happy everyone-wins scenario, why isn't egg dumping even more common than it already is? Tallamy suggests that the opportunity to dump eggs may be constrained by the size of the nest (if eggs have to be placed in a particular spot, for example, a stem, at some point there is no room left), the physiological capability of the female to keep producing eggs, or the synchrony among females in their reproductive stage. It does no good if you have eggs to offload if all the other nearby females are already half-way through the process. Finally, the kind of care given, for example, guarding, has to be such that it can't be much costlier to administer it to a large brood than a small one, kind of a cheaper-by-the-dozen effect.
Brothers, Sisters, All?
SOMETIMES even the most acute sibling rivalry has to take a back seat to cooperation, if cooperation is the only way to survival. A bizarre example of this perhaps begrudging all-for-one and one-for-all strategy occurs in a flightless blister beetle found in the sand dunes of the southwestern United States. The adult beetles eat, and lay their eggs on, an attractive purple-flowered plant called a milk vetch. But the larvae can't survive on the plant and instead are parasites in the nests of a solitary bee that also occurs in the desert. So how do you get to a bee's nest miles away over scorching sand when you are a baby beetle about the size of a poppy seed?
The answer is one of those You Couldn't Make This Stuff Up stories at which insects excel. Immediately after they hatch, the larvae make their way up to the tip of a plant stem, where they cluster together in a clump of anywhere from 120 to over 2,000 individuals. Viewed collectively, the clump resembles a female of the host bee species. They then emit a chemical that mimics the sex pheromone of the bee, which attracts a male bee eager for romance. When the bee lands and attempts to mate, some of the tiny larvae leap onto his back and are transported away from their siblings. They then transfer to a female bee during copulation when the hapless male eventually manages to find the real thing, and finally are taken to their goal of her nest when she flies back, replete with sperm and her minute beetle passengers. Once inside the nest, the larvae hop off and feed on the pollen and nectar the bee has brought back for her own offspring. Finally, they emerge as adults, to start the whole improbable cycle again.
Where did this bizarre life cycle come from, and why do the beetles rather than other—perhaps many other—kinds of animals exhibit it? We don't know for certain of course, but the sheer number of insect species may have provided a larger canvas on which to paint different pictures. Many of them probably died out before they became established, but a few, like this one, remained.
Although this scenario brings up many interesting issues, from the standpoint of family relationships it is like one of those conundrums of which the philosophers are fond. A male bee can hold only so many larvae, a fraction of the aggregation. But all of the members of the group have to cooperate to constitute a convincing mimic of a female bee. The remaining larvae can try again, to be sure, but as their numbers decrease, their portrayal of the bee loses its verisimilitude. This leads to what must be an increasingly uneasy alliance, as the beetles' only hope for survival arrives and the larvae must jostle for a chance to leap onboard. When should a larva stop working harmoniously, break rank with its brothers and sisters, and act solely in its own interests? Hollywood, take note. In my opinion, a movie based on this kind of drama has a lot more appeal than yet another tired take on the dysfunctional family reunion at Thanksgiving.