Sex on Six Legs Page 17

  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.

  Chapter 8

  Pirates at the Picnic

  Today it is accepted as proven that the ant is incontestably one of the noblest, most courageous, most charitable, most devoted, most generous and most altruistic creatures on earth.

  —MAURICE MAETERLINCK, 1930

  If ants had nuclear weapons, they would probably end the world in a week.

  —BERT HöLLDOBLER AND E. O. WILSON, 1994

  ANTS may inspire more emotional reactions than any other insect, reactions that go far beyond the revulsion of finding a cockroach scurrying across the kitchen counter or pleasure at seeing a butterfly light on a flower. As the two quotations above attest, ants can be paragons of harmony and virtue, or symbols of bloodthirsty violence. Honeybees come close to ants in serving as reflections of our own society, but we see bees singly, flying from blossom to blossom, rather than en masse, and the workings of the hive are not visible to most of us. Ants, however, stream across our driveways in glistening black ribbons. They seethe through our cereal boxes and bear crumbs triumphantly along edge of the shelf and out the door. With a few moments of casual observation, it's possible to see ants carrying their young from place to place, whereas no one other than beekeepers (and entomologists) ever sees much in the way of bee family life. And they walk, rather than fly, making them a little easier, perhaps, to identify with.

  Like many of the other social insects, ants seem to share food unhesitatingly, and they work tirelessly for their colony, as Maeterlinck notes above. Maeterlinck, a Belgian playwright and poet who won the 1911 Nobel Prize for literature, was particularly taken by the ants' practice of passing droplets of food from one individual to another, called trophyllaxis, a behavior not seen in most nonsocial insects. For reasons that are not altogether clear, at least to me, he seemed to think that this behavior was intensely pleasurable for the ants, somehow compensating the workers for their lack of sexual activity by a near-orgasmic sensation when the food was transferred.

  Solomon, of course, enjoined us to "Go to the ant, thou sluggard; consider her ways, and be wise." As historian Charlotte Sleigh writes in her entertaining book Ant, "The ant's supposed virtues of industry, prudence and mutual aid were extolled by a great number of people." The diligent ants that labor and save for the winter provide a smug contrast to the grasshopper that fritters the time away in a host of moralistic fables. A fair share of people probably sympathize with the indolent pleasure-seeking half of the story more than with the ants themselves, but the object lesson is clear: hard work is virtuous and will be rewarded. The Victorians seemed especially partial to the idealization of the ants' nobility and stressed the idyllic domestic activity supposedly taking place in the ant nest.

  But ants also have a dark side that is obvious to even a casual observer. Naturalists since ancient times noted the apparent wars that raged between ants of different colors, with battles that went on for hours. Army ants are so named for their rampaging behavior. And as Sleigh points out, "The commonly known fact that ants engage in warfare has given them a particular edginess in times of human conflict." And a handful of species of ants exhibit a behavior that is strikingly similar to slavery in humans: one kind of ant will make raids on a colony of another species and steal its young workers, to be reared in the nest of the invaders and put to work for the rest of their lives. Charles Darwin described part of such a raid in The Origin of Species, musing on "the wonderful instincts of making slaves." According to Bertrand Russell, "Ants and savages put strangers to death," although plenty of familial slaughter takes place as well. So-called killer bees are a close second, with plenty of media hype about enraged swarms pursuing hapless passersby. The pursuit is obvious (though the actual numbers of people attacked and injured is often exaggerated), and people stand at the ready to attribute rage and bloodlust to the pursuers.

  Hostility has also been linked with insects in novel ways. A now-defunct band from Houston, Texas, was called "Insect Warfare." Its album World Extermination is being re-released by the deliciously
named Earache Records, with apocalyptic cover art showing giant cockroaches, or possibly crickets (I am personally offended by this), fleeing a skeleton looming over a decaying cityscape. Humans are nowhere in sight.

  So which is it? Do ant wars and slave-taking raids mean that these, and perhaps the other social insects, are particularly aggressive, and hence that warmongering is natural in animals? Does the devotion and self-sacrifice so approvingly cited by Maeterlinck prevail? A closer look reveals that the real villainy takes place much more surreptitiously, and while less full of carnage, it is far more deadly.

  An Army of Savage Lace

  WHEN I was a child I went through a phase in which I told people I wanted to be a myrmecologist when I grew up. Although I did indeed spend time watching the ants in our backyard, along with the other insects, I was probably driven more by smug delight at knowing that the word means someone who studies ants than by any actual career motivation. Be that as it may, when we had an assignment in third or fourth grade to read a book and report on it to the rest of the class, I chose a book on ants, and happily launched into a litany of their amazing behaviors. Ants, I proclaimed, made gardens of fungus that they harvested for food. They stored honeydew in their own massively swollen abdomens and fed it to the other workers, droplet by droplet. Not only that, I cheerfully told my classmates, who were by that point probably unnerved if they were not simply bored, but army ants could swarm through entire jungle villages, consuming every living thing they encountered by tearing it to pieces. Cows, pigs, chickens, and people, all were subject to the advancing hordes with their bladed jaws. If one were caught unawares by the oncoming troops, the only recourse was to set one's bedposts in saucers of kerosene, get under the covers, and pray the ants didn't find a way to drop down onto the bed from the ceiling. I was slightly hampered in my explanation of this dire state of affairs by my uncertainty of exactly what kerosene was, but I was sure that if I lived in an area frequented by army ants, I would be able to procure some.