Sex on Six Legs_ Lessons on Life, Love, and Language From the Insect World - Marlene Zuk [75]
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