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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