Cannibals eat members of the same species as themselves. In many cases, the eaten individuals are already dead, from a variety of causes; eating them is more like scavenging. Another form of cannibalism might be called predatory cannibalism: the eaten individuals are killed (or badly damaged) just before being eaten. The animal kingdom includes many species that regularlyengage in predatory cannibalism, commonly among family members, and that is the subject of this essay. Here are a few examples; perceptive readers will notice that every example leads to more questions (stated or not):
–eating siblings: Occasional sib-eating is known to occur in owls and eagles and other species, but that is different from regular, customary sib-eating, which has evolved independently in many different kinds of animals, including spiders, sea stars, flour beetles, earwigs, ladybugs, and many more. A certain marine slipper snail, for instance, engages in multiple matings, after which the female broods her embryos in a capsule. Those embryos have multiple fathers, and the more fathers they have, the higher the intensity of sibling cannibalism. Sibling cannibalism usually results in the cannibalistic siblings growing faster, getting bigger, and surviving better than if they had not eaten their sibs.
Sand tiger shark sibs eat each other before they are born. Females of this species have two uteri, each one holding multiple eggs. Females mate with several males, so the embryos have different fathers. Embryos from the first eggs to hatch in each uterus grow faster than the others and soon gobble them up (and then eat any unfertilized eggs too). This begs the question of what determines which embryos (fathered by which males) win out and thus have the advantage of becoming extra-large, thanks to their special diet.
Spadefoot toads breed in ephemeral pools; the tadpoles commonly eat detritus. Some tadpoles develop bigger and better jaws and teeth and start to eat fairy shrimp; then they eat their sibs. As a result, they reach metamorphosis more quickly and are more likely to become toads with the adult form before the pond dries up. The cannibals can discriminate their own kin from the tadpoles of other mothers, and they prey mostly on non-kin (at least when well-fed).
Larval tiger salamanders sometimes also become cannibals, with bigger heads and teeth than the non-cannibals. They too can discriminate between kin and non-kin. Larvae of different maternal lineages differ in discriminatory ability; lineages with greater tendencies to become cannibals are better at discriminating kin than lineages with a lower tendency to become cannibals. However, in over-crowded conditions, even cannibals are less discriminating and may eat anyone.
–eating parents: Even in the few species in which males take over parental care, I have not (so far) found any examples of young ones killing and eating their attentive fathers. However, the young of some insects, nematodes, pseudo-scorpions, and spiders commonly eat their mothers. Females of a desert spider normally regurgitate food to their young. Digestive enzymes of mated females increase and eventually digest the guts themselves; all that material is stored in the females’ abdomens. When the young are born, they puncture their mothers and suck them dry in just a few hours, leaving just a husk. The black lace spider female signals to her young to approach her; then they jump on her repeatedly and eventually she lets them suck out her insides; they finish her off by poisoning her before sucking all the juices out. Very young spiders of this species will eat any female, but after they are about four days old, they only eat their real mother (!).
–eating mates: Female spiders and mantises get public attention for their mate-eating behavior, but it turns out that this behavior is not as common as the myths would have it. Depending on the species, a lot depends on male behavior, on inherent aggressiveness of females, on fighting ability of the male, on availability of other prey and hunger levels of females, on the probability of finding another mate, and other factors. Males of some species can sometimes avoid being eaten by fighting back vigorously (springbok mantis), by wrapping her up in a silken shroud (a North American nursery-web spider), or just by being very cautious when approaching a female. And then there’s the little biting midge that lets a male begin copulating, but then jabs his head, digests his insides, and sucks him dry. His body husk drops off, except for his genitalia, which remain in place for a while, preventing access by other males.
In most cases, females that are cannibalistic gain reproductive advantages by laying larger, bigger eggs that survive better than those of non-cannibalistic females. Therefore their deceased mates also gain reproductive advantages. In a species of praying mantis, amino acids from the male’s body have been detected both in the female and in her eggs, so he is more than just a meal. Those indirect advantages to males have to be weighed against the probability of finding a second mate: for some species, males have such rare opportunities to mate that investing (their lives) in the young of one mating is the better reproductive strategy (leading to more offspring).
–eating your own offspring: This happens sporadically in a variety of species, including spiders, rodents, snakes, fish, birds, and mammals: a parent eats one or more of the offspring in a brood, usually because it is not developing properly or adequately. It’s a way of regaining some of the resources initially spent on offspring production.
However, regular consumption of one’s own offspring is obviously counterproductive. Evolutionary success is generally measured by the number of surviving offspring (and grandchildren, etc.) produced, and any family lineage that customarily ate its own children would not exist for very long. Nevertheless, under certain conditions, eating your own babies may be advantageous. In some cases, parental male fish may eat the young ones they are tending, especially if there has been lots of cuckoldry by other males and many of the young might not be their own; then they can start over with a new brood and hope for increased paternity.
Eating offspring is also a source of energy in times of food scarcity. A freshwater amphipod becomes more cannibalistic when it is parasitized and thus has increased nutritional needs. Juveniles are preferred prey, presumably including offspring of the cannibalistic adults. Can they distinguish their own young from others and prey selectively on the others?
Polar bears are currently losing access to their usual marine mammal prey, because the sea ice is breaking up. Researchers have observed that, as a result, the hungry bears have turned increasingly to predatory cannibalism. Male bears have been known to kill cubs occasionally (which brings the mothers back into heat and the males then may sire another cub), but now polar bear males are attacking females with cubs and eating the cubs significantly more often. In some cases, they may be eating their own offspring.
Some butterflies have caterpillars that feed on milkweed plants, from which they take up some toxic chemicals that help protect them from would-be predators. However, researchers have recently noted that males of certain of those species spend time and effort scratching the skins of caterpillars, which could be their own offspring. The skin oozes those toxic chemicals, which are sucked up by the scratching males, perhaps to be used as courtship attractants or part of the nuptial gift transferred to females along with the sperm. The caterpillars are seriously wounded and sometimes die. I wonder, then, if the intensity of scratching caterpillars increases when mating competition is fiercest.