There are also serious drawbacks to being a cannibal.
In all likelihood, the most significant of these is a heightened chance of acquiring harmful parasites or diseases from a conspecific. Both parasites and pathogens are often species-specific and many of them have evolved mechanisms to defeat their host’s immune defenses. As a result, predators that consume their own kind run a greater risk of picking up a disease or a parasite than do predators that feed solely on other species. In the most famous example of cannibalism-related disease transmission, the Fore people of New Guinea were nearly driven to extinction as a result of their ritualized consumption of brains and other tissues cut from the bodies of their deceased kin, kin who had themselves been infected by kuru, an incurable and highly transmissible neurological disease. More on that topic later, but given its importance, the potential for disease transmission stands as a prime example that non-humans and humans alike share some of the negative consequences of cannibalism.
Cannibals—whether microbes or Methodists—who eat their own relatives can also experience decreases in a measure of evolutionary success known as inclusive fitness, in which the survival of an individual’s genes, whether they’re from an offspring or a collateral relative (like a brother or cousin) is the true measure of evolutionary success. A cannibal that consumes its own offspring, siblings, or even more distant relatives, removes those genes from the population, so it reduces its own inclusive fitness. Since this is bad juju, natural selection should favor cannibals that can discriminate between kin and non-kin, primarily because eating non-relatives results in no loss of inclusive fitness. In many instances, this is exactly what happens.
Because of the significance of inclusive fitness, it made perfect sense that David Pfennig and his colleagues had also worked on questions related to kin recognition, basically seeking to determine if some of their favorite cannibal species would avoid eating their own relatives. The researchers found that their study subjects did so by recognizing cues associated with their kin that were absent in non-kin.
“Most examples would fall under the heading of ‘the armpit effect,’ ” Pfennig told me. “Here, an individual forms a template for what its kin smell like based on what its own smell is.” He used the example of a species of paper wasps (subfamily Polistinae) that regularly raid the nests of conspecifics to provide food for their own broods. In these species, individuals learn that “If an individual smells like your nest or burrow . . . you don’t eat them.”
Similarly, tiger salamander larvae are more likely to eat the larvae of unrelated individuals than they are to consume relatives. Pfennig explained that he and his colleagues determined this experimentally by “preventing them from being able to smell.”
“How did you do that?” I wondered, envisioning a team of micro-surgeons hovering over a tiny, amphibious patient. Irrigation please, Nurse. Can’t you see this patient is dehydrating?
“By applying super glue under their nares,” he replied.
“Oh, right,” I said with an uncomfortable laugh, before Pfennig finished up by assuring me that the condition was temporary.
If you’re wondering whether or not spadefoot toads avoid eating their kin, Pfennig told me that omnivores school preferentially with their siblings, whereas cannibals generally associate only with non-siblings. In close encounters of the bitey kind, cannibal tadpoles release siblings unharmed and consume non-relatives. In the lab, though, apparently all bets are off if the cannibals are deprived of food and then placed in a tank with other tadpoles. In these cases, starvation becomes the great equalizer, and both kin and non-kin are eaten. As I would learn from researchers unearthing new evidence about the Donner Party, this particular aspect of cannibalism spans the entire animal kingdom.
On the plane ride back to New York, I thought a great deal about the cannibalism I’d seen in the temporary ponds below the majestic Chiricahua Mountains, and about the tiger salamanders I’d collected at Long Island golf courses as a kid.
Cannibal morphs.
I wondered whether H. G. Wells knew about their existence when he wrote The Time Machine in 1895. In Wells’s classic novel, the Time Traveler encounters two human species: the child-sized and docile Eloi, and the brutish Morlocks, who raise the Eloi in order to feed upon them. Wells explained the Morlocks’ cannibalistic behavior by suggesting that they were once members of a worker class, toiling underground for lazy, upper-class surface-dwellers. The Time Traveler speculates that a food shortage (i.e., an environmental change) forced the subterraneans to alter their diets—at first rats, but ultimately something a bit larger. Eventually this behavior resulted in a race of hulking cannibals, feeding on the surface-dwellers, whose own evolutionary path would produce the sheeplike Eloi, pampered, well-fed, and eventually slaughtered for food.