Guess who’s coming to dinner?
Humans are sometimes said to occupy a “pecking order,” but of course the term actually refers to chickens and other poultry. Mild pecking is normal behavior in the flock, employed by dominant birds (or “despots”) as a way to remind subordinates of their lower social position.
But the practice can turn gruesome when thousands of birds are packed wing to wing. Then, some bottom-of-the-order birds are pecked to death — and eaten. As poultry and egg farms increased in size in the 1920s and 1930s, feather-pecking and cannibalism, known in the trade as “pick out,” became serious threats.
In 1939, Joseph Haas, founder of the National Band and Tag Co., devised a fashionable method to deal with coopedup cannibals: mini-sunglasses equipped with red celluloid lenses on an aluminum frame. Poultry farmers were informed that having their chickens see the world through rose-tinted cheaters would “make a sissy of your toughest birds.”
Until recently, the party line among scientists was that cannibalism occurred in a few species in the wild, like black widow spiders and praying mantises. Cannibalism, researchers felt, was an aberrant behaviour resulting from a lack of alternative forms of nutrition or the stresses associated with captive conditions.
But over the decades, evidence has been gathering for an alternative view. Cannibalism, it turns out, occurs in hundreds of species, perhaps thousands. The behaviour varies in frequency between major animal groups — nonexistent in some, common in others. It varies from species to species and even within the same species, depending on local environmental conditions. As important, the behaviour serves a variety of functions, depending on the cannibal, and some of these have nothing to do with stress or captive conditions. There are even instances in which an individual being cannibalized receives a benefit.
Before his death in a boating accident in 2000, Gary Polis, an ecologist at the University of California, Davis, came up with a list of cannibalism-related rules for invertebrates. Immature animals are consumed more often than adults, he found, and many species do not recognize individuals of their own kind (especially eggs and immature stages) as anything other than food.
He noted that cannibalism was more common in females than in males, and that as alternative forms of nutrition decrease in availability, incidents of cannibalism will increase. Lastly, in a given population, cannibalism is often directly related to the degree of overcrowding.
By the 1990s, Polis’ generalizations had been observed among widely divergent animal groups, not just invertebrates. The benefits of consuming one’s own kind, it seemed, can outweigh the costs.
That price, though, can be substantial. Cannibals that consume their own relatives remove those genes from the population, reducing what scientists call their inclusive fitness. But the most significant drawback appears to be a greater chance of acquiring harmful, speciesspecific parasites or pathogens.
In the most famous example, 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 deceased kin. Many had died of kuru, a neurodegenerative condition similar to mad cow disease, and their tissues contained the pathogen, spreading it even further. The kids menu As a new generation of researchers builds upon the work of scientists like Polis and Laurel Fox, an evolutionary biologist at the University of California, Santa Cruz, cannibalism in nature has begun to seem almost normal.
We now know that a significant amount of cannibalism occurs in mollusks, insects and arachnids. Additionally, thousands of aquatic invertebrates like clams and corals have tiny eggs and larvae that are often a major food source for the filter-feeding adults — itself a form of indiscriminate cannibalism.
In many fish species, adults can be a million times as large as their own eggs. Fish eggs, larvae and fry are vast in number, minute in size and high in nutritional value. In sand tiger sharks ( Carcharias taurus), the babies doing the cannibalizing are not even born yet.
The young of sand tigers, like hammerheads ( Sphyrna zygaena) and blue sharks ( Prionace glauca), develop inside the females’ oviducts, a developmental strategy known as histotrophic viviparity.
Scientists who first looked at late-term sand tiger embryos in 1948, noticed that they were anatomically well developed, with mouths full of teeth — a point (or several) driven home when one researcher was bitten on the hand while probing the oviduct of a pregnant specimen.
Strangely, these late-term embryos also had swollen bellies, which were initially thought to be yolk sacs, a form of stored food. This was puzzling, since most of the nutrient-rich yolk should have been used up by this late stage of development. Further investigation showed that the abdominal bumps were not yolk sacs at all — they were stomachs full of smaller fetal sharks. These embryos had fallen victim to the ultimate in sibling rivalry, a form of in-utero cannibalism known as adelphophagy (from the ancient Greek for “brother eating”): sibling cannibalism.
Such behaviour is possible because sand tiger shark oviducts contain embryos at different developmental stages (a characteristic that also evolved in birds). Once the largest of the embryos run through their own yolk supply, they begin consuming eggs. And when the eggs are gone, the ravenous fetal sharks begin consuming their smaller siblings. Ultimately, only two pups remain, one in each oviduct.
This is similar to the “lifeboat strategy” seen in birds like vultures and egrets. Here, cannibalism is often the end result of asynchronous hatching: two eggs are laid, but one hatches several days before the other. The firstborn chick uses its extra bulk to win squabbles over food with its younger brother or sister.
In instances where the parents are unable to provide enough to eat, the firstborn will kill and consume the younger sibling. During times of stress, this is an efficient way to produce well-nourished offspring — albeit fewer of them.
Examples of animal cannibalism are as numerous as they are interesting, from spadefoot toad larvae who eat their own brood-mates to legless amphibians called caecilians whose hatchlings peel and con- sume their mothers’ skin. And they occur among mammals, too.
Polar bears consume other polar bears, and were doing so long before climate change impacted their hunting practices. And male lions, after taking over a pride, will eat the cubs that another male has sired. Both are examples of heterocannibalism — the eating of nonrelatives.
In lions, incoming males seek to terminate the maternal investment in unrelated cubs. More important, a lioness with cubs will not come into heat for a year and a half after giving birth. But, as has been observed in other mammals, like bears, a lioness that loses her cubs becomes sexually receptive almost immediately. It isn’t just for animals Are there instances where, as in the animal kingdom, human cannibalism makes sense? And if so, could this behaviour resurface in the future? Cannibalism may be gruesome, and repugnant to our current sensibilities, but it has been widely practiced for a variety of reasons.
Funerary cannibalism was practiced by groups like the Fore of New Guinea and the Wari’ of Brazil. These indigenous people were as mortified at the concept of burying their dead as newly arrived missionaries and anthropologists were at the thought of consuming their own departed loved ones.
From kings to commoners, Europeans, too, once routinely consumed human blood, bones, skin, guts and body parts. They did it without guilt, a form of medicinal cannibalism. They did it for hundreds of years, and then they made believe it never happened.
Throughout their long history, body parts were such important ingredients in Chinese culinary cannibalism that the historian and author Key Ray Chong devoted a13-page chapter in his book Cannibalism in China to “Methods of Cooking Human Flesh.” Rather than an emergency ration consumed as a last resort, there are many reports that exotic humanbased dishes were prepared for Chinese royalty and upper-class citizens.
In a procedure that had become known to seafarers as “the custom of the sea,” sailors cast adrift on the open ocean drew straws. The sailor who drew the short straw gave up his life so that the rest might eat. In perhaps the most famous case, in1765 a storm dismasted the American sloop Peggy, leaving it adrift in the middle of the Atlantic Ocean with its captain, nine crewmen and a single slave.
After consuming the ship’s cat, their uniform buttons and a leather bilge pump, and after the captain had retreated to his cabin clutching a pistol, the crew decided to draw lots. The loser was to be served up as dinner. By an incredible coincidence, the slave drew the short straw.
Although the man begged for his life, the captain was unable to prevent his murder, later writing that as the crew prepared to cook the body, one sailor rushed in, tore away the slave’s liver and ate it raw. This is the horrific origin of the term “lifeboat strategy,” co-opted by ornithologists over two centuries later to describe the fate of unfortunate nestlings.
As scientists have come to understand, factors like overpopulation and lack of alternative nutrition lead to cannibalism among animals, and it is clear that even modern humans have been driven to the behaviour. What, then, of the future?
Populations are growing. Resources are dwindling. Deserts are spreading. And the societal rules that bind us together are proving more fragile than we ever imagined they could be. Maybe it is wise to remember that human cannibalism, so unthinkable now, was not uncommon not so long ago. Bill Schutt is a professor at L.I.U.-Post, a research associate at the American Museum of Natural History and the author of the forthcoming Cannibalism: A Perfectly Natural History.
From kings to commoners, Europeans, too, once consumed human blood, bones, skin, guts and body parts