When a family dinner means something else

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 behaviour in the flock, employed by dominant birds 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.

Until relatively recently, the party line among scientists was that cannibalism
occurred in only a few species in the wild, like black widow spiders. 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.

Before his death in a boating accident in 2000, Gary Polis, an ecologist at the University of California, Davis, USA, 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 recognise individuals of their own kind (especially eggs) 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, Gary’s generalisations had been observed among widely divergent animal groups.

The kid’s menu
As a new generation of researchers builds upon the work of scientists like Gary and Laurel Fox, an evolutionary biologist at the University of California, Santa Cruz, USA, 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.

But the black lace-weaver spider takes the concept of prepackaged meals a step further. After their first molt, black lace-weaver spiderlings are too large for their mother to care for, though they are in dire need of additional food. In a sacrificial act of parenting, she calls the babies to her by drumming on the web and presses her body down into the gathering crowd. The ravenous spiderlings swarm over their mother’s body. Then they eat her alive.

In sand tiger sharks, the babies doing the cannibalising are not even born yet. The young of sand tigers, like hammerheads and blue sharks, 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 these specimens were anatomically well developed, with mouths full of sharp 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.

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.

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. 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 a chapter in his book Cannibalism in China to ‘Methods of Cooking Human Flesh’.

As scientists have come to understand, factors like overpopulation and a lack of alternative forms of nutrition lead to cannibalism among animals, and it is clear that even modern humans have been driven to the behaviour on many occasions. 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.