Significantly, Fox also determined that cannibalism was a far more widespread occurrence than anyone had previously imagined and that it took place in every major animal group, including many that were long considered to be herbivores . . . like butterflies. She emphasized that cannibalism in nature, which some researchers referred to as “intraspecific predation,” also demonstrated a complexity that seemed to match its frequency. Fox suggested that the occurrence of cannibalism in a particular species wasn’t simply a “does occur” or “doesn’t occur” proposition, but was often dependent on variables like population density and changes in local environmental conditions. Fox even followed cannibalism’s environmental connection onto the human branch of the evolutionary tree. After pondering reports that humans practicing non-ritual cannibalism lived in “nutritionally marginal areas,” she proposed that consuming other humans might have provided low-density populations with 5 to 10 percent of their protein requirements. Conversely, she suggested that cannibalism was rare in settlements where populations were dense enough to allow for the production of an adequate and predictable food supply.
In 1980, ecologist and scorpion expert Gary Polis picked up the animal cannibalism banner and began looking at invertebrates that consumed their own kind. Like Fox, he noted that while starvation could lead to increases in the behavior, it was certainly not a requirement. Perhaps Polis’s most important contribution to the subject of cannibalism in nature was assembling a list of cannibalism-related generalizations under which most examples of invertebrate cannibalism could be placed. 1) Immature animals get eaten more often than adults; 2) Many animals, particularly invertebrates, do not recognize individuals of their own kind, especially eggs and immature stages, which are simply regarded as a food source; 3) Females are more often cannibalistic than males; 4) Cannibalism increases with hunger and a concurrent decrease in alternative forms of nutrition; and 5) Cannibalism is often directly related to the degree of overcrowding in a given population.
Polis emphasized that these generalizations were sometimes found in combination, such as overcrowding and a lack of alternative forms of nutrition (a common cannibal-related cause and effect), both of which now fall under the broader banner of “stressful environmental conditions.”5
In 1992, zoologists Mark Elgar and Bernard Crespi edited a scholarly book on the ecology and evolution of cannibalism across diverse animal taxa. In it, they refined the scientific definition of cannibalism in nature as “the killing and consumption of either all or part of an individual that is of the same species.” Initially the researchers excluded instances where the individuals being consumed were already dead or survived the encounter—the former they considered to be a type of scavenging. Eventually, though, they decided these were variants of cannibalistic behavior observed across the entire animal kingdom. Although there are certainly gray areas (encompassing things like breastfeeding or eating one’s own fingernails), my fallback definition of cannibalism for this book is: The act of one individual of a species consuming all or part of another individual of the same species. In the animal kingdom, this would include behavior like scavenging (as long as the scavenged body was from the same species as the scavenger) and maternal care in which tissue (i.e., skin or uterine lining) was consumed. In humans, cannibalism would extend beyond the concept of nutrition into the realms of ritual behavior, medicine, and mental disorder.
As the study of cannibalism gained scientific validity in the 1980s, more and more researchers began looking at the phenomenon, bringing with them expertise in a variety of fields. From ecologists we learned that cannibalism was often an important part of predation and foraging, while social scientists studied its connection to courtship, mating, and even parental care. Anatomists found strange, cannibalism-related structures to examine (like the keratinous beak of the spadefoot toad), and field biologists studied cannibalism under natural conditions, thus countering the previous mantra that the behavior was captivity-dependent.
By the 1990s, Polis’s generalizations had been observed among widely divergent animal groups, both with and without backbones, supporting the conclusion that the benefits of consuming your own kind could outweigh the often substantial costs. Once these generalizations became established, and as a new generation of researchers built upon foundations constructed by pioneers like Fox and Polis, cannibalism in nature, with all of its intricacies and variation, began to make perfect evolutionary sense.
Arizona’s lowland scrub stood in stark contrast to the lush peaks and bolder-strewn valleys of the state’s Chiricahua Mountains. These “sky islands” (isolated mountains surrounded by radically different lowland environments) provided a spectacular backdrop for my afternoon wade through yet another transient pond.
The air temperature had risen to 95 degrees Fahrenheit, which kept most of the area’s terrestrial denizens hiding in shade or below ground, but the inhabitants of Horseshoe Pond reminded me of sugared-up kindergarteners tearing around a playground (albeit with fewer legs and more cannibalism). By this time, I had already begun to see distinct patterns of behavior in the spadefoot tadpoles that motored hyperactively just below the water’s surface.