Cannibalism: A Perfectly Natural History(5)
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.
I noticed that the smaller, omnivorous morphs generally stuck to the shallows bordering the shoreline. They buzzed through the brown water in a non-stop, seemingly random quest for food, changing direction abruptly and often. One explanation for the patternless swimming behavior became apparent as I waded farther away from the shore, for here in the deeper water was the realm of the cannibals. I stood quietly and watched as hundreds of conspicuously larger tadpoles crisscrossed the pond, making frequent excursions from the deeper water toward the shore in a relentless search for prey.
Immature animals get eaten more than adults, I thought. Certainly, although in this case the youngsters were eating each other.
“They remind me of killer whales hunting for seals,” said Ryan Martin, a former student of Pfennig’s, now a professor at Case Western Reserve, who was also studying spadefoot toads here in Arizona.
Martin’s comparison was spot on, and I threw him a nod, watching as a tiny hunter swam away from the shore with an even tinier pondmate clamped tightly between its serrated jaws.
So why did the local spadefoot larvae exhibit cannibalistic behavior? There certainly seemed to be enough organic matter suspended in these algae-tinted ponds to feed the entire brood and more.
As I spoke to Pfennig and his team of researchers, I learned that the answer was directly linked to the aquatic environments in which the adult amphibians deposited their egg masses.6 Formed by spring and early-summer monsoons, the transient ponds frequented by the spadefoots (spadefeet?) are often little more than puddles, and as such they can evaporate quite suddenly in the hot, dry environment of southeastern Arizona. Natural selection, therefore, would favor any adaptations enabling the water-dependent tadpoles to “get out of the pool” as quickly as possible (i.e., to grow legs). In this instance, the phenomenon that evolved can be filed under the rather broad ecological heading of phenotypic plasticity: When changing environmental conditions allow multiple phenotypes (observable characteristics or traits) to arise from a single genotype (the genetic makeup of an organism). To clarify this concept, here are a couple of non-cannibalism-related examples.
Water fleas (Daphnia) are tiny aquatic crustaceans, named for a swimming style in which they appear to jump like fleas. In response to the appearance of backswimmers (Laurel Fox’s favorite predatory insects), Daphnia develop tail spikes and protective crests. Although the genetic potential for body armor is always there (in the Daphnia’s genotype), it doesn’t exhibit itself until a specific environmental change occurs, in this case the arrival of Daphnia-munching backswimmers.