Cannibalism: A Perfectly Natural History(4)

“These look like two different species,” I said, examining a handful of tadpoles that I’d just scooped up. I also noted that the larger individuals were light tan in color while the little guys had bodies flecked with dark green.

“Initially, people thought they were different species,” Pfennig replied.

Using a magnifying glass to get a better look at my squirmy captives, I saw that the differences went beyond body size and color. The larger tadpoles were also sporting powerful tails and serious-looking beaks.

“Yikes, nice choppers,” I commented, always the scientist.

“They’re made of keratin,” Pfennig said. This was the same tough, structural protein found in our nails and hair.

Later, while comparing the two tadpole morphs under a dissecting ’scope, I saw that behind a set of frilly lips, the flat keratinous plates (which worked fine for detritus dining) had been transformed into a jack-o-lantern row of sharp-edged teeth in the cannibalistic forms. It was also evident that the jaw muscles were significantly enlarged in the cannibals, especially the jaw-closing levator mandibulae, whose bulging appearance reminded me of a kid with six pieces of Dubble Bubble jammed into each cheek (a dangerous behavior I only rarely attempt anymore ). Studies had shown that myofibers, the cells making up these muscles, were larger and greater in number (or hypertrophied and hyperplasious, respectively)—producing a more powerful bite. Of course, the extra bite force was necessary because, beyond latching onto the occasional unshaved human leg, these critters were using bulked-up bodies and the weaponry that accompanied it to subdue and consume their omnivorous pondmates.

Not quite so obvious was a significant shortening of the gastrointestinal (GI) tract in the cannibals, with the explanation relating to the dietary differences that accompanied the tadpole transformations. In the omnivores, a long GI tract is required for the breakdown of tough-to-digest plant matter, while a shorter GI tract works just fine when the diet is a fleshy one.4

Over a three-day period, I watched and captured tadpoles in bodies of water that ranged from tire-carved puddles to bovine swimmin’ holes of the double-wide Olympic variety. Accompanied at various times by Pfennig, his wife, Karin, their two young daughters, and a pair of extremely personable UNC grad students, Antonio Serrato and Nick Levis, I learned a great deal about the three species of Spea that laid their eggs in such dangerously unpredictable conditions. Much of this information centered on the ecology, behavior, and evolution of these creatures. Of course, the cannibalism angle was there as well, although these researchers (including the kids) treated that particular behavior as perfectly normal.

Until relatively recently though, and with a very few exceptions, cannibalism in nature would have been regarded as anything but normal. As a result, until the last two decades of the 20th century, few scientists spent time studying a topic thought to have little, if any, biological significance. Basically, the party line was that cannibalism, when it did occur, was either the result of starvation or the stresses related to captive conditions.

It was as simple as that.

Or so we thought.

In the 1970s, Laurel Fox, a University of California Santa Cruz ecologist, took some of the first steps towards a scientific approach to cannibalism. She had been studying the feeding behavior of predatory freshwater insects called backswimmers (belonging to the order Hemiptera, the “true bugs”). Fox determined that, while the voracious hunters relied primarily on aquatic prey, “cannibalism was also a consistent part of their diets.”

I contacted Fox and asked her about the transition that had taken place in the scientific community regarding this behavior. She told me that her observations in the field had sparked her interest and that, soon after, she began compiling a list of scientific papers in which cannibalism had been reported. Although there turned out to be hundreds of references documenting the behavior in various species, no one had linked these instances together or come up with any generalizations regarding the behavior. By the time Fox’s review paper came out in 1975, she had concluded that cannibalism was not abnormal behavior at all, but a completely normal response to a variety of environmental factors.

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.