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 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’ most important contribution 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 umbrella of “stressful environmental conditions.”
In 1992, evolutionary biologists 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.
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 cannibalism was often an important part of predation and foraging, while social scientists studied its relationship 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 dependent on captivity.
Arizona’s lowland scrub stood in stark contrast to the lush peaks and boulder-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 kindergartners 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.
Until relatively recently, though, and with very few exceptions, cannibalism in nature would have been regarded as anything but normal.
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.
So why did certain 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 laid their eggs. Formed by spring and early-summer monsoons, the transient ponds frequented by the spadefoots 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 phenotype plasticity: When changing environmental conditions allow multiple phenotypes (observable characteristics or traits) to arise from a single genotype (the genetic makeup of an organism).
The selection pressure lies in the temporary nature of the brood ponds, where the eggs are deposited and hatch, and where the tadpoles develop into toadlets. The period from egg to juvenile toad normally takes around 30 days — unless, that is, the pond dries out first, killing the entire brood. In response to this particular environmental selection pressure, what evolved was a means by which some of the tadpoles can mature in about two-thirds of the time. The increased growth rate occurs because the cannibal larvae are getting a diet high in animal protein as well as a side order of veggies, the latter in the form of nutrient-rich plant matter their omnivorous prey had consumed during what turned out to be their last meal.
Though the story of spadefoot toad cannibalism has been well researched, it is not fully resolved. No one has yet been able to identify the precise stimulus within these brood ponds that triggers the appearance of the cannibal morphs.
However, Pfennig and his co-workers did previously work on a completely different cannibalism-triggering stimulus in another amphibian. And this one happened to be one of North America’s most spectacular species.
The Small Get Eaten
Tiger salamanders (Ambystoma tigrinum) are the largest salamanders in the United States, reaching lengths of up to 13 inches. These thick-bodied, sturdy-limbed urodelans are widespread across much of the country. Their markings, yellow blotches against a black body, make them easy to identify, but they are rarely seen in the open except during annual marches to a nuptial pond. Tiger salamander eggs are laid in the late winter or early spring, and like other salamanders, and their cousins the frogs and toads, their larvae are fully aquatic with external gills and fishlike tails. They typically feed on zooplankton and other micro-invertebrates, but under certain environmental conditions, a small percentage develop traits that include huge heads, wide mouths and elongated teeth. Consequently, these toothy individuals exploit larger prey, among them other tiger salamander larvae.
Pfennig and his colleagues set up lab experiments on fertilized A. tigrinum eggs to investigate the stimuli that set these changes into motion. First, the researchers determined that the cannibal morphs only developed when larvae were placed into crowded conditions. Next, they used a variety of experiments to see whether the larval transformation might be triggered by visual cues (that didn’t work), smell (nope) or touch.
“It looks like they had to have the tactile cues,” Pfennig told me. “There’s something about bumping into each other that triggers the production of the cannibals.”