Though the story of spadefoot toad cannibalism has been well researched, it is not fully resolved. The reason is that no one has been able to identify the precise stimulus within these brood ponds that triggers the appearance of the cannibal morphs. Until recently, the prime candidates were a pair of microscopic fairy shrimp species (order Anostraca). David Pfennig and his colleagues proposed that the consumption of the shrimp by some of the spadefoot tadpoles served to trigger the cascade of genetically controlled developmental changes that transformed the shrimp-munchers into outsized cannibals.
But what was it about eating fairy shrimp that set this transformation into motion? Pfennig hypothesized that iodine-containing compounds found in the shrimp might cause the activation of specific genes in the tadpoles, genes that weren’t turned on in the individuals that didn’t consume shrimp. The prime candidate for a trigger substance turns out to be thyroxin, a thyroid hormone whose functions include stimulating metabolism and promoting tissue growth. A new set of experiments, though, have shown that even tadpoles that weren’t fed fairy shrimp could still undergo the transformation to cannibals, indicating that (at the very least) something besides thyroxin intake must initiate the changes.
“What if it’s not what they’re eating but the mechanism of chewing itself that serves as a trigger?” I made the suggestion while brainstorming the problem with biologist Ryan Martin. “What if chewing on something alive like a fairy shrimp, something larger or something that struggles when you clamp onto it, sets this developmental cascade into motion?”
Martin shot me a “not bad for a bat biologist” look. “Sounds like a good grad student project.”
“Hey, it’s all yours,” I said with a laugh. We then set to work, drawing up an outline for a potential experiment to test the hypothesis.
Although the jury is still out on the stimulus for the spadefoot transformations, Pfennig and his coworkers previously worked on a completely different cannibalism-triggering stimulus in another amphibian. And this one happened to be one of North America’s most spectacular species.
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.7 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 of them 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 determine 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.”
Movement and touch related, I thought, remembering my suggestion about a possible trigger for the spadefoot cannibals. But instead of speculating about my own half-baked ideas, the conversation turned toward the pros and cons of cannibalism, especially as it pertained to consuming kin.
One of Gary Polis’s general characteristics regarding cannibalism is that immature animals get eaten far more often than adults. Ultimately, this makes larvicide (or infanticide) the most common form of cannibalism in the animal kingdom. Intuitively, it doesn’t seem logical to eat the next generation, but the behavior can make evolutionary sense for several reasons. Young animals not only provide a valuable source of nutrition, but in most species they’re relatively defenseless. As such, they present instant nutritional benefits but little or no threat to larger members of the same species, most of which are invulnerable to attacks from immature forms.
But beyond acquiring a meal, and as we saw with spadefoot toads, cannibalism enables individuals of some species to accelerate their developmental process, thus allowing them to quickly outgrow a stage in which they might be preyed upon or perish due to unpredictable environmental conditions. In species like the flour beetle (Tribolium castaneum), the behavior may also impart a reproductive advantage, since studies have shown that cannibalistic individuals produce more eggs than non-cannibals. Finally, many animals maintain specific territories, within which they are intolerant to the presence of conspecifics (i.e., members of the same species). According to Polis, crowding increases the frequency with which individuals violate the space of others. By reducing overcrowded conditions, cannibalism can serve to decrease the frequency of territory violations.