Cannibalism: A Perfectly Natural History(6)

Here’s another example, unrelated to cannibalism. The reef-inhabiting bluehead wrasse (Thalassoma bifasciatum) is famous for its habit of removing parasites from much larger fish, even entering into their mouths. In this case, however, it’s the removal of a male wrasse from its harem of 30 to 50 females that alters their local environment. Rather than waiting for a new male to arrive, something extraordinary takes place in the harem. Within minutes, one of the females begins exhibiting male-typical behaviors. Relatively quickly, the former female transforms into a male, a form of phenotypic plasticity known in the trade as protogyny. The opposite occurs in protandry, in which individuals begin life as males and transform into females. Examples include the clownfish (Amphiprion), whose behavior could have offered an intriguing alternative resolution to the animated film Finding Nemo.



In spadefoot toads, though, it’s not the appearance of a predator or the loss of a harem’s personal sperm bank that initiates the alternate phenotype (i.e., cannibalistic larvae). 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 (20 days). 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.

In an interesting note, Spea couchii does not transform into cannibalistic morphs but has evolved an alternative solution to the transient pond problem. Couch’s spadefoot can go from egg to toad in only eight days—an amphibian record.

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.