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.
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.