The Red Tails of Gray Treefrog Tadpoles

Adult Gray Treefrogs are well known for their distinctive trills and charismatic appearance. But the tadpoles are just as remarkable as the adults. When you swipe a net through a pond in eastern North America, and find a tadpole with a bright red tail, you know that you have found a Gray Treefrog tadpole. No other tadpole in North America has such a distinctive tail.

Why are the tails of Gray Treefrog tadpoles so conspicuous? The wetlands that they live in often contain predators, including Giant Water Bugs and Predaceous Diving Beetles, that feast upon the tadpoles. Many tadpoles are eaten before they can metamorphose into froglets and leave the dangerous waters. Their bright red tail is one part of a suite of adaptive traits that help Gray Treefrog tadpoles survive attacks from predators.

Perhaps surprisingly, Gray Treefrog tadpoles don't always develop a red tail. The bright red color only develops when tadpoles are exposed to predators. This phenomenon is illustrated in the photo below. The upper tadpole developed in a pond full of predatory insects, including dragonflies, water bugs, and predaceous diving beetles. In fact, the big rip on the tadpole's tail suggests it had a narrow escape from one of these predators. The lower tadpole developed in an environment without any predators. The ability of a single genotype to develope alternative traits in different environments is known as "phenotypic plasticity." Phenotypic plasticity is one type of adaptation that can occur when organisms evolve in variable environments.

Adaptive phenotypic plasticity of tadpole tails has been shown in many species, including Pacific Chorus Frogs and Wood Frogs, but it was first experimentally investigated in Gray Treefrog Tadpoles. In their 1996 Evolution paper, biologists Andy McCollum and Josh Van Buskirk describef a set of experiment to test if the phenotypic in tadpole tails is actually adaptive. They first raised tadpoles in environments without predators, with free predatory dragonflies, and with caged predatory dragonflies. Compared to tadpoles raised without dragonflies, tadpoles raised with dragonflies had bright red tails, deeper tails, and were less active.

In the next step of their experiment, they placed tadpoles from the different environments into containers with dragonfly larvae and examined which tadpoles survived. Tadpoles with the predator-induced traits had the highest survival, and tadpole tail color was one of the best predictors of survival. The greater the amount of red in a tadpole's tail, the more likely it was to survive. In the final step of their experiment, they examined the survival of the tadpoles with the different traits when no predator was present. They found that when predators weren't able to attack tadpoles, the tadpoles with the predator-induced traits (bright red tail, deep tail, low activity) had lower survival than tadpoles without those traits. Thus, they found a trade-off! The traits induced by the predators (e.g., red tail and deep tail) provided a benefit of improved survival when the tadpoles were attacked by predators. But when there were no predators around, those same traits incurred a cost of reduced survival. This trade-off in survival across ponds with predators and without predators explains why these tail traits have evolved to be phenotypically plastic.