How Baby Poison Frogs Could Escape Cannibalism

Siblings can be annoying, but for young splash-back poison frogs, they’re also deadly. If placed in the same pool, tadpoles of this species will gladly eat their brothers and sisters.

Now a new study suggests tadpoles have a way to escape their cannibalistic kin: Hitchhiking on the backs of adults. (Read: "Cannibalism—the Ultimate Taboo—Is Surprisingly Common.")

Female poison frogs usually lay their eggs above water-filled plants, such as bromeliads. When the eggs hatch into tadpoles, prudent fathers often turn up and carry their hatchlings to different plant pools, one by one, so that their offspring won't eat each other as they develop into colorful adults.

Occasionally, dads accidentally drop off more than one tadpole in a pool, or forget to come back around to disperse their clutch. In most cases, the tadpoles will eventually dwindle to one satisfied youngster.

"They are pretty voracious," says Kyle Summers, an evolutionary biologist at East Carolina University who studies poison dart frogs but was not involved in the new study.

"Even if they're the same size, they can end up killing another tadpole and eating it."

The Kindness of Strangers

For the study, Lisa Schulte, a postdoctoral researcher at the Vrije Universiteit Brussel, and colleagues filled small plastic cups with rainwater and hatched 15 egg clutches of the splash-back frogs in a field laboratory in northeastern Peru.

The researchers kept each clutch of two to four siblings together, and the tadpoles had no contact with adults during this first part of the experiment. (Also see "New Poisonous Frog Species Discovered in Peru.")

Next, the team rotated nickel-size adult frogs from three separate species into each of the 15 cups for a daily session each. They did this in random order with either male or female Ranitomeya variabilis; male Ranitomeya imitator, a related species from the same genus; and male Hyloxalus nexipus, which belong to the wider family of poison dart frogs, Dendrobatidae.

They then recorded the amphibians' behavior on video and analyzed the clips. While the adult frogs mostly tried to escape the secured cups, tadpoles in every clutch tended to gather around the adult, regardless of its species.

Some tadpoles attempted to jump on the adults' backs; in two cases, they succeeded—once with an adult of their own species and another time with a R. imitator, according to the study, published May 5 in the Journal of Zoology.

Though the findings are limited to only a few animals, the authors suggest this behavior might show competition between tadpole siblings to be the first rescued from a pool. (Read about more cannibals in the animal world.)

In most wild situations, though, the adult frogs most likely to visit a pool would be the tadpoles' parents, since they return to the same pools to sleep.

No Fake Frogs, Thank You

Tadpoles did have some standards, however.

While the youngsters were content to hitch a ride with passing strangers, they balked at jumping on the backs of artificial clone frogs produced by a 3-D printer. In additional experiments, the scientists added these clone frogs to each cup of tadpoles.

This suggests the researchers' frog models were either not natural enough or that the tadpoles might use other senses, such as chemical cues, when recognizing a possible savior. (See "13 Gorgeous Pictures Remind Us Why Frogs Need Our Help.")

Juan Santos, a biologist at St. John's University in New York City, liked the study overall, but noted a major limitation: The scientists observed only a handful of observations of the tadpoles climbing on the backs of frogs.

Since the team mostly saw the tadpoles approaching the adult frogs, but not jumping on them, it's too soon to say the behavior is common.

"It's not like they actually found out that this is recurrent," Santos says.

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