Freak Frog Is an Evolutionary Find

You might call it the curious incident of the frogs in the night.

In fall 2005, at two tropical ponds in Panama, biology doctoral student Justin Touchon (GRS’09) was researching the eggs and tadpoles of the hourglass treefrog, known to breed at night and lay its eggs only on land. A third pond nearby was filled with the same species but lacked any visible egg clutches. So, one night Touchon and Karen Warkentin, a College of Arts and Sciences assistant professor of biology, staked out this third pond and were amazed to see frogs laying eggs in the water, on submerged vegetation.

The discovery has big implications for research into evolution. Until that moment, all 5,000 or so frog species were thought to lay either aquatic or terrestrial eggs. None was believed to do both. What’s more, laying eggs on land is an evolved behavior, meaning that all species of frogs once deposited eggs exclusively in water (as most still do), but at various points way back in the frog family tree, evolutionary pressures led some frogs to branch off and become terrestrial egg-laying species. Now, Touchon and Warkentin may have found a transitional species that could provide insights into exactly how and why these behavioral changes took place.

“I said, ‘Justin you have to work on this,’” Warkentin recalls. “This is so cool.”

First, they had to find out if this species truly could lay eggs both on land and in water. Because the two ponds where Touchon’s initial research was based were shaded by a thick forest canopy and the third pond, with aquatic egg-laying treefrogs, was much more exposed, the researchers tested whether this environmental cue was driving the frogs’ egg-laying behavior. They created a dozen mini-pond habitats using plastic kiddie pools, placing half of the mini-ponds in the forest and half in a nearby open field. Over a series of nights in 2006 and 2007, they put a single pair of breeding frogs into each mini-habitat, choosing frogs from all three ponds and covering the habitats with a cage to ensure that each breeding pair had exclusive use of the area.

Indeed, as reported last month in the Proceedings of the National Academy of Sciences, the frogs deposited about 75 percent of their eggs in water when in the exposed mini-habitats and about 85 percent their eggs on land in the more shaded habitats. It didn’t matter which of the three natural ponds the frogs originally came from, says Touchon. “Even a pair of frogs [from one of the shadier ponds] that we’d never seen lay eggs in the water were laying eggs in the water when placed in the sunnier areas,” he says. “It raises the possibility that there are other animals who can do this, but nobody ever thought to look before.”

In other words, in the search for how and why some frogs evolved from aquatic to terrestrial egg layers, it’s possible that Touchon and Warkentin have found a key transitional species in the evolutionary chain, and that such behavioral flexibility may play a role in evolution more generally. “This opens a new way of looking at this problem,” says Warkentin. “Before we found the dual behavior in this one species, it would never have even occurred to us to ask whether plasticity played a role in evolution. You would think maybe some genetic mutation occurred at some point that just switched their egg-laying behavior.”

There is a somewhat ironic twist. Most aquatic frog eggs have more delicate membranes and dry out and die on land, and the thicker gelatinous clutches of terrestrial eggs are starved of oxygen in water and drown. But the eggs of the hourglass treefrog survive on both land and water by not being extremely well suited for either environment. When deposited on land, for instance, they need rain to survive, but too much rain will flood them, and they drown in deeper water.

While there are many theories of what evolutionary pressures — from environment to predators — led to the change in the egg-laying behavior of some frogs, these theories have thus far been extremely difficult to validate. But, says Touchon, having a frog that can deposit eggs on land and in the water allows researchers to “manipulate the environment to test our hypotheses about these pressures” and how they affect both reproductive decision-making and egg mortality. He hopes to search for “other transitional species” among the handful of species in the same genus as the hourglass treefrog where terrestrial reproduction has evolved.

If he finds any, Touchon says, he’ll investigate whether “the selection pressures are the same on all these types of frogs, to see what causes the movement to land.” Another line of follow-up research suggested by Warkentin is to test how reliable the cues are that frogs are using in their reproductive behaviors, in terms of egg survival. “It’s particularly relevant in a changing climate or when a habitat is modified by development,” she says. “Maybe this flexibility will help these frogs cope with such changes, but maybe the cues they’ve been relying on will no longer be valid.”

Chris Berdik can be reached at cberdik@bu.edu.