Egged on

It was quite a game changer ~375 million years ago when animals with backbones first left the ocean to live on land. Plants, fungi and insects had already made the move, creating an amazing opportunity for the first animals that figured out how to colonize dry habitats. Those first animals were definitely still tied to the water, if nothing else for the purposes of breeding. Scientists working at the Smithsonian Tropical Research Institute (STRI) in Panama think maybe the ability of animals to lay eggs both in the water and on land may have given amphibians a leg up when it came to more fully moving away from water.

“We think that maybe this kind of flexible reproductive behavior came first,” said lead author Justin Touchon, associate professor of biology at Vassar University, “and that the ability to be flexible may have come before the evolution of other adaptations of eggs, which are necessary for developing on land.”

Naturalists love to learn from nature: Touchon and colleagues observed how more than a dozen closely related species of tree frogs (Dendropsophus spp.) from Central and South America lay their eggs.

Four species only laid eggs in water. Three species only laid eggs on land. But remarkably, six species laid eggs both in the water and on surfaces such as branches or leaves hanging over ponds. 

The species that only lay eggs on land had eggs that were larger than those that only laid eggs in the water. The eggs of the species that could do both were intermediate in size, demonstrating that as species become more fully terrestrial, they also evolve to have eggs better suited for land. And, the jelly surrounding the eggs laid on land could absorb more water than the other species, a feature that would be helpful for surviving dryness.

By comparing different populations of just one of the flexible species that lives in many places, from Mexico to Ecuador, they discovered that on average, the frogs living in rainier areas laid fewer eggs in water than frogs of the same species living in drier habitats; and that while some populations have evolved more terrestrial egg-laying behavior, they have not yet evolved the physical differences in eggs seen across species. This tells us that the behavior likely evolves first, and the other adaptations come second.

In an age of rapidly shifting climate patterns, it may be useful to understand what it takes to survive and adapt to changing conditions. And maybe being flexible is essential.

“We tend to think of evolutionary novelties in terms of physical characters that evolve and permit species to do new things, but this work shows that maybe those traits can actually evolve secondarily, after a new behavior has already evolved,” said Touchon.

The Smithsonian Tropical Research Institute, headquartered in Panama City, Panama, is a unit of the Smithsonian Institution. The institute furthers the understanding of tropical biodiversity and its importance to human welfare, trains students to conduct research in the tropics and promotes conservation by increasing public awareness of the beauty and importance of tropical ecosystems.