Perched on a stream bank in the mountains of central Costa Rica, a rufous-eyed brook frog (Duellmanohyla rufioculis) focuses its wide, red eyes on photographer Sean Graesser, who is crouched a few feet away. Suddenly, the frog’s nictitating membrane—a semi-transparent protective sheath that’s sometimes dubbed a third eyelid—slides up over those big red orbs, and the amphibian is at once starry-eyed.
Nictitating membranes are common in the animal kingdom: They’re found in reptiles, amphibians, birds, fish, and even some mammals. (In fact, humans have a homologous wedge of tissue in the inside corner of each eye—a vestigial trait that serves as a reminder of our evolutionary past.) In amphibians, nictitating membranes provide a layer of protection during activities such as swimming, resting, or handling prey. The membranes are usually translucent, so the animals are still able to see to some degree even when their third eyelids are deployed.
While the presence of nictitating membranes is nothing unusual among frogs, this gold-spangled variety is one-of-a-kind. Scientists aren’t entirely sure how rufous-eyed brook frogs might benefit from their glittery third eyelids, but one explanation—as counterintuitive as this may sound—is that the golden flecks help them hide from would-be predators. “This species has bright red eyes that stand out quite beautifully from its green- and gold-speckled body,” explains herpetologist Cameron Siler from the University of Oklahoma. “So it would not surprise me if these pigmentation patterns evolved in response to some selective pressure to better camouflage those eyes.” In other words, while most animals with such brilliant coloration use their bright-and-shiny features to attract attention, these frogs may use their glittery lids to blend in.
Horquetas, Costa Rica
Sean Graesser is a conservation photographer, storyteller, and biologist. In his scientific research, he specializes in birds, particularly hummingbirds, studying their habitats and migration patterns. He has spent the past ten years in Costa Rica and Panama doing research and teaching tropical rainforest field techniques.