Clumps of frog eggs are often easy meals for snakes and wasps. But some baby frogs have evolved a strategy to avoid being eaten.
Take the red-eyed treefrog (Agalychnis callidryas) for example.
These frogs — found in Neotropical rainforests of Panama, Mexico and Colombia — normally lay clusters of around 40 eggs on leaves or branches hanging over ponds and streams. If undisturbed, the tadpoles in the eggs take about a week to develop, before dropping into the water below.
But if a predator appears, the embryos can hatch ahead of schedule, in a few seconds, to escape the threat. Researchers at the Smithsonian Tropical Research Institute (STRI) have filmed and examined the baby frogs’ escape strategy.
The secret to the embryos’ quick-escape, according to the study published in the Journal of Experimental Biology, lies in their hatching glands — special glands that release enzymes which dissolve the egg membrane.
In many frogs, hatching glands are loosely scattered on the embryos’ bodies. These glands release the enzyme slowly, over several hours or days.
But in the embryos of the red-eye treefrogs, the hatching glands are concentrated around their snouts, the team found. Instead of releasing the enzyme gradually, these embryos can release them all at once. The team confirmed this using a high-speed video camera.
On sensing an approaching parrot snake, the embryos seemed to shake and release a jet of the membrane-dissolving enzyme from their glands, the team observed.
“That movement seems to push them snout-first against the hole the enzyme makes in the egg membrane. Then they muscle their way out by using big, S-shaped thrashing movements,” Karen Warkentin, STRI research associate and professor at Boston University, explained in a statement.
Using this mechanism, the embryos can take between six and 50 seconds to hatch, the team found.
This study adds to the evidence that embryos of many species are not passive, but actively interact with their environment, Warkentin said.
(Video credit: Michael S. Caldwell and Karen M. Warkentin)
- Kristina L. Cohen, Marc A. Seid, Karen M. Warkentin (2016) How embryos escape from danger: the mechanism of rapid, plastic hatching in red-eyed treefrogs. Journal of Experimental Biology. 219: 1875-1883; doi: 10.1242/jeb.139519