Scientists from Boston University have discovered that the embryos of red-eyed tree frogs (Agalychnis callidryas) use up most of the oxygen within their eggs before they hatch. Undergraduate Jessica Rogge and associate professor Karen Warkentin found that tree frogs—whose eggs are ready to hatch about four days after they have been laid—delay hatching for several additional days. During that time, the tree frog embryos continue to grow and consume oxygen, all the time increasing their risk of asphyxiation.


By delaying hatching, the frogs balance the odds of survival in their favor: the larger the frog embryos grow before hatching, the better their chances of survival after hatching.

The red-eyed tree frog inhabits the tropical rainforests of Central America. Female red-eyed treefrogs lay their eggs on leaves that overhang ponds. When the eggs hatch, the tadpoles fall into the water below. Once in the water, the tiny tadpoles are vulnerable to predation by fish, and larger tadpoles are better able to fend for themselves than smaller tadpoles.

Rogge and Warkentin found that the frog embryos orient themselves so that their gills are positioned in the area of the egg with the richest supply of oxygen, such as near the surface of the egg that is exposed to the air. This ensures that the embryos can take advantage of every scrap of oxygen the egg has to offer before hatching.

That embryos are capable of doing this is quite remarkable—the developing frogs have no gills, blood, or capacity for muscle movement.

Yet they somehow they are able to maintain their head in the position of optimal oxygen supply within the egg.

Rogge and Warkentin adjusted the position of the embryos within the eggs to see what would happen if the embryos were reoriented away from the oxygen 'sweet spot'. The researchers gently probed the embryos, nudging them into different positions and moving the head away from the oxygen-rich part of the cell. They found that the embryo soon drifted back to its original position, with its head oriented towards the area of richest oxygen concentration.

References

Red-eyed Treefrog Embryos Actively Avoid Asphyxiation Inside Their Eggs. Eurekalert. 2008 Accessed at http://www.eurekalert.org/pub_releases/2008-11/stri-rte110308.php

Rogge JR and Warkentin KM. 2008. External gills and adaptive embryo behavior facilitate synchronous development and hatching plasticity under respiratory constraint. The Journal of Experimental Biology, 211: 3627-3635 DOI: 10.1242/jeb.020958