Embryos of the red-eyed tree frog have developed an interesting strategy to survive on a patchy supply of oxygen. To permeate the normally oxygen-deficient eggs, oxygen must first pass through a strong outer membrane. But even though tiny hairs called cilia stir the fluid inside these quarter-inch-diameter eggs, most of the oxygen is near the eggs' exposed surface. To get at the oxygen, these four- [above] and five-day-old next page embryos position their heads and external gills [red strands] near the air-exposed surface for minutes at a time; conversely, if the frogs' gills end up in a low-oxygen region in the eggs, the frogs will reposition them within 15 seconds on average. Biologist Karen Warkentin of Boston University, the senior researcher on the study, was surprised that the frogs were capable of this preferential orientation so early in their development—they showed the behavior as soon as one day old.