Behavioural trade offs
A LIFE history strategy is a set of blueprints according to which a plant or animal regulates its life from birth to death. How long to remain in an embryonic state inside the egg, how to face up to the risks that follow weaning, how to allocate one's time between feeding, reproduction and care of the young - all form part of the life history strategy.
While many aspects of a life history strategy are determined genetically, there are others which may be determined non -genetically. When that is the case, one speaks of phenotypic plasticity, meaning a variability in behaviour (or form or function) in response to contingent circumstances.v This has been brought out in a persuasive study of the process of hatching in the tree frog, Agalychnis callidryas, by Karen M Warkentin of the University of Texas (Proceedings of the us National Academy of Sciences, Vol 92).
The young of the A callidryas start life inside eggs laid on vegetation overhanging ponds. On hatching, the young fall into the water, and are exposed to the risk of attack by aquatic predators - fish and shrimp. The more delayed is the process of hatching, the higher the probability of survival of a tadpole. But staying inside the egg for a long time subjects the frog to a different hazard - that of being eaten by the cat-eyed snake, Leptodeira sep ten trio nalis. It appears that the sensible thing to do would be to delay hatching until the threat from the snake is imminent. And that is exactly what the frog does.
Warkentin observed that frog embryos normally hatched over a period of 2-3 days, but did so much faster upon perceiving the presence of a snake nearby. Her findings refer to whole egg clutches; the response is dramatic at the level of single individuals, which can hatch within seconds (on occasion even from eggs held in the snake's mouth). This premature hatching is elicited specifically by the predator and not by any general mechanical disturbance. It increases the probability of survival considerably. To quote Warkentin, "The ability to hatch rapidly over a broad time period allows A callidryas embryos to respond phenotypically to the actual and immediate risk of being eaten by a snake."