Many animals, such as amphibians and insects, undergo metamorphosis when they mature, in which their body is remade. The size they are when they metamorphose affects how many offspring they will have, because body size of males affects how many matings they will get and body size of female affects how many eggs they can lay. But growing to a large size takes time, during which larvae (e.g. tadpoles and caterpillars) are vulnerable to being eaten by predators. It is usually presumed that if the larvae detect they are more risk they will mature earlier (and at a smaller size), so reducing their adult fitness. However, this is not what we see when we increase the apparent risk to tadpoles and caterpillars.
We use a computational model in which larvae grow in size and decide when to mature, but also decide how much energy to invest in avoiding being eaten, such as defensive spines. The results show that under lots of situations the best thing for larvae to do if the predation risk increases is to grow strong defences and grow to a larger size before maturing. This is especially the case when there is lots of food and no good hiding places, which is usually the case in laboratory experiments. In contrast, animals should only metamorphose at a smaller size if there is little food. We collected published results on how age and size at metamorphosis changed with higher risk in tadpoles and caterpillars, and we get broad agreement with our predictions.
Our work shows that the intuition about what larvae should do will usually be wrong. However, the results show that the often observed response to predators of growing to a larger size and metamorphosing later may often be an effect of doing experiments in laboratories. In future, researchers should try to make experiments more realistic by having restricted food (as would be true in natural situations) and provide lots of hiding places. Our model suggests that in this case animals will mature smaller but at a later time.
Posted by: Andrew Higginson Posted Fri Sep 01 2017