The mole salamander, a close relative to the axolotl, displays both paedomorphosis and peramorphosis. The larva can develop in either direction. Population density, food, and the amount of water may have an effect on the expression of heterochrony. A study conducted on the mole salamander in 1987 found it evident that a higher percentage of individuals became paedomorphic when there was a low larval population density in a constant water level as opposed to a high larval population density in drying water. This had an implication that led to hypotheses that selective pressures imposed by the environment, such as predation and loss of resources, were instrumental to the cause of these trends. These ideas were reinforced by other studies, such as peramorphosis in the Puerto Rican Tree frog. Another reason could be generation time, or the lifespan of the species in question. When a species has a relatively short lifespan, natural selection favors evolution of paedomorphosis (e.g. Axolotl: 7–10 years). Conversely, in long lifespans natural selection favors evolution of peramorphosis (e.g. Irish Elk: 20–22 years).