is seen in insular (island) rodents. Their characteristics include gigantism, wider cheek and teeth, reduced litter size, and longer lifespan. Their relatives that inhabit continental environments are much smaller. Insular rodents have evolved these features to accommodate the abundance of food and resources they have on their islands. These factors are part of a complex phenomenon termed Island syndrome or Foster's rule. 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). Across the animal kingdom Heterochrony is responsible for a wide variety of effects such as the lengthening of the fingers by adding extra phalanges in dolphins to form their flippers, sexual dimorphism, and the polymorphism seen between insect castes. Garstang's hypothesis Walter Garstang suggested the neotenous origin of the vertebrates from a tunicate larva, in opposition to Darwins opinion that tunicates and vertebrates both evolved from animals whose adult form was similar to (frog) tadpoles and the 'tadpole larvae' of tunicates. According to Richard Dawkins, Garstang's opinion was also held by Alister Hardy, and is still held by some modern biologists. However, according to others, closer genetic investigation rather seems to support Darwin's old opinion: In humans Several heterochronies have been described in humans, relative to the chimpanzee. In chimpanzee fetuses, brain and head growth starts at about the same developmental stage and grow at a rate similar to that of humans, but growth stops soon after birth, whereas humans continue brain and