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In Other Species
Heterochrony is extensively referenced in biological literature, but inconsistence with its uses remains. This is due to lack of universal definition. To exacerbate the confusion, subcategories of heterochrony were and are currently being used differently. The term neoteny is one of three types of paedomorphosis, that was originally used by Kolman (1885) to describe the truncation of development in an organism. Later on, neoteny was used by De Beer (1930) to describe decreased growth in somatic character and accelerated growth in gonadal development. To alleviate the confusion for simplicity purposes, heterochrony in other species will be described in the form of paedomorphosis and peramorphosis. In addition, species will be described in comparison to similar species or relatives along its lineage.

Paedomorphosis can be observed when an organism/species mature (sexual maturation is most typical) while retaining juvenile traits. One specie commonly studied is the Axolotl. The adult Axolotl looks very similar to its larval form. Some say it looks like a giant larvae. Adult Axolotls are sexually mature, but retain their gills and fins. They remain in aquatic environments. In three species of bagworm moths, females become flightless when their wings become degenerate, reduced, or wingless due to apoptosis. The reduction in their wings begins in their late larval stage to pupal stage. A common misconception of insects is the importance of acquiring the ability of flight. From an evolutionary perspective, if there is no demand for flight, then nature selection will favor flightlessness.

Contrary to paedomorphosis, peramorphosis is delayed maturation and extended periods of growth. The extinct Irish elk is an example of peramorphosis. From fossil record, it’s antlers spanned up to 12 feet wide, which is about a third larger than it’s closer relative’s antlers, the moose. The Irish elk’s antlers have a larger antler due to extended development during its period of growth. In addition, these huge antlers exemplify outweighed benefits of sexual selection by ecological selection. Due to the nutrient costs of maintaining these antlers combined with rapid climate change, the Irish elk could not adapt and evolve lesser antlers fast enough, which led to their extinction. Another example of peramorphosis is the insular rodents, also known as island rodents. Their characteristics include gigantism, wider cheek and teeth, reduced litter size, and longer life span. Their relatives that inhabit the continental environments are much smaller. These insular rodents have evolved gigantism, wider cheek and teeth to accommodate the abundance of larger food and resource they have on the island. This factor is part of a complex phenomenon termed Island Syndrome. With less predation and competition for resources, selection favored overdevelopment of these species. Reduced litter sizes enable overdevelopment of its bodies into larger ones. In some species of frogs, such as the Puerto Rican tree frog, they skip their entire larval stage. These frogs hatch out of their eggs into froglets with limbs, severely reduced gills (or lack gills), and gill slits. Their habitat includes forests, gardens, under rocks, and logs, which are non-aquatic.

A trend commonly seen is that paedomorphic species have the tendency to inhabit or stay in aquatic environments and peramorphic species tend to inhabit terrestrial environments. The mole salamander, which is a close relative to the Axolotl, surprisingly displays both paedomorphosis and paramorphisis. The larva can develop into both directions, but not backwards. Some research supposes that population density, food, and the amount of water had 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 were low larval population density in constant water level versus high larval population density in drying water. This had an implication that led to hypotheses claiming that selective pressures imposed by the environment, such as predation and 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 have a relatively short lifespan, natural selection will favor evolution of paedomorphosis (e.g. Axolotl: 7-10 years). On the flip side, in long lifespans, natural selection will favor evolution of peramorphosis (e.g. Irish Elk: 20-22 years).