The term neoteny is derived from Latin neotenia; neo + Greek teinein = to extend, meaning when larval life is extended. Neoteny also called juvenilization, is the retention, by adults in a species, of traits previously seen only in juveniles. Neoteny has two forms depending upon capability of individuals to breed.
Pedomorphosis was first proposed by WalterGarstang in 1922. The underlying mechanisms for this include heterochrony (change in features during development). Pedomorphosis is common in many animal species domesticated by humans, including dogs, chickens, pigs and cattle. It is believed to be a side-effect of the selective pressure of human-directed breeding for juvenile behavioral characteristics such as docility and cuteness. Pedomorphosis also occurs in termites and several species of cockroach. Humans are considered by some scientists to be pedomorphic, due to their flattened face, short jaw, and bulbous forehead compared to other adult primates.
Paedogenesis is the act of reproduction by an organism that has not achieved physical maturity. In other words paedogenesis is the production of offspring by an organism in its larval or juvenile form and elimination of the adult phase of the life cycle. It is associated with progenesis, where sexual maturity is achieved in the juvenile form and further physical maturity is not achieved. Paedogenesis is found in insects in which the larval stage reproduces without achieving maturity. It occurs in the females of certain beetles, Strepsiptera, bagworms, scale insects and gall midges.
EXAMPLES OF NEOTENY
Flightless birds such as ostriches, emus, cassowaries and kiwis are believed to have evolved by retaining characteers of chicks and losing ability to fly. Physical proportions of these birds resemble those of the chicks of flighted birds.
With human traits such as sparse body hairs and enlarged heads are thought to be reminiscent of baby primates. Lactose tolerance in adults is a form of neoteny now considered normal in certain populations that traditionally consume milk while most other humans are lactose intolerant as adults. Although females mature at an earlier age, women do not go on to acquire the toughened skin, coarse body hair, thyroid cartilage, bony eye ridges, or deepened voices which are the common inheritance of most adult hominoids and other primates. Paedomorphic characteristics in women are widely acknowledged as desirable by men. For instance, vellus hair is a juvenile characteristic. However, while men develop longer, coarser, thicker, and darker terminal hair through sexual differentiation, women do not, leaving their vellus hair visible.
Stephen Jay Gould was an advocate of the view that humans are a neotenous species of chimpanzee. The argument is that juvenile chimpanzees have an almost-identical bone structure to humans, and that the chimpanzee’s ability to learn seems to be cut off upon reaching maturity.
Another theory suggests that humans’ neotenous characteristics were an evolutionary strategy that enabled Cro-Magnons to gain predominance over neanderthals (and possibly H. erectus and H. heidelbergensis) by appealing to these species’ nurturing instincts through paedomorphic cuteness and to avoid territorial aggression.
Bolk (1926) provided an abbreviated list of human neotenic characters:
Origin of chordates
It is possible that chordates originated by neoteny. Molecular evidence suggests that the nearest relatives of the chordates are the tunicates, marine filter feeders. Although sessile in their adult, sexually mature form, tunicates have a motile larval dispersal form, which has a notochord similar to that found in chordates. At some point, the motile larvae of the tunicate became sexually mature before metamorphosis and gave rise to free swimming chordates.
Neoteny in amphibia
Natural pedomorphosis occurs in many species of amphibians, especially ambystomatid and protean salamanders. In amphibians it can be obligate or facultative. For example, some salamanders retain their gills during adulthood, unlike most other amphibians.
Most neotenic populations belong to the Tiger Salamander complex – Ambystoma tigrinum, Ambystoma velasci, Ambystoma mavortium, and their close relatives. Wholly neotenic Ambystoma species include the Axolotl, Ambystoma taylori, Ambystoma andersoni, and Ambystoma dumerilii. Neotenes retain ability to regenerate limbs, tails, and nearly every organ in their body. Neoteny, sometimes called paedomophism, is apparent in Urodela, except Rhyacotritonidae.
There are three types of neoteny, obligate, inducible obligate, and facultative.
All members of the families Amphiumidae, Sirenidae, Cryptobranchidae, and Proteidae are obligate neotenes, meaning they never fully metamorphose, and retain larval characteristics in varying degrees into adulthood. Most of these species are insensitive to thyroid hormone doses.
Inducible obligate neoteny
Inducible obligate neotenes of the family Ambystomatidae, and some species of the family Plethodontidae are unique in that metamorphosis can be induced by manipulating the thyroid function in the laboratory. The most famous inducible obligate neotene is perhaps Ambystoma mexicanum (Ambystomatidae).
Facultative neoteny is observed in Salamandridae, Dicamptodontidae, Ambystomatidae, Hynobiidae, and Plethodontidae. Facultative neoteny occurs in individuals or entire populations as a result of environmental factors. For example, in extremely cold temperatures, where a terrestrial existence would be inhospitable, individuals or populations may remain aquatic, retaining larval characteristics into adulthood.
The Axolotl larva and Neoteny
A sexually mature axolotl, at age 18–24 months, ranges in length from 15–45 centimetres. Axolotls possess features typical of salamander larvae, including external gills and a caudal fin extending from behind the head to the vent. Their heads are wide, and their eyes are lidless. Their limbs are underdeveloped and possess long, thin digits. Males are identified by their swollen cloaca lined with papillae, while females are noticeable for their wider bodies full of eggs. Axolotls have barely visible vestigial teeth which would have developed during metamorphosis. Three pairs of external gills are used for respiration, although buccal pumping (gulping air from the surface) may also be used in order to provide oxygen to their lungs.
Factors affecting neoteny
In the axolotl, metamorphic failure is caused by a lack of thyroid stimulatinghormone, which is used to induce the thyroid to produce thyroxine in transforming salamanders. Axolotls can be induced to metamorphose by an injection of iodine (used in the production of thyroid hormones) or by shots of thyroxine hormone.
Another method for inducing metamorphosis is to keep them in shallow water tanks. They will then, over a period of weeks, slowly metamorphose into adult salamanders. However, most attempts at inducing metamorphosis lead to death. This is likely due to the strong genetic basis for neoteny in pet axolotls. Artificial metamorphosis also dramatically shortens the axolotl’s lifespan, if they survive the process.
Research indicates that neoteny occurs because the hypothalamus of the brain fails to produce the hormone that causes the pituitary to stimulate the thyroid gland to produce growth hormones that trigger metamorphosis. Some scientists think that neoteny may have evolved as a response to the hazards of life on land.