Definitions

neoteny

neoteny

[nee-ot-n-ee]
neoteny, in biology, sexual maturity reached in the larval stage of some animals. Certain environmental conditions can inhibit the completion of metamorphosis; low temperature or lack of available iodine retard the action of the thyroid gland, the larval form may mature sexually, mate, and produce fertile eggs. If environmental conditions improve, neoteny is reversible; i.e., the larvae can complete metamorphosis and attain normal maturity. When neoteny occurs in some salamanders (see axolotl), they remain aquatic. In insects, reproduction in the larval stages is known as paedogenesis; it occurs in certain beetles and gall midges. In the midges, the daughter larvae produced within a mother larva consume the mother and escape; the process may continue for several generations.
Neoteny also called juvenilization, is the retention, by adults in a species, of traits previously seen only in juveniles (pedomorphosis/paedomorphosis), and is a subject studied in the field of developmental biology. In neoteny, the physiological (or somatic) development of an animal or organism is slowed or delayed (alternatively, seen as a dilation of biological time). Ultimately this process results in the retention, in the adults of a species, of juvenile physical characteristics well into maturity. The English word neoteny is borrowed from the German Neotenie, the latter constructed from the Greek νέος (young) and τείνειν (tend to). The standard adjectival form is "neotenous", although "neotenic" is often used.

In vertebrate biology, neoteny is most easily identified when sexually mature, completely viable juveniles or larvae are found.

Specific individual traits that differ in descendant organisms, when compared to ancestors, are sometimes called neotenies; humans, for example, appear to have several neotenies in comparison to chimpanzees.

In evolution

Neoteny plays a role in evolution, as a means by which, over generations, a species can undergo a significant physical change. In such cases, a species’ neotenous form becomes its “normal” mature form, no longer dependent upon environmental triggers to inhibit maturity. The mechanism for this could be a mutation in or interactions between genes involved in maturation, changing their function to impede this process.

Neoteny is not the only contributing factor affecting maturation in species that may have undergone neotenous changes over the course of their evolution, and its actual involvement in the following examples is not well understood:

  • flightless birds—physical proportions resemble those of the chicks of flighted birds;
  • humans—with traits such as sparse body hair and enlarged heads 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. It corresponds to a mutation that permits the digestion of lactose beyond the lactation period.
  • pets, such as dogs—which share many physical features with the immature wolf (these same traits were found during the development of the tame silver fox). Such puppy-like traits may have made early dogs seem "cute" and less threatening than wolves, leading to both natural and artificial selection of such dogs.

It is possible that the origin of the chordates (the phylum including all vertebrates) was the result of an instance of 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. As a sexually active pelagic organism it had considerable feeding and habitat colonization advantages over the sessile form, so was at an evolutionary advantage. However, the alternative - that the sessile form developed later and the pelagic form was ancestral - is also thought possible (based on the fact that many jellyfish undergo the reverse: a sessile polyp phase, followed by the familiar soft, highly mobile adult form).

In humans

see:The Naked Ape
Neoteny in humans can be seen in different aspects. It can be compared with other great ape species, between the sexes and between individuals. Some examples include:

  • A chimpanzee completes brain growth at about 1 year of life while a human doesn't fully complete growth until about 23, also at birth a human has only 32% of the brain size it will eventually have.
  • the flatness of the human face compared to other primates
  • late arrival of the teeth

Compared to other species

There is controversy over whether adult humans exhibit certain neotenous features, or juvenile characteristics, that are not evidenced in other great ape species. 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 (Homo sapiens) to gain predominance over H. neanderthalensis (and possibly H. erectus and H. heidelbergensis) by appealing to these species' nurturing instincts through paedomorphic cuteness to avoid territorial aggression. Noted anthropologist Björn Kurtén explores this concept in his paleofictional Dance of the Tiger (1980).

Between sexes

While neoteny is not necessarily a physical state experienced by humans, 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.

Desmond Morris discusses the importance of neoteny in human biology in The Naked Ape and The Human Zoo.

Between individuals

Paedomorphic variations also not only exist between the sexes, but also between individuals, with some people displaying more characteristics of neoteny than others. This trend carries over to variations among ethnic groups as well.

Bruce Charlton, Reader in evolutionary psychiatry at Newcastle University U.K., suggests that there may be such a thing as "psychological neoteny." Due to recent changes in culture, he says, “In a psychological sense, some contemporary individuals never actually become adults.” Delayed maturity might be a consequence of later parenthood, itself caused by more prolonged formal education

In other animals

One example of a neotenic trait in vertebrates is the salamander species axolotl, which usually remains fully aquatic as it matures. Other salamanders, such as the widespread tiger salamander of North America, may retain the external gills usually only present in immature individuals, as adults in some populations in marginal habitats.

Neoteny and progenesis

Neoteny and progenesis are both mechanisms that result in paedomorphosis. Neoteny delays physiological, but not sexual, maturity. Comparatively, progenesis speeds up sexual, but not physiological, maturity. Progenetic organisms achieve sexual maturity in their juvenile state. This is most commonly found among certain amphibians and insects.

See also

References

External links

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