Neoteny (pronounced /niːˈɒtɨniː/), also called juvenilization, is the retention, by adults in a species, of traits previously seen only in juveniles (a kind of pedomorphosis), 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 (fallaciously, 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 invertebrate 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.
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 cow's 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.
- domesticated silver foxes — For almost 60 years, researchers in Siberia have bred a population of silver foxes that they have selected only for friendliness toward humans. The foxes not only developed pedomorphic characteristics like shortened faces and floppy ears, but also many other features common in the domesticated dog but not the dogs' wolf ancestors, such as multi-colored coats, wavy hair or fur, and curly tails.
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