Neoteny

Toy dogs often display an extreme level of neoteny, resembling not just infant, but fetal wolves.[1]

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",[2] 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.

Contents

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:

In humans

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:

Compared with other species

The idea that adult humans exhibit certain neotenous (juvenile) features, not evinced in the great apes, is about a century old. Louis Bolk made a long list of such traits,[3] and Stephen Jay Gould published a short list in Ontogeny and Phylogeny.[4] "Man, in his bodily development, is a primate foetus that has become sexually mature" (Bolk). The human capacity for long continued learning may be construed as a juvenile trait greatly extended. However, there are, of course, significant differences between juvenile chimpanzees and adult humans, most obviously in human abstract rational thought and language and, less obviously, in the human female sexual cycle.[5] Therefore, neoteny is just one aspect of the story of human evolution.

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 discussed the importance of neoteny in human biology in The Naked Ape and The Human Zoo.

Between races

In Ashley Montagu's list of "[n]eotenous structural traits in which Mongoloids... differ from Caucasoids", Montagu lists "Larger brain, larger braincase, broader skull, broader face, flat roof of the nose, inner eye fold, more protuberant eyes, lack of brow ridges, greater delicacy of bones, shallow mandibular fossa, small mastoid processes, stocky build, persistence of thymus gland into adult life, persistence of juvenile form of zygomatic muscle, persistence of juvenile form of superior lip muscle, later eruption of full dentition (except second and third molars), less hairy, fewer sweat glands, fewer hairs per square centimeter [and] long torso"[6] "Mongoloid subjects were found to have approximately 20% higher bone density at the angle of mandible than Caucasoid subjects."[7]

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. The amphibian Rough-skinned Newt exhibits neoteny in numerous populations, with noted preference in certain geographic areas; moreover, the phenomenon of gill retention in this newt manifests gradations of neoteny in some populations, such that partial gill retention is seen in some individuals.[8]

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

  1. ↑ Budiansky, Stephen (1999). The Covenant of the Wild: Why animals chose domestication. Yale University Press. ISBN 0300079931. 
  2. ↑ http://www.thefreedictionary.com/neotenous
  3. ↑ Bolk, L. 1926. Das Problem der Menschwerdung. Fischer, Jena.
  4. ↑ short-list of 25 characters reprinted in Gould, Stephen Jay 1977. Ontogeny and phylogeny. Harvard. p357
  5. ↑ Young J.Z. 1957; 2nd ed 1975. The life of mammals. Oxford.
  6. ↑ Montagu, Ashley. Growing Young. Published by Greenwood Publishing Group, 1989 ISBN 0897891678
  7. ↑ Ong. R.G. Evaluation of bone density in the mandibles of young Australian adults of Mongoloid and Caucasoid descent. PubMed. 1999. Accessed September 10, 2008. [1]
  8. ↑ C. Michael Hogan (2008) Rough-skinned Newt (Taricha granulosa), Globaltwitcher, ed. N. Stromberg [2]

Further reading

External links