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Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the
physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemica ...
, or somatic, development of an organism, typically an animal. Neoteny is found in
modern humans Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
compared to other primates. In progenesis or paedogenesis, sexual development is accelerated. Both neoteny and progenesis result in paedomorphism (as having the form typical of children) or paedomorphosis (changing towards forms typical of children), a type of
heterochrony In evolutionary developmental biology, heterochrony is any genetically controlled difference in the timing, rate, or duration of a developmental process in an organism compared to its ancestors or other organisms. This leads to changes in the ...
. It is the retention in adults of traits previously seen only in the young. Such retention is important in
evolutionary biology Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life ...
,
domestication Domestication is a sustained multi-generational relationship in which humans assume a significant degree of control over the reproduction and care of another group of organisms to secure a more predictable supply of resources from that group. ...
and
evolutionary developmental biology Evolutionary developmental biology (informally, evo-devo) is a field of biological research that compares the developmental processes of different organisms to infer how developmental processes evolved. The field grew from 19th-century beginn ...
. Some authors define paedomorphism as the retention of
larval A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...
traits, as seen in salamanders.Schell, S. C. ''Handbook of Trematodes of North America North of Mexico'', 1985, pg. 22


History and etymology

The origins of the concept of neoteny have been traced to the Bible (as argued by
Ashley Montagu Montague Francis Ashley-Montagu (June 28, 1905November 26, 1999) — born Israel Ehrenberg — was a British-American anthropologist who popularized the study of topics such as race and gender and their relation to politics and development. He ...
) and to the poet
William Wordsworth William Wordsworth (7 April 177023 April 1850) was an English Romantic poet who, with Samuel Taylor Coleridge, helped to launch the Romantic Age in English literature with their joint publication '' Lyrical Ballads'' (1798). Wordsworth's ' ...
's " The Child is the father of the Man" (as argued by Barry Bogin). The term itself was invented in 1885 by Julius Kollmann as he described the
axolotl The axolotl (; from nci, āxōlōtl ), ''Ambystoma mexicanum'', is a paedomorphic salamander closely related to the tiger salamander. Axolotls are unusual among amphibians in that they reach adulthood without undergoing metamorphosis. I ...
's maturation while remaining in a
tadpole A tadpole is the larval stage in the biological life cycle of an amphibian. Most tadpoles are fully aquatic, though some species of amphibians have tadpoles that are terrestrial. Tadpoles have some fish-like features that may not be found ...
-like aquatic stage complete with gills, unlike other adult
amphibians Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
like frogs and toads. The word ''neoteny'' is borrowed from the
German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ge ...
''Neotenie'', the latter constructed by Kollmann from the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
νέος (''neos'', "young") and τείνειν (''teínein'', "to stretch, to extend"). The
adjective In linguistics, an adjective ( abbreviated ) is a word that generally modifies a noun or noun phrase or describes its referent. Its semantic role is to change information given by the noun. Traditionally, adjectives were considered one of the ...
is either "neotenic" or "neotenous". For the opposite of "neotenic", different authorities use either "gerontomorphic"Henke, W. (2007). Handbook of paleoanthropology, Volume 1. Springer Books, NY.Hetherington, R. (2010). The Climate Connection: Climate Change and Modern Human Evolution. Cambridge University Press. or " peramorphic".Hall, B.K., Hallgrímsson, B. Monroe, W.S. (2008). Strickberger's evolution: the integration of genes, organisms and populations. Jones and Bartlett Publishers: Canada. Bogin points out that Kollmann had intended the meaning to be "retaining youth", but had evidently confused the Greek ''teínein'' with the Latin ''tenere'', which had the meaning he wanted, "to retain", so that the new word would mean "the retaining of youth (into adulthood)". In 1926, Louis Bolk described neoteny as the major process in humanization. In his 1977 book '' Ontogeny and Phylogeny'',
Stephen Jay Gould Stephen Jay Gould (; September 10, 1941 – May 20, 2002) was an American paleontologist, evolutionary biologist, and historian of science. He was one of the most influential and widely read authors of popular science of his generation. Goul ...
noted that Bolk's account constituted an attempted justification for "scientific" racism and sexism, but acknowledged that Bolk had been right in the core idea that humans differ from other
primates Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...
in becoming sexually mature in an infantile stage of body development.


In humans

Neoteny in humans is the slowing or delaying of body development, compared to non-human
primates Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...
, resulting in features such as a large head, a flat face, and relatively short arms. These neotenic changes may have been brought about by sexual selection in human evolution. In turn, they may have permitted the development of human capacities such as emotional communication. However, humans also have relatively large noses and long legs, both peramorphic (not neotenic) traits. Some evolutionary theorists have proposed that neoteny was a key feature in
human evolution Human evolution is the evolutionary process within the history of primates that led to the emergence of '' Homo sapiens'' as a distinct species of the hominid family, which includes the great apes. This process involved the gradual developmen ...
.
J. B. S. Haldane John Burdon Sanderson Haldane (; 5 November 18921 December 1964), nicknamed "Jack" or "JBS", was a British-Indian scientist who worked in physiology, genetics, evolutionary biology, and mathematics. With innovative use of statistics in biolo ...
states a "major evolutionary trend in human beings" is "greater prolongation of childhood and retardation of maturity." Delbert D. Thiessen said that "neoteny becomes more apparent as early primates evolved into later forms" and that primates have been "evolving toward flat face."Thiessen, D.D. (1997). Bittersweet destiny: the stormy evolution of human behavior. Transaction Publishers, N.J. Doug Jones argued that human evolution's trend toward neoteny may have been caused by sexual selection in human evolution for neotenous facial traits in women by men with the resulting neoteny in male faces being a "by-product" of
sexual selection Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (in ...
for neotenous female faces.


In domestic animals

Neoteny is seen in domesticated animals such as dogs and mice. This is because there are more resources available, less competition for those resources, and with the lowered competition the animals expend less energy obtaining those resources. This allows them to mature and reproduce more quickly than their wild counterparts. The environment that domesticated animals are raised in determines whether or not neoteny is present in those animals. Evolutionary neoteny can arise in a species when those conditions occur, and a species becomes sexually mature ahead of its "normal development". Another explanation for the neoteny in domesticated animals can be the selection for certain behavioral characteristics. Behavior is linked to genetics which therefore means that when a behavioral trait is selected for, a physical trait may also be selected for due to mechanisms like linkage disequilibrium. Often, juvenile behaviors are selected for in order to more easily domesticate a species; aggressiveness in certain species comes with adulthood when there is a need to compete for resources. If there is no need for competition, then there is no need for aggression. Selecting for juvenile behavioral characteristics can lead to neoteny in physical characteristics because, for example, with the reduced need for behaviors like aggression, there is no need for developed traits that would help in that area. Traits that may become neotenized due to decreased aggression may be a shorter muzzle and smaller general size among the domesticated individuals. Some common neotenous physical traits in domesticated animals (mainly dogs, pigs, ferrets, cats, and even foxes) include floppy ears, changes in the reproductive cycle, curly tails,
piebald A piebald or pied animal is one that has a pattern of unpigmented spots (white) on a pigmented background of hair, feathers or scales. Thus a piebald black and white dog is a black dog with white spots. The animal's skin under the white backgro ...
coloration, fewer or shortened vertebra, large eyes, rounded forehead, large ears, and shortened muzzle.Bertone, J. (2006). Equine geriatric medicine and surgery. Saunders, MI. When the role of dogs expanded from just being working dogs to also being companions, humans started
selective breeding Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant ...
dogs for morphological neoteny, and this selective breeding for "neoteny or paedomorphism" "strengthened the human-canine bond." Humans bred dogs to have more "juvenile physical traits" as adults, such as short snouts and wide-set eyes which are associated with puppies because people usually consider these traits to be more attractive. Some breeds of dogs with short snouts and broad heads such as the Komondor, Saint Bernard and
Maremma Sheepdog The Maremmano-Abruzzese Sheepdog, ( it, ), also called Maremmano, the Maremma Sheepdog or Abruzzese Sheepdog, and Cane da Pastore Abruzzese or Pastore Abruzzese (''Abruzzese Sheepdog'' and ''Abruzzese Shepherd'', respectively) is an Italian ...
are more morphologically neotenous than other breeds of dogs.
Cavalier King Charles spaniel The Cavalier King Charles Spaniel is a British breed of toy dog of spaniel type. Four colours are recognised: Blenheim (chestnut and white), tricolour (black/white/tan), black and tan, and ruby; the coat is smooth and silky. The lifespan is u ...
s are an example of selection for neoteny because they exhibit large eyes, pendant-shaped ears and compact feet, giving them a morphology similar to puppies as adults.McGreevy, P.D. & Nicholas, F.W. (1999). Some Practical Solutions to Welfare Problems in Dog Breeding. In Animal Welfare. 8: 329-341. In 2004, a study that used 310 wolf skulls and over 700 dog skulls representing 100 breeds concluded that the evolution of dog skulls can generally not be described by heterochronic processes such as neoteny, although some pedomorphic dog breeds have skulls that resemble the skulls of juvenile wolves. By 2011, the findings by the same researcher were simply "Dogs are not paedomorphic wolves."


In other species

Neoteny has been observed in many other species. It is important to note the difference between partial and full neoteny when looking at other species, to distinguish between juvenile traits which are advantageous in the short term and traits which are beneficial throughout the organism's life; this might provide insight into the cause of neoteny in a species. Partial neoteny is the retention of the larval form beyond the usual age of maturation, with possible sexual development (progenesis) and eventual maturation into the adult form; this is seen in the frog '' Lithobates clamitans''. Full neoteny is seen in '' Ambystoma mexicanum'' and some populations of ''
Ambystoma tigrinum The tiger salamander (''Ambystoma tigrinum'') is a species of mole salamander and one of the largest terrestrial salamanders in North America. Description These salamanders usually grow to a length of with a lifespan of around 12–15 years. ...
'', which remain in larval form throughout their lives. ''Lithobates clamitans'' is partially neotenous; it delays maturation during the winter as fewer resources are available; it can find resources more easily in its larval form. This encompasses both of the main causes of neoteny; the energy required to survive in the winter as a newly-formed adult is too great, so the organism exhibits neotenous characteristics until it can better survive as an adult. ''Ambystoma tigrinum'' retains its neoteny for a similar reason; however, the retention is permanent due to the lack of available resources throughout its lifetime. This is another example of an environmental cause of neoteny. Several avian species, such as the
manakin The manakins are a family, Pipridae, of small suboscine passerine birds. The group contains some 54 species distributed through the American tropics. The name is from Middle Dutch ''mannekijn'' "little man" (also the source of the different bird ...
s '' Chiroxiphia linearis'' and '' Chiroxiphia caudata'', exhibit partial neoteny. The males of both species retain juvenile plumage into adulthood, losing it when they are fully mature. In some bird species, the retention of juvenile plumage is linked to the molting time in each species. To ensure no overlap between molting and mating times, the birds may exhibit partial neoteny in plumage; males do not attain their bright, adult plumage before the females are prepared to mate. Neoteny is present because there is no need for the males to molt early, and trying to mate with immature females would be energy-inefficient. Neoteny is commonly seen in flightless insects, such as the females of the order Strepsiptera. Flightlessness in insects has evolved separately a number of times; factors which may have contributed to the separate evolution of flightlessness are high altitude, geographic isolation (islands), and low temperatures. Under these environmental conditions, dispersal would be disadvantageous; heat is lost more rapidly through wings in colder climates. The females of certain insect groups become sexually mature without metamorphosis, and some do not develop wings. Flightlessness in some female insects has been linked to higher
fecundity Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to pr ...
.
Aphid Aphids are small sap-sucking insects and members of the superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white woolly aphids. A t ...
s are an example of insects which may never develop wings, depending on their environment. If resources are abundant on a host plant, there is no need to grow wings and disperse. If resources become diminished, their offspring may develop wings to disperse to other host plants. Two environments which favor neoteny are high altitudes and cool temperatures, because neotenous individuals have more fitness than individuals which metamorphose into an adult form. The energy required for metamorphosis detracts from individual fitness, and neotenous individuals can utilize available resources more easily. This trend is seen in a comparison of salamander species at lower and higher altitudes; in a cool, high-altitude environment, neotenous individuals survive more and are more fecund than those which metamorphose into adult form. Insects in cooler environments tend to exhibit neoteny in flight because wings have a high surface area and lose heat quickly; it is disadvantageous for insects to metamorphose into adults. Many species of salamander, and amphibians in general, exhibit environmental neoteny.
Axolotl The axolotl (; from nci, āxōlōtl ), ''Ambystoma mexicanum'', is a paedomorphic salamander closely related to the tiger salamander. Axolotls are unusual among amphibians in that they reach adulthood without undergoing metamorphosis. I ...
and olm are salamander species which retain their juvenile aquatic form throughout adulthood, examples of full neoteny. Gills are a common juvenile characteristic in amphibians which are kept after maturation; examples are the tiger salamander, peninsula newt, and rough-skinned newt, both of which retain gills into adulthood.
Bonobo The bonobo (; ''Pan paniscus''), also historically called the pygmy chimpanzee and less often the dwarf chimpanzee or gracile chimpanzee, is an endangered great ape and one of the two species making up the genus '' Pan,'' the other being the comm ...
s share many physical characteristics with humans, including neotenous skulls. The shape of their skull does not change into adulthood (only increasing in size), due to
sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...
and an evolutionary change in the timing of development. Juveniles became sexually mature before their bodies had fully developed as adults and, due to a selective advantage, the skull's neotenic structure remained. In some groups, such as the insect families Gerridae,
Delphacidae Delphacidae is a family of planthoppers containing about 2000 species, distributed worldwide. Delphacids are separated from other "hoppers" by the prominent spur on the tibia of the hindleg. Diet and Pest species All species are phytophagous, m ...
and Carabidae, energy costs result in neoteny; many species in these families have small, neotenous wings or none at all. Some cricket species shed their wings in adulthood; in the genus '' Ozopemon'', males (thought to be the first example of neoteny in
beetle Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...
s) are significantly smaller than females due to
inbreeding Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders a ...
. In the termite ''
Kalotermes flavicollis ''Kalotermes flavicollis'', the yellownecked dry-wood termite, is a species of dampwood termites belonging to the family Kalotermitidae, one of the most primitive families of termites. Distribution This species is mainly present in France, ...
'', neoteny is seen in molting females. In other species, such as the
northwestern salamander The northwestern salamander (''Ambystoma gracile'') is a species of mole salamander that inhabits the northwest Pacific coast of North America. These fairly large salamanders grow to 8.7 in (220 mm) in length. It is found from southeastern ...
(''Ambystoma gracile''), environmental conditionshigh altitude, in this casecause neoteny. Neoteny is also found in a few species of the crustacean family Ischnomesidae, which live in deep ocean water.


Subcellular neoteny

Neoteny is usually used to describe animal development; however, neoteny is also seen in the cell
organelles In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
. It was suggested that subcellular neoteny could explain why sperm cells have atypical centrioles. One of the two sperm centrioles of fruit fly exhibit the retention of “juvenile” centriole structure, which can be described as centriolar “neoteny”. This neotenic, atypical centriole is known as the
Proximal Centriole-Like The proximal centriole-like or PCL is an atypical type of centriole found in the sperm cells of insects. The PCL name is due to some similarity to the Proximal centriole found in Vertebrates sperm and the hypothesis that the two structures are homo ...
. Typical centrioles form via a step by step process in which a cartwheel forms, then develops to become a procentriole, and further matures into a centriole. The neotenic centriole of fruit fly resembles an early procentriole.In The Golgi Apparatus and Centriole: Functions, Interactions and Role in Disease. M. Kloc, editor. Springer International Publishing, Cham. 3-15, https://link.springer.com/chapter/10.1007%2F978-3-030-23173-6_1


See also

*
Ageing Ageing ( BE) or aging ( AE) is the process of becoming older. The term refers mainly to humans, many other animals, and fungi, whereas for example, bacteria, perennial plants and some simple animals are potentially biologically immortal. ...
* Cuteness *
Kawaii ''Kawaii'' is the culture of cuteness in Japan. It can refer to items, humans and non-humans that are charming, vulnerable, shy and childlike.Kerr, Hui-Ying (23 November 2016)"What is kawaii – and why did the world fall for the ‘cult of c ...
* Larviform female * Neotenin


References


Further reading

* Bergstorm, Carl T. & Dugatkin, Lee Alan (2012). ''Evolution'', W.W. Norton


External links

* {{genarch Developmental biology Evolutionary biology Taxonomy (biology)