Neoteny (// // or //), also called juvenilization, is one of the two ways by which paedomorphism can arise. Paedomorphism or paedomorphosis is the retention by adults of traits previously seen only in the young, and is a subject studied in the field of developmental biology. Neoteny is found in modern humans. Paedomorphism can also be the retention of larval traits which is commonly studied in salamanders. In neoteny, the physiological (or somatic) development of an organism (typically an animal) is slowed or delayed. In contrast, in progenesis, sexual development occurs faster. Both processes result in paedomorphism, a type of heterochrony. Ultimately this process results in the retention, in the adults of a species, of juvenile physical characteristics well into maturity and pedogenesis (paedogenesis), the reproduction in a neotenized state.
Neoteny is one of three dimensions of heterochrony, or the change in timing of developmental events: acceleration (faster) vs. neoteny (slower), hypermorphosis (further) vs. progenesis (not as far), and predisplacement (begins earlier) vs. postdisplacement (begins later).
The word neoteny is borrowed from the German Neotenie, the latter constructed from the Greek νέος (neos, "young") and τείνειν (teínein, "to extend"). The adjective form of the word is either "neotenic" or "neotenous". For the opposite of "neotenic", different authorities use either "gerontomorphic" or "peramorphic".
- 1 In humans
- 1.1 Neotenic traits in humans
- 1.2 Human evolution
- 1.3 Growth pattern of children
- 1.4 Neotenous features elicit help
- 1.5 Brain
- 1.6 Between sexes
- 1.7 Between races and among primates
- 1.8 Specific neotenies
- 1.9 Down syndrome
- 2 Neoteny in dogs
- 3 Neoteny in other species
- 4 See also
- 5 References
Neotenic traits in humans
Various sources identify the following:
Many prominent evolutionary theorists propose that neoteny has been a key feature in human evolution. Stephen Jay Gould believed that the "evolutionary story" of humans is one where we have been "retaining to adulthood the originally juvenile features of our ancestors". J. B. S. Haldane mirrors Gould's hypothesis by stating 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." However, in light of some groups using arguments based around neoteny to support racism, Gould also argued "that the whole enterprise of ranking groups by degree of neoteny is fundamentally unjustified" (Gould, 1996, pg. 150).
Doug Jones, a visiting scholar in anthropology at Cornell University, said 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 for neotenous female faces. Jones said that this type of sexual selection "likely" had a major role in human evolution once a larger proportion of women lived past the age of menopause. This increasing proportion of women who were too old to reproduce resulted in a greater variance in fecundity in the population of women, and it resulted in a greater sexual selection for indicators of youthful fecundity in women by men.
Ashley Montagu said that the fetalized pithecanthropine represented by the juvenile Mojokerto skull and the fetalized australopithecine represented by the juvenile Australopithecus africanus skull would have had skulls with a closer resemblance to those of modern humans than to those of the adult forms of their own species. Montagu further listed the roundness of the skull, thinness of the skull bones, lack of brow ridges, lack of sagittal crests, form of the teeth, relative size of the brain and form of the brain as ways in which the juvenile skulls of these human ancestors resemble the skulls of adult modern humans. Montagu said that the retention of these juvenile characteristics of the skull into adulthood by australopithecine or pithecanthropine could have been a way that a modern type of human could have evolved earlier than what actually happened in human evolution.
Stanley Greenspan and Stuart G. Shanker proposed a theory in The First Idea of psychological development in which neoteny is seen as crucial for the "development of species-typical capacities" that depend upon a long period of attachment to caregivers for the opportunities to engage in and develop their capacity for emotional communication. Because of the importance of facial expression in the process of interactive signaling, neotenous features, such as hair loss, allow for more efficient and rapid communication of socially important messages that are based on facially expressive emotional signaling.
Other theorists have argued that neoteny has not been the main cause of human evolution, because humans only retain some juvenile traits, while relinquishing others. For example, the high leg-to-body ratio (long legs) of adult humans as opposed to human infants shows that there is not a holistic trend in humans towards neoteny when compared to the other great apes. Andrew Arthur Abbie agrees, citing the gerontomorphic fleshy human nose and long human legs as contradicting the neoteny hominid evolution hypothesis, although he does believe humans are generally neotenous. Brian K. Hall also cites the long legs of humans as a peramorphic trait, which is in sharp contrast to neoteny.
On the balance, an all or nothing approach could be regarded as pointless, with a combination of heterochronic processes being more likely and more reasonable (Vrba, 1996).
Growth pattern of children
Physical anthropologist Barry Bogin said that the pattern of children's growth may intentionally increase the duration of their cuteness. Bogin said that the human brain reaches adult size when the body is only 40 percent complete, when "dental maturation is only 58 percent complete" and when "reproductive maturation is only 10 percent complete". Bogin said that this allometry of human growth allows children to have a "superficially infantile" appearance (large skull, small face, small body and sexual underdevelopment) longer than in other "mammalian species". Bogin said that this cute appearance causes a "nurturing" and "care-giving" response in "older individuals".
Neotenous features elicit help
The Multiple Fitness Model proposes that the qualities that make babies appear cute to adults additionally look "desirable" to adults when they see other adults. Neotenous features in adult females may help elicit more resource investment and nurturing from adult males. Likewise, neotenous features in adult males may similarly help elicit more resource investment and nurturing from adult females in addition to possibly making neotenous adult males appear less threatening and possibly making neotenous adult males more able to elicit resources from "other resource-rich people". Therefore, it could be adaptive for adult females to be attracted to adult males that have "some" neotenous traits.
Caroline F. Keating et al. tested the hypothesis that adult male and female faces with more neotenous features would elicit more help than adult male and female faces with less neotenous features. Keating et al. digitally modified photographs of faces of African-Americans and European Americans to make them appear more or less neotenous by either enlarging the size of their eyes and lips or decreasing the size of their eyes and lips. Keating et al. said that the more neotenous white male, white female and black female faces elicited more help from people in the United States and Kenya, but the difference in help from people in the United States and Kenya for more neotenous black male faces was not significantly different from less neotenous black male faces.
Helmuth Nyborg said that a testable hypothesis can be made using the GTC-A/E model with regards to "neoteny". Nyborg said that the hypothesis is that "feminized", slower maturing, "neotenic" "androtypes" will differ from "masculinized", faster maturing "androtypes" by having bigger brains, more fragile skulls, bigger hips, narrower shoulders, less physical strength, live in cities (as opposed to living in the countryside) and by receiving higher performance scores on ability tests. Nyborg said that if the predictions made by this hypothesis are true, then the "material basis" of the differences would be "explained". Nyborg said that some ecological situations would favor the survival and reproduction of the "masculinized," faster maturing "androtypes" due to their "sheer brutal force" while other ecological situations would favor the survival and reproduction of the "feminized," slower maturing, "neotenic" "androtypes" due to their "subtle tactics."
Aldo Poiani who is an evolutionary ecologist at Monash University, Australia, said that he agrees that neoteny in humans may have become "accelerated" through "two-way sexual selection" whereby females have been choosing smart males as mates and males have been choosing smart females as mates.
Neoteny has been important to human evolution, because it has increased the maturation period and the size of the human brain. Two to three million years ago, there was an "incomplete segmental duplication of [the] ancestral SRGAP2" gene in the ancestors of humans. This new gene, SRGAC2, slowed spine maturation and allowed for more neuronal migration. As a result, the dendrite spines increased in number and length, and they became "more complex". This accounts for the greater synaptic densities in humans when compared to other primates and rodents.
Somel et al. said that 48% of the genes that affect the development of the prefrontal cortex change with age differently between humans and chimpanzees. Somel et al. said that there is a "significant excess of genes" related to the development of the prefrontal cortex that show "neotenic expression in humans" relative to chimpanzees and rhesus macaques. Somel et al. said that this difference was in accordance with the neoteny hypothesis of human evolution.
Humans have been evolving toward greater "psychological neoteny." Dr. Bruce Charlton, a Newcastle University psychology professor, said what looks like immaturity — or in his terms, the "retention of youthful attitudes and behaviors into later adulthood" — is actually a valuable developmental characteristic, which he calls psychological neoteny. Highly educated people and eminent scientists demonstrate more neotenous psychological traits. In fact, the ability of an adult human to learn is considered a neotenous trait. Physical neotenization in humans has, likewise, caused psychologically neotenous traits in humans: curiosity, playfulness, affection, sociality and an innate desire to cooperate.
Ashley Montagu said that the following neotenous traits are in women when compared to men: more delicate skeleton, smoother ligament attachments, smaller mastoid processes, reduced brow ridges, more forward tilt of the head, narrower joints, less hairy, retention of fetal body hair, smaller body size, more backward tilt of pelvis, greater longevity, lower basal metabolism, faster heartbeat, greater extension of development periods, higher pitched voice and larger tear ducts.
Cro-Magnon humans (40,000 to 10,000 years ago) differed from co-existing Neanderthals in the following ways: "higher forehead, less prominent brow ridges, smaller teeth, less robust bodies, and reduced sexual dimorphism."
Attractive women's faces
In a cross-cultural study, more neotenized female faces were the most attractive to men while less neotenized female faces were the least attractive to men, regardless of the females' actual age. Using a panel of Asian, Hispanic and White judges, Michael R. Cunningham found that the Asian, Hispanic and white female faces found most attractive were those that had "neonate large eyes, greater distance between eyes, and small noses" and his study led him to conclude that "large eyes" were the most "effective" of the "neonate cues". Cunningham also said that "shiny" hair may be indicative of "neonate vitality".
Cunningham said that there was a "difference" in the preferences of Asian and White judges. Cunningham said that Asian judges preferred women with "less mature faces" and smaller mouths than the White judges. Cunningham hypothesized that this difference in preference may stem from "ethnocentrism" since "Asian faces possess those qualities", so Cunningham re-analyzed the data with "11 Asian targets excluded" and he concluded that "ethnocentrism was not a primary determinant of Asian preferences." Using a panel of Blacks and Whites as judges, Cunningham said that more neotenous faces were perceived as having both higher "femininity" and "sociability".
In contrast, Cunningham said that faces that were "low in neoteny" were judged as "intimidating". Upon analyzing the results of his study Cunningham concluded that preference for "neonate features may display the least cross-cultural variability" in terms of "attractiveness ratings". In a study of Italian women who have won beauty competitions, the study said that the women had faces characterized by more "babyness" traits compared to the "normal" women used as a reference. In a study of sixty Caucasian female faces, the average facial composite of the fifteen faces considered most attractive differed from the facial composite of the whole by having a reduced lower facial region, a thinner jaw, and a higher forehead.
Doug Jones, a visiting scholar in anthropology at Cornell University, said that there is cross-cultural evidence for preference for facial neoteny in women, because of sexual selection for the appearance of youthful fecundity in women by men. Jones said that men are more concerned about women's sexual attractiveness than women are concerned about men's sexual attractiveness. Jones said that this greater concern over female attractiveness is unusual among animals, because it is usually the females that are more concerned with the male's sexual attractiveness in other species. Jones said that this anomalous case in humans is due to women living past their reproductive years and due to women having their reproductive capacity diminish with age, resulting in the adaption in men to be selective against physical traits of age that indicate lessening female fecundity. Jones said that the neoteny in women's faces may be a "by-product" of men's attraction to indicators of "youthful fecundity" in "adult females".
A similar study was conducted on the attractiveness of males with the subject of the skull and its application in human morphology Paul Wehr and team (2001) utilized psychology and evolutionary biology to understand selection on facial features. Averageness has been a result of stabilizing selection whereas facial paedomorphosis or juvenile traits are a result of directional selection (Wehr, 2001). It is necessary at this point to define several terms to put them into practical time and space, as they pertain to this argument. The idea of directional selection as defined by Bergstrom and Dugatkin (2012) is when a single phenotypic trait is driven by selection toward fixation in a population (Bergstrom and Dugatkin, p. 218). In contrast, stabilizing selection is defined as being a scenario wherein both alleles are driven toward fixation in a population (or polymorphic, both alleles reaching equilibrium at a fixation) (Bergstrom and Dugatkin, p. 221). To compare the effects of directional and stabilizing selection on facial paedomorphosis Wehr used graphic morphing to alter appearances to make faces appear more or less juvenile. The results concluded that the effect of averageness was preferred nearly twice over juvenile trait characteristics which indicates that stabilizing selection influences facial preference, and averageness was found more attractive than the retention of juvenile facial characteristics (Wehr, 2001). This follows the conclusions of the other study presented previously. It was perplexing to find that women tend to prefer the average facial features over the juvenile, because in animals the females tend to drive sexual selection by female choice and the Red Queen hypothesis. Some may hypothesize this is because in humans women tend to focus on social features including education, job status, family background, humor, and courtships. Experiments have found males tend to focus on the attractiveness of women, whereas women focus of overall quality and verbal interactions.
Between races and among primates
Stephen Jay Gould objected to the ranking of races as more or less neotenous, but Gould argued that if one used the terms set forth by 1920s proponents of racial neoteny, "Asians" are "clearly" the most neotenized human race.
Ashley Montagu said that the "Mongoloid skull, whether Chinese or Japanese" is the most neotenized human skull. Montagu further said that the "European" skull was less neotenized than the Mongoloid, with the "Australian Aborigine" skull less neotenized than the European and the Neanderthal skull even less neotenized than the Australian Aborigine skull. Montagu said that humans have more neotenized skulls than Australopithecus.
Delbert D. Thiessen said that Homo sapiens are more neotenized than Homo erectus, Homo erectus was more neotenized than Australopithecus, Great Apes are more neotenized than Old World monkeys and Old World monkeys are more neotenized than New World monkeys.
Nancy Lynn Barrickman said that Brian T. Shea concluded by multivariate analysis that Bonobos are more neotenized than the common chimpanzee, taking into account such features as the proportionately long torso length of the Bonobo. Ashley Montagu said that part of the differences seen in the morphology of "modernlike types of man" can be attributed to different rates of "neotenous mutations" in their early populations.
Ashley Montagu said that the San have the following neotenous traits relative to Caucasoids: large brain, light skin pigment, less hairy, round-headed, bulging forehead, small cranial sinuses, flat roof of the nose, small face, small mastoid processes, wide eye separation, median eye fold, short stature and horizontal penis.
M. R. Drennan of the Department of Anatomy, University of Cape Town, said that the Bushman's skull retains infantile morphological characteristics into adulthood that are only transiently present in the juvenile forms of other races. Drennan further said that the common description of an infant's skull from anatomy textbooks "epitomizes" the characteristics of the Bushman's skull.
Phillip V. Tobias of the Department of Anatomy, University of Witwatersrand, said that there are two phenotypical patterns in occipital curvature of "African crania": one for "Negroes" and one for Bushmen. Tobias said that the skulls of Bushmen retain strongly curved occiputs from youth into adulthood, but the curved occiputs of "Negro" skulls start to flatten when their first permanent teeth erupt. Tobias said that this flattening process in "Negroes" continues until their occiputs have flattened as adults. Tobias said that there are "infantile" features in the cranial morphology of Bushmen.
Marina L. Sardi of the anthropological division at the University of La Plata, Argentina, and Fernando V. Ramírez Rozzi said South African adults have neotenized relative facial heights and nose shapes in comparison to European adults, because "Europeans" develop relatively taller faces and relatively taller and narrower noses as they mature whereas "South Africans" do not undergo this ontogenic change as they mature. However, the relative length of both the tibia and the femur to the torso becomes greater in South Africans as they mature to a greater extent than in Europeans, so the relatively shorter legs of European adults are neotenous in comparison to the greater limb-to-torso ratio of South African adults.
Frederick S. Hulse said that aboriginal Australians have retained "similar" "skeletal characteristics" to those "which most men possessed in earlier times" (gerontomorphic characteristics) that are "contrary" to the "pedomorphic qualities" which the Bushmen have evolved.
Ashley Montagu said, "The Mongoloid skull has proceeded further than in any other people." "The Mongoloid skull, whether Chinese or Japanese, has been rather more neotenized than the Caucasoid or European." "The female skull, it will be noted, is more pedomorphic in all human populations than the male skull."
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".
Ashley Montagu said, "the skeleton of the classic Mongoloid type is very delicately made, even down to the character of the sutures of the skull which, like those of the infant skull, are relatively smooth and untortuous. In fact the Mongoloid presents so many physical traits which are associated with the late fetus or young infant that he has been called a fetalized, infantilized or pedomorphic type. Those who have carefully observed young babies may recall that the root of the nose is frequently flat or low as in Mongoloids, and that an internal epicanthic fold in such instances is usually present. The smaller number of individual head hairs and the marked hairlessness of the remainder of the body are infantile traits, as are likewise the small mastoid processes, the shallow fossa into which the jawbone fits (the mandibular fossa), the rather stocky build, the large brain-pan and brain, lack of brow ridges, and quite a number of other characters."
An interpretation of a claim by zoologist Clive Bromhall is that "Mongoloid races are explained in terms of being the most extreme pedomorphic humans."
Richard Grossinger said, "The intuition that advanced human development was pedomorphic rather than recapitulationary and accelerated was disturbing to many Eurocentric nineteenth century anthropologists." "If juvenilization was the characteristic for advanced status, then it was clear that the Mongoloid races were more deeply fetalized in most respects and thus capable of the greatest development."
Stephen Oppenheimer said, "An interesting hypothesis put forward by paleontologist Stephen Jay Gould many years ago was that the package of the Mongoloid anatomical changes could be explained by the phenomenon of neoteny, whereby an infantile or childlike body form is preserved in adult life. Neoteny in hominids is still one of the simplest explanations of how we developed a disproportionately large brain so rapidly over the past few million years. The relatively large brain and the forward rotation of the skull on the spinal column, and body hair loss, both characteristic of humans, are found in foetal chimps. Gould suggested a mild intensification of neoteny in Mongoloids, in whom it has been given the name pedomorphy. Such a mechanism is likely to involve only a few controller genes and could therefore happen over a relatively short evolutionary period. It would also explain how the counterintuitive retroussé [turned up at the end] nose and relative loss of facial hair got into the package." "[D]ecrease unnecessary muscle bulk, less tooth mass, thinner bones and smaller physical size; ...this follows the selective adaptive model of Mongoloid evolution."
Paul Storm of the Naturalis Biodiversity Center, Netherlands, said that in Australasia there are two types of cranial morphologies—the "Sunda" (Mongoloid) and "Sahul" (Australoid) types. Storm said that the "Sunda" (Mongoloid) type includes Chinese and Javanese people, and he said that the "Sahul" (Australoid) type includes Papuans and Australian aborigines. Storm said that the "Sunda" (Mongoloid) type has a flat face with high cheek bones, and Storm said that this "flat face" of the Chinese and Javanese is known as the "mongoloid face". Storm further said that the "Sunda" (Mongoloid) type has a more rounded skull, "feminine (juvenile) characters", a "retention of juvenile characters" and a limited outgrowth of superstructures such as the supraorbital region. Storm said that "Sunda" (Mongoloid) skulls resemble female skulls more than "Sahul" (Australoid) skulls resemble female skulls. Storm said that the skulls of "Asian" males ("Chinese and Javanese") have "more feminine characteristics", and he said that they have "many feminine characters in contrast with Australians".
Paul Storm said that Asia contained humans with "generalized" cranial morphology, but between 20,000 BP and 12,000 BP this generalized type disappeared as a new type emerged. This new type had a flatter face with more pronounced cheekbones, a more rounded head, reduced sexual dimorphism (male skulls started to resemble female skulls), a reduction of supestructures such as the supraorbital region and an increased "retention of juvenile characters". Storm said that this new type of skull that emerged is called the "Proto-Sunda" (Proto-Mongoloid) type, and it is distinguished from the "Sunda" (Mongoloid) type by being more "robust". Storm said that the "Mongoloid" or "Asian" type of skull developed relatively fast during a population bottleneck in Asia that happened during the Late Pleistocene or Early Holocene through a microevolutionary trend that involved a "continuation of neoteny and gracilisation trends". Due to different courses of evolution, Storm said that these two types of skulls, the "Sunda" (Mongoloid) type and the "Sahul" (Australoid) type, are now clearly recognizable at the present time.
Andrew Arthur Abbie who was an anatomist and anthropologist at the University of Adelaide talked about leg-to-torso length being related to neoteny. Abbie said that women normally have shorter legs than men, and he said that shorter legs are the normal condition in some ethnic groups such as Mongoloids. Abbie said that Mongoloids of whom he listed the people of "China, Japan and the Americas" have proportionately larger heads and shorter legs than Europeans, and he said that this is a case of "paedomorphism". Abbie said that aboriginal Australians and some African ethnic groups such as the "Negro", the "Hottentot" and the "Nubian" peoples have proportionately longer legs than Europeans, and he said that this is a case of "gerontomorphism". Abbie said that ethnic groups with proportionately shorter legs than Europeans are relatively "paedomorphic" in terms of leg-to-torso ratios when compared to Europeans, and he said that ethnic groups with proportionately longer legs than Europeans are relatively "gerontomorphic" in terms of leg-to-torso ratios when compared to Europeans.
William Ernest Castle said that the difference in limb proportions between the relatively short-limbed "Chiriguan" amerindian and the relatively long-limbed "Dinka negro" is the "same" as the difference in limb proportions between "boy and man". He said that there could be an ontogenic cause that produces "long-continued growth" in populations that characteristically have relatively longer limbs, and he said that the differences in height between the races could be due to "interruptions at different stages of the general growth process".
Leonard Halford Dudley Buxton who taught physical anthropology at Oxford University said that in the "Yellow man" the depression of the nose is below the nasion rather than at the place where the nasal bones meet the frontal bone like in the "European races". Buxton said that in the "Yellow man" the nasal bones form a wider angle rather than the narrower angle of the "European races". Buxton said that these features of the nose of the "Yellow man" make it "flatter" and "not unlike that found in European children". Buxton said that "Yellow men" have the "Mongolian fold", and Buxton said that this "fold occurs occasionally in European children, and sometimes even in adults". Buxton said that the presence of the "Mongolian fold" in "Yellow men" is possibly due to the shape of the nose of "Yellow men" that "in some cases resemble that of European children".
Michael L. McKinney, professor in the Ecology & Evolutionary Biology Department of the University of Tennessee, said that the "larger brain of the Mongoloids" is "not neotenic but hypermorphic".
Ashley Montagu said that Negroids have the following neotenous traits relative to Caucasoids: flattish nose, flat roof of the nose, small ears, narrower joints, frontal skull eminences, later closure of the premaxillary sutures, less hairy, longer eyelashes and cruciform pattern of the lower second and third molars.
He notes both the physical neoteny of people with Down syndrome: "round in shape," "bowed legs which tend to be short," "slanty eyes," a "long tongue" and "short fingers," and their mental neoteny: "unsexual," "playful," "affectionate," "mischievous" and "imitative".
Neoteny in dogs
When the role of dogs expanded from just being working dogs to also being companion dogs, humans started selective breeding dogs for morphological neoteny, and this selective breeding for "neoteny or paedomorphism" had the effect of enhancing the bond between humans and dogs. 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 are more morphologically neotenous than other breeds of dogs.Cavalier King Charles spaniels 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.
Neoteny in other species
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Neoteny is seen more in domesticated animals like dogs and mice. Neoteny has been observed in many other species. Neoteny in amphibians seems to be the most widely studied aside from humans; many examples of neoteny in amphibians stem from studies done mainly on salamanders. There is also a general prevalence of increased neoteny within domesticated animals like dogs. Neoteny has also been noted in species similar to humans, like chimpanzees. The neotenous traits in chimpanzees that resemble those within humans may give some insight into the evolutionary history of humans. What can be gathered from many studies on neoteny is that organisms with similar lineages tend to neotenize similar features, meaning they retain related features from the juvenile form into adulthood. Finally, there are two main reasons for the occurrence of neoteny: it can either be a result of the benefit of retaining juvenile characteristics due to an environment that favors those characteristics over the adult form, or the retention of juvenile characteristics leads to greater survival because those characteristics are less costly in terms of energy expenditure.
It is important to note the difference between partial and full neoteny when looking at other species in order to distinguish between juvenile traits that are only advantageous in the short term and traits that provide a benefit throughout the organism’s life; this might then provide some insight into the cause of neoteny in those species. Partial neoteny is the retention of the larval form beyond the usual age of maturation with the possibility of the development of sexual organs progenesis, but eventually the organism still matures into the adult form; this can be seen in Lithobates clamitans. Full neoteny is seen in Ambystoma tigrinum. This species is part of a larger group, perrenobranchiates, which remain in their larval form for the duration of their life. This means that they are capable of reproducing and they do not mature into adult forms. The species Rana clamata exhibits partial neoteny when it delays its maturation through the winter season because it is not advantageous for it to metamorphose into the adult form until there are more resources available since it can find those resources much more easily in the larval form. This would fall under both of the main causes of neoteny; the energy required to survive in the winter as a newly formed adult is too costly, so the organism exhibits neotenous characteristics until a time when it is capable of better survival as an adult. Ambystoma tigrinum retains its neotenous features for a similar reason, however the retention is permanent due to the lack of resources available throughout its lifetime. This is another example of an environmental cause of neoteny in that the species retains juvenile characteristics because the environment limits the ability of the organism to fully come into its adult form. A few species of birds show partial neoteny. A couple examples of such species are Chiroxiphia linearis and Chiroxiphia caudata. The males of both species retain their juvenile plumage into adulthood, but they eventually lose it once they are fully mature. In certain species of birds the retention of juvenile plumage is often linked to the molting times within each species. In order to ensure there is no overlap between the molting and mating times, the birds may show partial neoteny in regards to their plumage so that the males do not attain their bright adult plumage before the females are prepared to mate. In this instance, neoteny is present because there is no need for the males to molt early and it would be a waste of energy for them to try to mate when the females are not yet prepared.
Neoteny is seen more in domesticated animals like 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 reproductive cycle, curly tails, piebald coloration, fewer or shortened vertebra, large eyes, rounded forehead, large ears, and shortened muzzle.
Neoteny is commonly seen in flightless insects like the females in the order Strepsiptera. The flightless trait in insects has evolved many separate times; environments that may have contributed to the separate evolution of this trait are: high altitudes, isolation on islands, and insects that reside in colder climates. These environmental factors may be responsible for the flightless trait, because in these situations it would be disadvantageous to have a population that is more dispersed, so flightlessness would be favored due to the boundaries it poses to dispersal. Also, in cooler temperatures heat is lost more rapidly through wings, thus the circumstance favors flightlessness. Another couple of main points to note about insects are that the females in certain groups become sexually mature without metamorphosing into adulthood, and some insects which grow up in certain conditions do not ever develop wings. Flightlessness in some female insects has been linked to higher fecundity, this would increase the fitness of the individual because the female is producing more offspring and therefore passing on more of her genes. In those instances, neoteny occurs because it is more advantageous for the females to remain flightless in order to conserve energy which thereby increases their fecundity. Aphids are a great example of insects that may never develop wings due to their environmental setting. If resources are abundant there is no need to grow wings and disperse. When the nutrition of a host plant is abundant aphids may not grow wings and stay there for the duration of their life, however if the resources become diminished the offspring may develop wings in order to disperse to other host plants.
Two common environments that tend to favor neoteny are high-altitude and cool environments because neotenous individuals have a higher fitness than those that metamorphose into the adult form. This is because the energy required for metamorphosis is too costly for the individual’s fitness, also the conditions favor neoteny due to the ability of neotenous individuals to utilize the available resources more easily. This trend can be seen in the comparison of salamander species of lower and higher altitudes. The neotenous individuals have higher survivorship as well as higher fecundity than the salamanders that had gone to the adult form in the higher altitude and cooler environment. Insects in cooler environments tend to show neoteny in flight because wings have a high surface area and lose heat quickly, thus it is not advantageous for insects in that environment to metamorphose into adults.
Many species of salamander, and amphibians in general, are known to have neotenized characteristics because of the environment they live in. The axolotl is a species of salamander that retains its juvenile aquatic form throughout adulthood, which is an excellent example of full neoteny. Gills are a common juvenile characteristic in amphibians that are kept after maturation; an example of this would be a comparison of the tiger salamander and the rough-skinned newt, both of which retain gills into adulthood. These species are better able to survive when they retain certain juvenile features, like gills, that allow them access to both aquatic and land environments.
Pygmy chimpanzees (Bonobos) share many physical characteristics with humans. A prime example are their neotenous skulls. The shape of their skull does not change into adulthood; it only increases in size. This is due to sexual dimorphism and an evolutionary change in timing of development. Juveniles became sexually mature before their bodies had fully developed into adulthood, and due to some selective advantage the neotenic structure of the skull remained in later generations.
More examples of neoteny can be seen in these other species:
1. Species in which energy costs result in neoteny-
- The families: Gerridae, Delphacidae, and Carabidae. Many of the insect species within these families have smaller, neotenous wings or no wings at all.
- Some cricket species which shed their wings in adulthood.
- Beetles of the genus Ozopemon; the males are thought to be the first known example of neoteny in the order Coleoptera. They are significantly smaller than the females and it is thought that the neoteny in males is linked to the inbreeding within species.
- Kalotermes flavicollis: a termite that shows neoteny in females during molting.
- The rook, Corvus frugilegus: the evolution of birds from dinosaurs may be linked to dimorphism through neoteny.
2. Species in which the environmental conditions cause neoteny-
- The Northwestern Salamander: higher altitude causes greater neotenic tendencies (this can be to help conserve energy as mentioned above).
- A few species of the family Ischnomesidae, crustaceans. These species live in deep ocean waters; this fact may be linked to their neotenous characteristics for the same reasons as in insects at high altitude and in cold environments; which may lead us to conclude that neoteny arises for similar reasons between completely different species.
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