Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological or somatic development of an organism, typically an animal. Neoteny in modern humans is more significant than in other primates. In progenesis or paedogenesis, sexual development is accelerated.Volkenstein, M. V. 1994. Physical Approaches to Biological Evolution. Springer-Verlag: Berlin, .
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. It is the retention in adults of traits previously seen only in the young. Such retention is important in evolutionary biology, domestication, and evolutionary developmental biology. Some authors define paedomorphism as the retention of larval traits, as seen in salamanders.Schell, S. C. Handbook of Trematodes of North America North of Mexico, 1985, pg. 22Ginetsinskaya, T.A. Trematodes, Their Life Cycles, Biology and Evolution. Leningrad, USSR: Nauka 1968. Translated in 1988, .
History and etymology
Julius Kollmann created the term "neoteny" in 1885 after he described the axolotl's maturation while remaining in a tadpole-like aquatic stage complete with gills, unlike other adult amphibians like frogs and toads. From pp. 397–398: "Dann drängt sich die Frage auf, ob das Latenzstadium der Eier, das einerseits bei Fischen, Vögeln and Säugethieren in so höchst überraschenden Formen vorkommt, anderseits das Latenzstadium bei den Wirbellosen ¹) nicht eine Variante derselben Eigenschaft der Organismen sei, welche ich Neotenie genannt habe, und die auf irgend einer Entwichlungsstufe in Kraft treten kann." (Then the question arises whether on the one hand the latency stage of eggs — which occurs in such highly surprising forms in fish, birds and mammals — [and] on the other hand the latency stage in invertebrates ¹) be not a variant of the same property of the organisms, which I have called "neoteny" and which can come into force at any stage of development.)
The word neoteny is borrowed from the German Neotenie, the latter constructed by Kollmann from the Greek νέος (neos, "young") and τείνειν (teínein, "to stretch, to extend"). The adjective is either "neotenic" or "neotenous".Neoteny, The Free Dictionary. 2011. Accessed April 30, 2011. 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 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 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, 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. Some evolutionary theorists have proposed that neoteny was a key feature in human evolution. J. B. S. Haldane 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 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 rabbits, dogs, pigs, ferrets, cats, and even foxes) include floppy ears, changes in the reproductive cycle, curly tails, piebald 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 include being companions, humans started selectively breeding dogs for morphological neoteny, which "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 are more morphologically neotenous than other breeds of dogs.Beck, A.M. & Katcher, A.H. (1996). Between Pets and People: The Importance of Companionship. West Lafayette, Indiana: Purdue University Press. 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.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.Drake, Abby Grace, "Evolution and development of the skull morphology of canids: An investigation of morphological integration and heterochrony" (January 1, 2004). Doctoral Dissertations Available from Proquest. Paper AAI3136721. link By 2011, the findings by the same researcher were simply "Dogs are not paedomorphic wolves."
In other animals
Neoteny has also been observed in many other species in the wild. 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 (the axolotl) and some populations of Tiger salamander (Ambystoma tigrinum), which remain in larval form throughout their lives.
Neoteny is an ancient, pervasive phenomenon: in urodeles, many extant taxa are neotenic, and both morphological and histological data suggest that the Middle Jurassic taxon Marmorerpeton was neotenic.
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; in those habitats, it is disadvantageous for insects to metamorphose into adults.
Amphibians
Many species of salamander, and amphibians in general, are environmentally neotenous. Axolotl and olm retain their gills and juvenile aquatic form throughout adulthood, in full neoteny. Gills are a common juvenile characteristic in amphibians which are kept after maturation; examples are the tiger salamander and rough-skinned newt, both of which retain gills into adulthood.
Lithobates clamitans is partially neotenous, delaying maturation during the winter as fewer resources are available, and 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.
Many species of mole salamander are neotenous. In the northwestern salamander (Ambystoma gracile), high altitude is the environmental cause of neoteny.
Neoteny occurs in all salamander families. It seems to be a survival mechanism in environments with little iodine. In these circumstances, salamanders can reproduce and survive in the form of a smaller larval stage, which is aquatic and requires less food compared to terrestrial adults. If salamander larvae ingest a sufficient amount of iodine, they quickly begin metamorphosis and transform into bigger terrestrial adults, with higher dietary requirements, but an ability to disperse across dry land.
Arthropods