Neoteny, also called juvenilization, is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compared to other primates. In progenesis or paedogenesis, sexual development is accelerated.
Neoteny and reduction in skull size – grey wolf and chihuahua skulls
The axolotl is a neotenous salamander, often retaining gills throughout its life.
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 size, shape, characteristics and even presence of certain organs and features. It is contrasted with heterotopy, a change in spatial positioning of some process in the embryo, which can also create morphological innovation. Heterochrony can be divided into intraspecific heterochrony, variation within a species, and interspecific heterochrony, phylogenetic variation, i.e. variation of a descendant species with respect to an ancestral species.
Giraffes acquired their long necks through heterochrony, extending the development period of the seven neck vertebrae's growth in the embryo to add length to the bones, not by adding more bones.
Axolotls retain gills and fins as adults; these are juvenile features in most amphibians.
Irish elk skeleton with antlers spanning 2.7 metres (8.9 ft) and a mass of 40 kg (88 lb)
