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 "Lithobates clamitans". Full neoteny is seen in "Ambystoma mexicanum" and some populations of "Ambystoma tigrinum", which remain in larval form throughout their lives. "Lithobates clamitans" is partially neotenous; it delays maturation during winter season because fewer resources are available, and it can find existing resources more easily in 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 manakins "Chiroxiphia linearis" and "Chiroxiphia caudata", exhibit partial neoteny. The males of both species retain juvenal 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.