 Not all species of salamanders follow this path . Neoteny , also known as paedomorphosis , has been observed in all salamander families , and may be universally possible in all salamander species . In this state , an individual may retain gills or other juvenile features while attaining reproductive maturity . The changes that take place at metamorphosis are under the control of thyroid hormones and in obligate neotenes such as the axolotl ( Ambystoma mexicanum ) , the tissues are seemingly unresponsive to the hormones . In other species , the changes may not be triggered because of underactivity of the hypothalamus @-@ pituitary @-@ thyroid mechanism which may occur when conditions in the terrestrial environment are too inhospitable . This may be due to cold or wildly fluctuating temperatures , aridity , lack of food , lack of cover , or insufficient iodine for the formation of thyroid hormones . Genetics may also play a part . The larvae of tiger salamanders ( Ambystoma tigrinum ) , for example , develop limbs soon after hatching and in seasonal pools promptly undergo metamorphosis . Other larvae , especially in permanent pools and warmer climates , may not undergo metamorphosis until fully adult in size . Other populations in colder climates may not metamorphose at all , and become sexually mature while in their larval forms . Neoteny allows the species to survive even when the terrestrial environment is too harsh for the adults to thrive on land . 
