Neoteny in Amphibians

In some amphibians, the larva becomes sexually mature without metamorphoses or delayed metamorphoses. In some amphibians, the character of neoteny is shown as the failure or delay of the larva to metamorphose while becoming sexually mature. Neoteny emphasizes the retention of embryonic or larval features in the adult body.

Examples of Neoteny:
Axolotl larva of Ambystoma (tiger salamander):

The tiger salamander goes through a gilled aquatic larval stage, then metamorphoses and transforms into adult air-breathing landforms. However, under certain circumstances, the larva does not go through metamorphoses, retain the gills and aquatic habitat but become sexually mature.

  • It is the best example, Axolotl larva is aquatic, consisting of gills.
  • The larva develops gonads, lays eggs, and attains a large size. On the other hand, the Ambystoma is terrestrial and does not possess gills.
  • In the beginning, the axolotl larva was considered separate genera.
  • Axolotl larva lost its gills and develops lungs while thyroxine is administered. It metamorphosed into an adult.
  • The metamorphosis of axolotl larva can be induced by reducing the water level.
Axolotl larva - Neoteny in Amphibia

                                                      Axolotl larva

Proteus and Necturus: They are permanently found in neotenous forms. These retained the larval features. Proteus and Necturus reproduce sexually like mature animals.

The Factors affecting neoteny

The exact cause and significance of neoteny are not exactly understood. The metamorphosis is affected by the environment in several ways:

  • The abundance of food, temperature, or insufficient iodine may cause the failure of metamorphoses and retention of larval features.
  • Drying up of swamps,
  • Lack of food, and rise in temperature in surrounding water induce axolotls to metamorphose.
  • Deepwater and coldness inhibit the secretion of thyroxin (Shufeldt)
  • Saline water is responsible for neoteny (Weismann)
  • Low temperature is responsible for the arrest of metamorphosis (Huxley, 1929).
Types of Neoteny:
  1. Partial Neoteny
  2. Intermediate Neoteny
  3. Total Neoteny

Partial Neoteny: In this type of neoteny the metamorphosis is delayed due to temporary ecological changes in the environment. Partial neoteny is shown by tadpole and larva tiding over winter.

Intermediate Neoteny: This type of neoteny is shown in axolotls which reproduce sexually but undergo metamorphosis under suitable conditions.

Total Neoteny (Permanent neoteny): It is shown in several salamanders like Necturus, Siren, Proteus, etc. They retain larval features. They do not metamorphose even after thyroxin treatment.

Types of Neoteny described by Kollman:

According to Kollman, there are two types of neoteny, these are

  • Partial Neoteny- During the winter season, the simple retardation of metamorphosis beyond the normal period is shown in the tadpole of Hyla arborea, Rana esculenta.
  • Total Neoteny- In this form, the gills are retained and become sexually mature. Example- Ambystoma.
Significance of Neoteny:

The exact significance of neoteny is not clear, but a few scientists through some experiments suggested its significance. According to Weismann (1875), the neoteny was the case of retarded evolution or atavism, i.e. reversion to ancestral condition. Now it is regarded as a secondary specialization, as a physiological adaptation of advantage.


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