Neoteny is when an animal reaches adulthood but continues to retain features from its younger stage. In axolotls, for example, they can grow up and reproduce while still having gills, just like when they were larvae. The word itself comes from Latin and Greek roots meaning “extended youth,” which describes the way juvenile traits are carried forward instead of disappearing. In simple terms, neoteny is the process of becoming an adult while still looking and functioning like the young form.
Forms of Neoteny
Neoteny can take two main forms, distinguished by whether an organism is able to reproduce while still retaining juvenile traits.
I. Pedomorphosis
Pedomorphosis is the process in which an animal carries its youthful features into adulthood. This is explained by heterochrony, a change in the timing of development. Pedomorphosis is especially common in animals domesticated by humans, such as dogs, chickens, pigs, and cattle, where selective breeding for juvenile qualities like tameness and cuteness has encouraged these traits.
II. Paedogenesis
Paedogenesis refers to reproduction by an organism that has not yet reached full physical maturity. In this form, the juvenile is capable of producing offspring while still in its larval stage, and the adult phase of the life cycle is skipped entirely. This is often connected with progenesis, where sexual maturity appears early, before the body has completed its physical growth. Paedogenesis is the form seen in axolotls, and it is also found in certain insects, including beetles, bagworms, scale insects, gall midges, and members of the group Strepsiptera.
Further Classification of Neoteny
Neoteny can further be divided into three types: partial, full, and quasi. These categories describe how long larval features are retained and whether metamorphosis into the adult stage is still possible.
A. Partial Neoteny
Partial Neoteny occurs when metamorphosis is delayed beyond the usual time. The organism remains in its larval stage for longer than normal, often due to environmental changes such as cold winters or lack of resources. The larvae do not usually reach sexual maturity while delayed, but they eventually transform into adults once conditions improve.
A good example is the frog Lithobates clamitans, whose tadpoles can remain in their larval form through the winter and only metamorphose when survival as an adult becomes easier. Other examples include tadpoles of Rana esculenta, Rana temporaria, and Bufo vulgaris.
B. Full Neoteny
Full Neoteny describes species that remain in the larval form for their entire lives. These organisms keep traits such as gills, tail fins, and weak limbs but still reach sexual maturity, allowing them to reproduce without undergoing metamorphosis.
The mudpuppy (Necturus maculosus) and the olm are classic examples. Axolotls also exhibit this form, as they retain their larval aquatic body while becoming sexually mature adults.
C. Quasi Neoteny
Quasi Neoteny is a more flexible form. In this case, the larva normally remains in its juvenile form and can reproduce without metamorphosing, but it is still capable of changing into the adult stage if conditions or treatments allow.
Axolotls are the best-known example. In the wild, they often remain in their larval state for life, but under special conditions, such as exposure to certain hormones or environmental changes, they can undergo metamorphosis and develop into adult salamanders.
Juvenile Characteristics Axolotls Retain into Adulthood
One of the most striking juvenile traits that axolotls keep throughout their lives is their external gills. These feathery structures, common to amphibian larvae, remain in adult axolotls and allow them to breathe underwater. Other amphibians, such as the tiger salamander and the rough-skinned newt, also retain gills into adulthood, but axolotls are the best-known example.
In addition to gills, axolotls preserve several other features that are normally lost during metamorphosis.
- They keep a long dorsal fin running along the back and tail, similar to that of a tadpole.
- Their lateral line system, used to sense vibrations in the water, also persists into adulthood, even though it usually disappears in other adult amphibians.
- They never develop eyelids, giving them their wide-eyed appearance
- Parts of their skeleton remain cartilaginous rather than becoming fully hardened bone.
- Their teeth stay small and larval in form, suited to suction feeding rather than chewing.
- For breathing, they continue to rely not only on gills but also on skin and buccal pumping, a juvenile-style method of drawing air.
Together, these traits explain why axolotls are often described as “forever babies.” They are salamanders that never complete metamorphosis, reaching sexual maturity while keeping the look and lifestyle of their larval stage.
Cause of Neoteny in Axolotls
Neoteny in axolotls is primarily the result of a missing thyroid-stimulating hormone. Without this hormone, the thyroid gland does not produce enough thyroxine, which is essential for triggering metamorphosis in other salamanders. As a result, axolotls remain in their aquatic, larval-like form throughout life.
Under human care, this process can be artificially completed. When axolotls are given thyroid hormones, or substances like iodine that stimulate hormone production, they undergo metamorphosis and transform into fully terrestrial salamanders. In fact, the study of axolotl neoteny was key to the scientific discovery of thyroid hormones.
Research also points to the hypothalamus as the deeper cause. In axolotls, the hypothalamus does not produce the hormone needed to tell the pituitary gland to stimulate the thyroid. Without this signal chain, metamorphosis fails to occur. Some scientists suggest that this neoteny may be an evolutionary adaptation, allowing axolotls to avoid the dangers of life on land by remaining permanently aquatic.
Why Neoteny is Important in Axolotls
Neoteny gives axolotls several survival advantages:
- Helps them live fully in water – By keeping their gills and tail fin, axolotls can remain aquatic for life, thriving in lakes and ponds without needing to move onto land.
- Adapts to safe environments – When water is rich in food and free of threats, there’s no pressure to transform into a land-dwelling salamander. Staying aquatic is simpler and safer.
- Conserves energy – By skipping metamorphosis, axolotls avoid the costly process of developing lungs, eyelids, and stronger limbs.
- Preserves useful features – Traits like gills, fins, and their lateral line system remain perfectly suited for breathing, swimming, and hunting underwater.
- Demonstrates evolutionary flexibility – While many salamanders transition to life on land, axolotls have taken a different path, thriving by staying “forever young.”
Put simply, neoteny allows axolotls to remain aquatic, conserve energy, and keep the features that make them perfectly adapted to their environment.