Why do animals change their coats in the fall?

Many animals, especially those living in temperate or cold regions, change their coats as winter approaches.

This phenomenon, known as "seasonal molting," is a natural adaptation essential to their survival in the face of winter conditions. But why and how does this change in coat occur?

A response to temperature changes

The main reason animals change their coats in the fall is to prepare for colder winter temperatures. During this season, the ability to retain body heat becomes crucial for their survival, and a thicker coat, often called the "winter coat," helps them stay warm.

An animal's summer coat is generally lighter and thinner, allowing their body to regulate its temperature when it's hot outside. As fall approaches, the days get shorter and temperatures begin to drop, triggering biological signals in many animals.

These signals are largely influenced by photoperiod, which refers to the length of daylight. When daylight decreases, light receptors in the animals' eyes send a signal to their brain to activate the production of new layers of hair or feathers.

The winter coat: a natural insulator

The new coat that grows in the fall is much denser than the summer coat.

In many mammals, such as foxes, rabbits, or deer, this winter coat consists of two layers: an outer layer of long, stiff hairs, which protects against the elements like snow and rain, and a soft, dense undercoat, which traps air and acts as an insulator to keep warmth close to the body.

This system is particularly efficient in helping animals stay warm without expending too much energy to maintain their body temperature.

Coat color

In addition to its thickness, some animals also change the color of their coat in the fall.


Species such as the mountain hare (Lepus timidus) or the stoat (Mustela erminea) switch from a brown or gray coat in the summer to a white coat in the winter. This color change helps camouflage the animal in the snow, protecting it from predators.

The mechanism behind this color change is also triggered by the reduced daylight, which influences the production of melanin, the pigment responsible for hair color.

A vital adaptation for survival

Fall molting is a critical evolutionary adaptation.

Without a thicker coat, animals would lose too much body heat and risk hypothermia. The winter coat is thus a natural strategy to reduce heat loss and increase their chances of survival during the colder months.

In conclusion, animals change their coats in the fall in response to lower temperatures and decreased daylight hours. This new thicker and sometimes lighter coat helps protect them from the cold, conserve energy, and, for some, blend into their winter environment.