Why do horses have chestnuts on their legs?

Answered by Tom Adger

Horses have chestnuts on their legs, also known as “night eyes,” which are hard, calloused bumps located on the inside of their legs, just above the knees on the front legs and just below the hocks on the hind legs. These chestnuts are actually remnants of the evolutionary leg and foot structure of horses’ ancestors, known as Eohippus.

Chestnuts are thought to be vestigial structures, meaning they are remnants of organs or structures that had a function in a previous evolutionary ancestor, but have lost their original purpose over time. In the case of chestnuts, they are believed to be remnants of the pads that Eohippus and other early equids used for support and traction on different terrains.

During the evolutionary process, horses’ legs and feet have undergone significant changes to adapt to different environments and modes of locomotion. The chestnuts, which were once larger and more functional, have gradually reduced in size and lost their original function. However, they have persisted in horses’ anatomy as small, hardened bumps.

The exact purpose of chestnuts in modern horses is still not fully understood. Some theories suggest that they may have a protective function, helping to reduce friction and prevent injury when horses move through dense vegetation. They may also have a sensory role, providing horses with additional tactile information about their surroundings. Additionally, chestnuts have been observed to grow larger in response to changes in hormonal levels, such as during pregnancy or in stallions entering breeding season, suggesting a potential hormonal influence on their development.

While chestnuts are commonly found on most horses, there are some individuals that may have smaller or even absent chestnuts. This variation can occur due to genetic factors or individual variation within a population.

In my personal experience with horses, I have noticed that chestnuts can vary in size, texture, and appearance between different horses. Some horses have very prominent and well-defined chestnuts, while others may have smaller, smoother ones. I have also observed that chestnuts can accumulate dirt and debris, so regular cleaning and grooming are necessary to maintain their hygiene.

It is important to note that chestnuts are completely normal and should not be confused with any injury or abnormality. They are a natural part of a horse’s anatomy and do not require any specific treatment or removal unless they become irritated or inflamed.

Chestnuts on horses’ legs are vestigial remnants of the pads that their evolutionary ancestors used for support and traction. While their exact function in modern horses is not fully understood, they are believed to have a protective or sensory role. Chestnuts vary in size and appearance among individuals, but their presence is completely normal and does not require any specific intervention.