Biology
Genetic Basis of Coat Color Polymorphism in Snowshoe Hares
Quick fact
While most snowshoe hares molt to white in winter, about 1-5% of individuals in some populations remain brown year-round, and this variation is linked to a single gene variant that reduces the production of pigment-regulating signaling protein.
Why this is interesting
Snowshoe hares are famous for turning white in winter to blend in with snow, but in some places, they don't. Why do some hares stay brown all year?
Read the full explanation
Understanding Genetic Basis of Coat Color Polymorphism in Snowshoe Hares
Snowshoe hares (Lepus americanus) live in northern North America, where winters bring snow. To avoid predators, they change their fur color from brown in summer to white in winter. This change is triggered by day length. However, not all hares do this. In some populations, a few individuals keep their brown fur all winter. That’s because they have a different version of a gene that controls pigment. The gene is called Agouti signaling protein (ASIP). Normally, in winter, the gene is turned off in hair follicles, allowing the production of white fur. But in hares with a certain genetic variant, the gene stays on, so they keep producing brown pigment. This genetic variation is what we call polymorphism—multiple forms of a trait in the same population.
A deeper explanation
The key gene, ASIP, codes for a protein that inhibits melanocortin 1 receptor (MC1R) on melanocytes. When ASIP binds to MC1R, it blocks the production of eumelanin (dark pigment), leading to lighter fur. In snowshoe hares, a specific variant of ASIP (called ASIPW) is associated with winter whitening. This variant is dominant over the wild-type allele that allows brown winter fur. The mechanism: during winter, the expression of the whitening allele is upregulated in response to shorter day length, causing the hair follicles to produce unpigmented hair. Hares without this allele continue to produce brown fur. The polymorphism is maintained because in areas with less snow, brown fur may actually provide better camouflage in the absence of snow cover. As climate change reduces snow duration, the frequency of the brown-winter allele may increase, illustrating natural selection on a single gene that affects fitness. This serves as a model for understanding how genetic variation and environmental selective pressures interact.