Color-Changing Rabbits and Photoperiodism

Looking outside my window, across the snow-cover vista otherwise known as Ann Arbor, it amazes me that anything can live (or, even thrive) in such a frozen environment. Since I was a kid I was fascinated with animals, such as the snowshoe hare, that can change the color of their fur from brown in the summer to white in the winter. The process occurs over about 10 weeks, with the white fur appearing first on the ears and feet and gradually moves towards the body. During the spring, brown fur replaces the white fur in the reverse process.

Certainly this increased camouflage provides the hare with an improved ability to hide from predators, survive the winter and reproduce in warmer times. But how does this drastic change occur? What environmental cues trigger the biological changes in the hare?

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My guesses as to what could cause the change were:
- a change in diet due to available plant food altering with the seasons
- the lengthening and shortening of the light cycle might trigger the change via circadian rhythms
- a direct response to the change in temperature (body or ambient?)

After a bit of research, I found that my second guess--that the change is related to the length of the day's light cycle--was the correct one. Since snowshoe hares do not hibernate and are active in the nighttime, they are positioned to detect these changes in the light cycle over the seasons (ie, the photoperiod). But, I wanted a bit more of a biological explanation.

I guessed that these rabbits' melanin production in their fur hair cells was related to the melanopsin, a photo-sensitive pigment in the retina that helps regulate circadian rhythms in the brain. When the days are long and the nights are short, the hypothalamus and suprachiasmatic nucleus receives stimulation from melanopsin-containing retinal ganglion cells, which in turn stimulates melanin production in the fur. When the days become shorter, the brain receives less stimulation from these cells, and melanin production slows, resulting in all new growth of fur lacking pigment (white).

As I mentioned, the hares are nocturnal, and this graph (below) represents the monthly hours of activity in darkness across the year. Hares have much more dark-time activity in the winter months, on average, and the summertime months have much less. Makes sense: the hares spend more time in the dark when the days are shorter (in winter). This chart is a rough representation of its cycle from brown to white and back to brown.

The dependence of the fur color on the photoperiod has been duplicated indoors in experiments by shortening the light that the rabbits receive, and monitoring the change in coat color. Shortening the photoperiod induces hormone production and the growth of the winter coat without pigment (white), and lengthening it artificially simulates the springtime phase of the cycle, with pigmented fur (brown). Furthermore, snowshoe hares that exist in permanently cold and snowy environments remain white, and hares that reside in Oregon and Washington (where the photoperiod is longer) stay permanently brown.