These two snowshoe hares were on my way into work this morning and it was pretty hard not to stop and take a photo of the difference in their coat colours. Admittedly, neither is especially camouflaged on the manicured greenway they were hanging out on, although the white one could be mistaken for a plastic bag from a distance (*sigh). But in more forested areas this colour changing is a pretty wicked adaptation. So what triggers them to change colour and what does that mean in the face of climate change?
First of all, what was fascinating when I started looking up snowshoe hares is how far back the research dates. In 1932 Grange wrote “the color changes of the snowshoe hare have been a subject of scientific discussion for almost 150 years” (p. 99). He then goes on to describe some of this historical research which seemed to flip back and forth regarding whether the change in coat colour was due to new fur growth or changes in the existing fur. These older papers are fascinating to read as the language is so different from the sterile reports of today. For example, Grange states “the protective coloration of the snowshoe hare can scarcely fail to elicit the comment and speculation of persons who live intimately with these animals in their native haunts…Throughout the snowy and blustery winter period the snowshoe hare hops stealthily over the crust and loose snow, almost like some phantom creature, rendered relatively inconspicuous by its white coat” (p. 101). Take that prof who told me that I had to be less poetic in my scientific writing!
While Grange was not able to determine what triggered the hare’s coat change, he did make some valuable observations. 1) The coat colour does not necessarily match the specific conditions the hare is experiencing, ie they might change to white before there is sufficient snow cover, or back to brown before the snow is gone. 2) They do seem to change colour at times that match the average climatic conditions, that is they match what would be expected based on the ~30 year average. And 3) Hares living at northern latitudes have a white coat for longer than those living in more southern locations. (The Grange article is well worth the read just for the fluidity of the language.)
If we fast forward to the present day, we will have learned a few things. We have significant evidence to indicate that it is the length of the day (photoperiod) that affects when hares will start to change their coat colour. We can make this conclusion because over the course of three years with very different snow conditions Mills, et al (2013) found that the date of both the fall and spring colour molts was basically fixed, as was the rate of colour change in the fall. The rate of colour change in the spring however, did appear to be affected by surrounding snow levels with longer snow seasons resulting in slower molts. However, this one change was not enough to deal with changing conditions to climate change, leaving Mills et al to conclude that “the compelling image of a white animal on a brown snowless background can be a poster child” for the education and research needed to investigate the fitness consequences of climate change.
In one last study, Jones et al (2018) found that in some areas where there is less snow hares will molt in spring and fall but both coats will be brown. Interestingly, it appears that this lack of colour change is a result of interbreeding with jackrabbits rather than the adaptation of the hares on their own. This has interesting implications for species and sexual selection as interspecific (between species) gene flow becomes one possible means of dealing with the rate of change from human-induced climate change.
So what does this mean for the two hares I saw this morning? Well, it likely indicates genetic variation between the two individuals and with a bit more time the white hare will likely be just as brown as the brown one. And, if the temperatures continue to drop and we get our seemingly annual May snowstorm, the white one might enjoy a few more days of camo this year.
Grange, W. (1932). The Pelages and Color Changes of the Snowshoe Hare, Lepus americanus phaeonotus, Allen. Journal of Mammalogy, 13(2), 99-116. doi:10.2307/1374046
Jones, M., Mills, L. S., Alves, P., Callahan, C., Alves, J. M., Lafferty, D. (2018). Adaptive introgression underlies polymorphic seasonal camouflage in snowshoe hares. Science, 360(6395), 1355-1358. DOI: 10.1126/science.aar5273
Lincoln, GA, Clarke, IJ, Hut, RA, & Hazlerigg, DG. (2006). Characterizing a mammalian circannual pacemaker. Science 314(5807):1941–1944.
Mills, L. S., Zimova, M., Oyler, J., Running, S., Abatzoglou, J. T., & Lukacs, P. M. (2013). Camouflage mismatch in seasonal coat color due to decreased snow duration. PNAS, 110(18), 7360-7365. https://doi.org/10.1073/pnas.1222724110