White Rabbits Imperiled by Climate Change! — Well, maybe not.

Guest Essay by Kip Hansen

L. Scott Mills, of the Wildlife Biology Program, Department of Ecosystem and Conservation Sciences, University of Montana, has been working on the seasonal dimorphism of the snowshoe hare (seasonal coat color polyphenism) for many years now. In 2013 he produced a much publicized study that purported to show that snowshoe hares (Lepus americanus) were imperiled by climate change — being unable to keep their coats properly lined up with the seasons so as to be white when the ground was snow covered and brown when there was no snow.  [ Original study link, in pdf here ]

Mills et al. captured and radio-collared about 150 hares over a three year period and then tracked them down at spring and fall molt times, photographed them against the ambient background and evaluated if there was a color mismatch (white on brown or brown on white).  Then Mills and his team, using CMIP5 climate models to project probable snow-cover season length (and dates) for mid-century and end-of-century, found that “the reduced snow duration will increase the number of days that white hares will be mismatched on a snowless background by four- to eightfold by the end of the century.”  Since predation is the major factor determining snowshoe hare population, increased exposure through visibility would mean increased predation and plummeting populations.

“We took advantage of a serendipitous triplet of consecutive winters (2010–2012) at our US Northern.  Rockies study site in western Montana that spanned among the shortest and longest snow years in the recent past. We monitored 148 different snowshoe hares over the study period (43 different hares in 2010, 63 in 2011, and 58 in 2012), using radiotelemetry to locate hares weekly to quantify coat color phenology and the snow around each hare..”.

“The hares couldn’t adjust the date that they started to change color very much, Mills and his colleagues report online today in the Proceedings of the National Academy of Sciences. Each year, the animals started to molt around the same time—10 October in fall and 10 April in spring. “That happened regardless of whether there was a ton of snow on the ground or not,” Mills says. In the fall, switching to white took 40 days. In the spring, the changeover to brown took between 30 and 50 days and lasted longer in the colder years, Mills says. “They do have some ability to speed up or put on the brakes [on color change].”” 

The onset of the fall molt — brown to white — is driven by photoperiodism (day length) and is apparently independent of temperature or presence or absence of snow.  The duration of the fall molt — start to finish — was the same in the three studied years, even though the three winters were vastly different.  Spring molt — white to brown — also started around the same date each year, but duration varied taking longer in the colder of the three years.  (NB:  Three years is a very small sample, so we should not make too much out of small differences between years.)

Mills’ assertion that “They do have some ability to speed up or put on the brakes [on color change]” is merely anecdotal given the small sample size and may be coincidental — or the spring molt duration could depend not on snow cover or temperature at all, but on availability of food by quantity or type, for instance, which would also change in step with length of winter.  I can think of many factors that would affect the Spring molt in particular — body condition of each individual after a long winter might slow the molt, health of each animal and amount of “spare” energy available to it for the molt.

An interesting note,  that will be important later on in this essay, is that none of the hares radio-collared in the first year survived to the third year:

Wild snowshoe hares generally have low annual survival rates, limiting the expression of individual plasticity across >1 y. Of our 148 different animals monitored, only 7 survived for >1 fall or spring molt and only one survived for >1.5 y after collaring; because of incomplete detection and temporary emigration of radiocollared animals, we were not able to document consecutive spring or fall molts for any of these 7 hares.”

In effect, nearly all of their test subjects died after being observed for 1 Fall or 1 Spring molt.  They were not testing/observing the same hares multiple times, but different hares at different times.   This also implies a fantastic reproductive recruitment rate necessary to maintain the population size with individual lifespans similar to that of many insects or rodents — 1 to 2 years (or two winters – the lifespan of a hamster).

Almost nothing need be said here of the pretense that winter length and extent of snow cover can be predicted 50 to 100 years into the future on a regional level by existing CMIP5 climate runs.    However,  the paper claims just that for this small area.  The time period of the study itself illustrates the silliness of this — they did the study over three back-to-back years which had a snow-cover seasons that varied from 160 to 190 days.  Their projected mid-century seasons, for both RCP4.5 and 8.5 have overlapping confidence intervals with the entire span of the three years studied:

As we see in their Figure 5 above, the posited “color mismatch” grows as they allow the models to predict shorter winters with less snow cover. [Green lines added to illustrate that all of the future mismatch derives from shortening the projected winter snow cover.]

The concluding paragraph of the paper is an interesting study itself:

“As a threshold trait with distinct initiation and rate components that determine crypsis [the ability of an animal to avoid observation or detection by other animals], coat color mismatch is a more direct climate change-induced phenological stressor than the trophic-level asynchronies usually discussed. The compelling image of a white animal on a brown snowless background can be a poster child for both educational outreach and for profound scientific inquiry into fitness consequences, mechanisms of seasonal coat color change, and the potential for rapid local adaptation.”

The authors actually recommend using the following image as a “poster child” for climate change propaganda (which they call “educational outreach”).

That was then — 2013.  And now, in June 2018, to be fair, L. Scott Mills, lead author of the 2013 paper just discussed, appears as a co-author of a new paper.  [The Author Contributions section of the new paper shows that Mills (along with five others) “designed the study” and supplied input to those actually writing the paper, as well as approving the final manuscript as part of the team.]  The new paper  is a study of the genetics of this color-changing ability of the snowshoe hare using “whole-genome sequences for a winter-white hare” is titled “Adaptive introgression underlies
polymorphic seasonal camouflage in snowshoe hares”.  This new paper is a follow-up called for by  Mills’ (2013); a “profound scientific inquiry into …. mechanisms of seasonal coat color change, and the potential for rapid local adaptation.”

Those who have studied gene-driven evolution in the last twenty years, or who actually understood the differences between gene-driven evolution and strict-Darwinistic “survival-of-the-fitness evolution”, might wonder about Mills’ 2013 work.  I certainly did — wonder, that is.   Did he and his team really think that the hares would change color depending on the background of the 60 foot circle of land on which they were photographed at some random times? Did it not occur to them that in the Spring, hares would be most often found where there was a food source, meaning ground not snow-covered?  —  thus appearing in early spring to be a color-mismatch (judged by 10-meter circle).   Did they think that they were chameleons who change color based on background?  Were they surprised to find that the hare’s molts were keyed by day-length (photoperiodism)?

The truth is that hares all over the northwestern United States undergo various degrees-of and variously-timed polymorphism. The featured image of this essay shows a partially-molted hare in the Spring or Fall. The 2018 paper includes this illustration:

[ Link to new 2018 paper.  Pdf is at DOI: 10.1126/science.aar5273 ]

This is all detailed in the new paper. L. Scott Mills deserves kudos for his dedication to scientific rigor — as we shall see — the findings of the new paper eliminate climate change as a direct cause and move the driver back where it belongs — to breeding patterns and survival rates.  More specifically:

“We have shown that introgression has shaped locally adaptive seasonal camouflage in snowshoe hares. Recurrent introgression of coat color variants could facilitate evolutionary responses to environmental change within populations as well as the long-term maintenance of adaptive variation among species….. The evolution of winter-brown coats in snowshoe hares may have enabled their persistence in environments with more ephemeral seasonal snow after the end of the last glacial maximum. Temperate snow-cover duration is predicted to dramatically decrease over the next century under most models of climate change, which may further intensify directional selection for winter-brown camouflage. Thus, the establishment of this dynamic color polymorphism through introgression is likely to be a critical component of ongoing adaptation to rapidly changing seasonal environments in this iconic ecological model.”

Translating into Plain Language, snowshoes hares have been through periods of longer and shorter winters, and more and less snowpack and cover, and polymorphism has been retained in the much of the species — at the same time, other segments of the species have less polymorphism, either in degree of color change or length of change.   The interbreeding of these populations, which co-exist in area, at least at the edges, drives rapid allele frequency shifts manifesting differing color-shifting schemes — that is introgressive hybridization (which generally results in a complex mixture of parental genes).

The authors supply charts of the “Exome SNP associations” but for the rest of us, this little chart segment, ala Gregor Johann Mendel, should be explanation enough.

All of this means that the snowshoe hare is not going to be wiped out by climate change — but rather, as a species, it easily adapts, rolling with the punches of Nature, as it has always done in the past through the mechanism of introgressive hybridization allowing the best, pro-survival mix of polymorphism alleles to manifest in the wider population for current regional conditions over fairly short periods of time, especially effective given its short lifespan, heavy predation and rapid reproductive recruitment cycle.

Self-correcting science, advancing one step at a time:  L. Scott Mills called for, and eventually got,  a more profound investigation of the seasonal polymorphic color change of the snowshoe hare  — and signed on as an author, helping to design the genetic sequencing experiment despite the fact that the new findings would contradict his earlier paper’s prediction that “Without evolution in coat color phenology, the reduced snow duration will increase the number of days that white hares will be mismatched on a snowless background by four- to eightfold by the end of the century.” Maybe the new paper could be looked at as confirming his earlier prediction.  But it doesn’t require more or future “evolution” — the necessary traits already exist in the genetic makeup of the snowshoe hare population which allow it to adjust to shorter-than climate-scale (30 years) changes in conditions, guaranteeing their long-term survival as a species.

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 Sample Press Coverage:

Science Magazine (2013):  Color-Changing Hare Can’t Keep Up With Climate Change

NPR (2013):  Climate Change Leaves Hares Wearing The Wrong Colors

NatGeo (2014):  Can Snowshoe Hares Evolve to Cope With Climate Change?

Peninsula Clarion — Alaska (2016):  Refuge Notebook: Climate change does not bode well for snowshoe hares

Science Magazine (2018):  How the snowshoe hare is losing its white winter coat

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Authors Comment Policy:

 Evolution is always a tricky subject to discuss with the general public — there are a lot of opinions and as many flavors of evolutionary theory as there are of ice cream — discussions can become heated. Let’s try to avoid that here.  In this case, the controversial aspects don’t present themselves as the problem and its solution, already enacted by Nature,  turn out to be based on fairly simple dominate/recessive genes (alleles) that regulate seasonal coat color in snowshoe hares.

I will point out that the science press did not seem to understand that the second, most recent, paper eliminates the threat to the snowshoe hares…they all link back to the 2013 paper as if its conclusions were still valid.

I am always happy to see when some bit of science gets set pretty well straight.  What do you think?

Lead with my first name, Kip,  if you are speaking to me in your comment.

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