For years now, we’ve known that wolverines are found in regions of deep spring snowpack and low summer temperatures – Copeland et al’s 2010 paper elucidated this elegant model of wolverine distribution by mapping known wolverine locations from all over the globe and placing these locations onto a map of global snowpack on May 15th, and maximum August temperatures of less than 22 degrees Celsius. The paper showed that wolverines are tied to the cold regions of the northern hemisphere, and linked this dependence on cold to the fact that wolverines give birth in snow dens. The paper was groundbreaking and its publication eagerly anticipated, because it provided enough evidence of climate change threat to support the USFWS’ 2010 decision that wolverines are warranted for listing.
A new paper in the Journal of Mammalogy by Bob Inman, Audrey Magoun, Jens Persson, and Jenny Mattisson expands our understanding of the links between wolverines and the cold, exploring the complex reasons for the evolution of wolverine reproductive timing and behavior. If the Copeland paper told us that wolverines are indeed climate sensitive due to denning requirements and a cold-adapted physiology, this paper asks why those denning requirements and physiological limits are so strict – in other words, what adaptive advantage does cold-climate specialization offer to the species?
Inman and his co-authors suggest that the wolverine’s strategy is driven by the nutritional needs of the species, and of reproductive females in particular. Pregnancy and nursing are the most nutritionally demanding activities that any wolverine – any mammal, in fact – undertakes, and the successful rearing of young requires a secure source of food. The timing of wolverine births early in the year, according to the paper, allows females to take advantage of a flux of winter-killed ungulates that they have cached, and the nursing and weaning periods encompass the spring surge in baby ungulates and the brief summer explosion of small mammal populations. Persistently cold climates allow wolverines to cache food in an environment where it won’t go bad, allowing them to store sparse nutritional sources and “be efficient in channeling available food resources towards reproduction.” And, suggest the researchers, by living at the outer – or upper, in the case of the Rockies – margins of the ranges of other predators such as wolves and bears, wolverines avoid direct competition with much larger and better equipped rivals. Put all of this together, and wolverines obtain a neat set of advantages by living in a harsh and desolate landscape.
The paper relies on synthesis of existing research, and contains a great section reviewing all the data on reproduction. Wolverines mate in summer, but implantation of the fertilized embryos is delayed until winter. We generally say that wolverines give birth around February 14th (Valentine’s Day) and that the kits are weaned by May 15th (Mother’s Day), because these dates are easy to remember and are, on average, accurate. But in the details of known wolverine birth dates, we see a much wider range, with some wolverines giving birth as early as January, others as late as April. This means that implantation – nidation, in scientific parlance – also occurs over a range of dates, from November through January, with a 45-day gestation. Most of the births do occur mid-February to early March, but the range offers the possibility that female wolverines are responding to environmental factors on a year-by-year basis (several of the females were monitored over several years and had different chronologies in different years.) Does this mean that wolverines might possess some latitude in timing births to correspond to changing snow and nutrition availability over the longer term? Like almost everything else about wolverines, we have no definite answer, but it’s interesting to think about.
The paper also summarizes reports in some studies of very high levels of wolverine pregnancy (implanted embryos). This, too, is interesting, since female wolverines seem to raise very few litters. The data suggest that many females who give birth lose their litters early. Nutritionally, this is more adaptive than struggling to keep a litter alive and then losing it later, since it represents a much smaller investment of resources. Losing a litter early during a year when conditions are sub-optimal gives females a chance to maintain better body condition for next year’s litter, when conditions might be better. All of the attention to this question of reproduction is critical, since we absolutely must understand these dynamics in order to determine effective conservation strategies.
This paper received a fair amount of attention in the press, most of it focusing on food storage: here at the Examiner, at the Huffington Post, at LiveScience, at USA Today (there is, perhaps, a wolverine fan on staff there, because this makes two articles on gulos in two months), at National Geographic, and at MSNBC. I’m sure that there are others out there, too. Most of these articles quote lead author Bob Inman, of the Wildlife Conservation Society, sounding very respectable and scientific – but his secret identity is revealed in an article discussing whether or not the ubiquitous presence of Wolverine the X-Men superhero may also be an adaptation to climate change, in which Wolverine uses his extensive social connections as a food storage strategy: “Understanding why and how Wolverine exists where he does and the various adaptations he has evolved to eke out a living will better inform population management strategies and conservation of the comic industry,” said researcher Robert Inman. I’d heard that Bob was away in Sweden for several months, but now we know the truth.
My own summary of this paper is delayed; I usually like to talk to the scientists to make sure I have all of the implications correct, but the paper came out just as I left for Mongolia, and I haven’t had the chance to talk to anyone. So this is just a basic summary. More later.