Territoriality in Female Wolverines

For reasons that I’ll get into in a future post, I feel suddenly compelled to write up a quick literature review on the topic of territoriality in female wolverines.

Let’s get this out of the way immediately: female wolverines are territorial. This is evident throughout the recent literature. Out of concern for the safety of resident adult wolverines in the US, researchers generally don’t publish maps of wolverine home ranges, but anyone who has had the chance to look at the minimum convex polygons of study animals can see a pretty clear pattern. Adult female wolverines don’t overlap with other adult female wolverines. The territorial boundaries are pretty strict, often down a drainage that both animals will approach and traverse, but that neither will cross. There are instances of incursions and small amounts of overlap, but this is generally less than 2% of any female’s home range. Adult males are also territorial, but with varying degrees of territorial incursion or excursion. The territoriality of wolverines is one of the most important factors for understanding wolverine distribution, their natural rarity, and, importantly, conservation concerns and strategies as we look at a warming world.

Strictly speaking, a territory is a patch of ground that an animal defends, and a home range is a patch of ground that an animal occupies and uses for subsistence. In the wolverine research community, including in published papers, these terms are used in a conflating fashion, although some papers do make a distinction. In presentations and talks and casual conversation, we often use the words “home range” to refer to the area that a wolverine (male or female) occupies, but we mean this as a defended home range, which is actually a territory.

Territoriality in the Literature

While the territoriality of wolverines is widely understood at this point, I was surprised to note that there are few American or Canadian papers or book chapters that deal explicitly with the question of how territoriality functions, or the ways in which it potentially limits population in areas of restricted habitat where the population fragments into a meta-population structure. The Scandinavians, on the other hand, do tend to deal with territoriality, probably because the management imperatives are more urgent in places where depredation on domestic reindeer is a major problem. A Norwegian master’s thesis from 2014 looks at genetic sampling as a method to determine territoriality, in comparison to GPS collar data. Jens Persson’s dissertation deals in some depth with possible explanations for territoriality, and how those explanations may differ between males and females. Proving that the Scandinavians more or less own this topic, a 2017 thesis by Malin Aronsson closely focuses on the dynamics of territoriality and dispersal among female wolverines (and lynx). But the territoriality of the species is taken for granted in most publications, especially the North American publications.

I was also reminded of the absence of peer-reviewed publications focusing specifically on several of the major American wolverine research projects. Doug Chadwick’s popular-science book The Wolverine Way remains the best published account of the Glacier wolverine project. The Rocky Mountain Research Station has put out a number of impressive papers that draw on the Glacier data to ask large-scale questions about habitat relationships, population genetics, dispersal, and climate change effects, but the annual project reports remain the best source of information for the park population itself. The same is true of the Absaroka-Beartooth project, the project with which I got my start in the wolverine world, working as a volunteer. Peer-reviewed papers for that project were somewhat limited by the low numbers of detected wolverines within the study area – sample sizes and statistics being the perennial priority for most journals these days – but the eerie emptiness of prime modeled wolverine habitat deserves some consideration, hopefully in future publications.

Back in 1981, Hornocker et al published a study asserting that wolverines were not territorial. This was one of the earliest studies of wolverines in the lower 48, lasting from 1972-1977, on the Flathead National Forest. It was a telemetry study in which the researchers observed overlap among many different wolverines, and concluded that wolverines were tolerant of fellow wolverines. They reported “no intraspecific strife” and discussed how a wounded female wolverine, whose injuries they first attributed to another wolverine, were likely caused by a mountain lion. (Despite assertions that wolverines are not territorial, the home range maps that are included in this paper do show a familiar pattern – two male wolverines who don’t overlap, one female wolverine who sticks to a fairly tight home range, and a second female, overlapping the first, who makes wider movements that include the home range of the male who overlaps the first female. If I were to guess, many years later, what was up with this scenario, I’d suggest a male-female pair and their juvenile daughter, preparing to disperse. The second male looks like an unrelated individual as his range does not overlap with any of the other animals.) Earlier observational studies of wolverines, and books about the species, also fail to make note of territorial behavior. The same is largely true for the Mongolian hunters and herders who I interviewed; they were well aware of the wolverine’s rarity and ability to travel over long distances, but only a few noted that a wolverine would reappear in a particular spot at intervals. None of those interviewees made a leap to territoriality as an explanation, but some did refer (maybe jokingly) to a wolverine’s nutag, which is a Mongolian concept denoting an individual’s homeland.

Home range maps showing seasonal movements for four wolverines in NW Montana, from Hornocker et al 1981

Home range maps for four wolverines in NW Montana, showing seasonal movements, from Hornocker et al 1981

Observation, track surveys, and radio telemetry, of course, are limited in important ways. These methods allow glimpses of an animal only at the moments when the observer or listener happens to have their eyes or telemetry antenna trained on the animal. Finding dens is more difficult, which – in the absence of DNA techniques – makes understanding relatedness more difficult as well. With the advent of GPS collars and DNA analysis, we were able to observe wolverines more closely and consistently. Hundreds of locations for multiple animals, taken over months, in combination with VHF locations over the course of years, made the territorial behavior of the species clear. Den locations and kit collaring showed that wolverines will tolerate their own offspring within their territories for up to two years after birth. This could account for Hornocker’s and others’ observations of multiple wolverines sharing the same home range, and photos like those that Igor Shpilenok took in Kamchatka, of up to six wolverines on a bear carcass at the same time.

The Wildlife Conservation Society’s Yellowstone Wolverine Project likewise relied on annual reports and white papers to convey results for many years. The project director, Bob Inman, was in the process of getting his PhD, so his dissertation eventually yielded several published papers. Again, though, these deal mostly with larger-scale questions about habitat relationships and conservation priorities. Several of his papers do discuss territoriality, but again, for obvious reasons, most of the published papers don’t include home range maps.

What all of these papers and sources do include, however, is an assumption that territoriality is important. Some imply territoriality; for example, in a summary of draft papers from 2007, the WCS project notes in the abstract for a chapter on wolverine space use:

Mean annual (1 Mar–28 Feb) 95% fixed kernel home range size was 453 km2 for adult females (n = 15 wolverine years) and 1,160 km2 for adult males (n = 13 wolverine-years). Mean percent area overlap of same-sex adults was < 1% (SE = 0.00, range = 0–2%, n = 12 pairs) using annual 100% minimum convex polygon home ranges.

The “<1% overlap” suggests territoriality, although it’s not explicitly stated here. In a 2012 review of wolverine reproductive chronology, however, Inman et al do note this:

Throughout its distribution, the wolverine displays extremely large home ranges, territoriality, low densities, and low reproductive rates (Copeland 1996; Inman et al. 2012; Krebs et al. 2007; Lofroth and Krebs 2007; Magoun 1985; Mattisson et al. 2011a; Persson et al. 2006, 2010). These adaptations are necessary for exploiting a cold, low-productivity niche where growing seasons are brief and food resources are limited (Inman et al. 2012).”  

Another 2012 Inman et al paper on spatial ecology does deal explicitly with the question of territoriality vs. undefended home ranges and is probably the most extensive discussion of this topic in the literature on wolverines in the lower 48. This paper does include some home range maps, with the locations stripped out.

Spatial distribution patterns of the Mustelidae are typically described as intra-sexual territoriality, where only home ranges of opposite sexes overlap (Powell 1979). Wolverine-specific reports exist for both intra-sexual territoriality (Magoun 1985, Copeland 1996, Hedmark et al. 2007, Persson et al. 2010) and for a high degree of spatial overlap but with temporal separation (Hornocker et al. 1983). Arguments against territoriality by wolverines include the lack of ability to defend such a large home range (Koehler et al. 1980). Our data on movement rates in relation to home range size, temporal development of the home range, minimal overlap of same-sex adults, and relatively immediate shifts upon a death suggest that wolverines are capable of patrolling a large territory and provide further support for intra-sexual territoriality. Reproductive success is closely correlated to the amount of energy that a female wolverine can obtain (Persson 2005), and for predators that are capable of individually acquiring prey, the presence of conspecifics reduces foraging efficiency (Sandell 1989). Since wolverines feed on individually obtainable prey and occupy relatively unproductive habitats, it follows that behaviors for maintaining exclusive access to resources would likely have selective advantage. Frequent marking behavior (Pulliainen and Ovaskainen 1975, Koehler et al. 1980) is likely part of an adaptive strategy that involves maintenance of exclusive territories within sexes so that feeding and breeding opportunities are monopolized by dominant individuals and their immediate offspring.

Female wolverine territories in Wyoming, with takeover of one home range by a female kit after the death of the resident adult female. From Inman et al 2012

The Scandinavian literature is also rife with references to territoriality among both male and female wolverines. Jens Persson, who works on wolverines in Sweden, reflects in his dissertation on the reasons for territorial behavior in female wolverines, and concludes that it’s related both to food, and also potentially to the need to protect kits against infanticide. Historically, it was widely believed that male wolverines would kill any kits they encountered, even their own; this has since been proven false, but the idea that males kill unrelated kits persists. Persson is the first researcher I’m aware of to suggest that female territoriality may actually be a defense against other females intent on infanticide.

Females could also gain from infanticide by eliminating non-related progeny to decrease future competition for territories or denning areas for her and her progeny. In addition, the death of an unrelated infant could also reduce the net reproductive success of a competitor (Hrdy & Hausfater, 1984). Competition for territories determine dispersal behaviour in female wolverines (Paper IV), suggesting that there is strong competition for territories among female wolverines.

 Wolff and Peterson (1998) hypothesized that a primary function of female territoriality in solitary mammals could be to protect vulnerable young from infanticidal conspecific females. Four predictions can be deduced from their offspring-defence hypothesis: 1) Female territoriality should be associated with young that are vulnerable to infanticide. 2) Female territoriality should be associated with defence of offspring, and therefore most pronounced during the offspring-rearing season. 3) Defence will be greatest against the segment of thepopulation that commits infanticide and against those individuals that females can dominate. 4) Optimal territory size should be a function of intruder pressure, intruder detectability, female response distances and offspring vulnerability, and changes in food abundance and distribution should not affect territory size directly unless they are correlated with the other factors. In concordance with predictions 1-3, wolverines have altricial young that are vulnerable from late winter until late summer (March – August) and female territoriality seem to be strongest during this period (Magoun, 1985; Landa, Lindén & Kojola, 2000). We lack data to evaluate prediction 4. However, in contrast to prediction 4, I believe that food actually is an important determinant of territory size in wolverine females (see Banci, 1994).

Malin Aronsson’s 2017 thesis examines the territorial dynamics of female wolverines, using the vast dataset from Swedish studies dating back to the 1990s. She makes some interesting observations about the counter-intuitive conclusion that wolverines are territorial despite living in low-resource environments, which is the opposite of what studies on other carnivores would suggest:

Wolverines are highly territorial (Persson et al. 2010), and by comparing space use overlap between years for the same individual I found that wolverines show high territorial fidelity resulting in a stable distribution of resident individuals. Interestingly, territorial fidelity in general is predicted to be low in habitats where food resources are low, variable, unpredictable or deplete fast (Wauters et al. 1995; Kirk et al. 2008; Edwards et al. 2009), which corresponds to the characterization of wolverine habitat in general (Inman et al. 2012b), and particularly in this study area (Person 2005). However, scavenging and caching are integral parts of wolverine biology (Inman et al. 2012b; Mattisson et al. 2016), which increase resource predictability, decrease depletion rate and create a valuable resource (i.e. cache sites) to defend, promoting high territorial fidelity despite the unpredictable environment (Tye 1986; Eide et al. 2004). In addition, occurrence of more efficient predators, such as the Eurasian lynx (Lynx lynx), provide carcasses for direct consumption and caching (Mattisson et al. 2011b). Furthermore, both males and females showed higher between-year fidelity at the territory level (i.e. 90% isopleth) compared to the core areas (i.e. 50% isopleth). That fidelity was lower at the core area compared to territory level suggests that it is critical to maintain the outer territory boundary to secure long-term resources, while the most used area within the territory may vary between years due to spatial fluctuations in key resources, or, for females, location of den sites may vary between years.

Aronsson also documents a few cases of territorial adult females shifting territories after successfully reproducing. Why this happened, and how frequently such moves occur, would be interesting questions for further investigation.

Another Scandinavian paper, a master’s thesis from 2014 by Espen Gregersen, compares home range and territoriality derived from GPS data to those derived from scat analysis. The question in this thesis was not so much “are wolverines territorial?” as “can wolverine territoriality be detected using non-invasive methods like scat analysis?” It’s an interesting question and one of relevance to those of us who have limited resources for large-scale trapping efforts in places like, say, Mongolia. Gregersen concludes that yes, we can indeed determine home range size and observe territoriality using scat samples – but it takes a very large number of samples. This thesis also addressed the question of territorial turnover after the death of a resident adult, which is particularly interesting at the southern edge of distribution, where wide separation of habitat patches makes recolonization less certain.

Finally, a forthcoming book chapter from Copeland et al. proposes a slightly different take on territoriality among male and female wolverines. That chapter will be out soon and I’ll look at it in depth once it’s published, but it too reinforces the idea that female wolverines are highly territorial, and maybe even more strictly territorial than males.

There are many other papers out there that include brief mentions of territoriality and intrasexual exclusion in home ranges. These are just a sample, and this write-up fairly cursory, but I hope they’re adequate to illustrate that wolverines – both male and female – are territorial.

Territoriality and Conservation

At this point, territoriality in wolverines is accepted as an important feature of their life history and ecology. The question of why hasn’t yet been answered, but the fact that wolverines require such large territories, and the fact that their reproductive rates are so low, accounts for their natural scarcity on any landscape – let alone one in which suitable habitat is located only at certain elevations in widely scattered patches across a sea of non-habitat.

Female territories structure the wolverine population. Females must have adequate resources to meet their needs and, hopefully, to reproduce. They occupy and defend territories that allow them to do this. Males in turn seem to select for territories that overlap with resident females. Whether this represents a territorial strategy for sexual monopoly, or whether it’s defined by the male’s capacity for paternal investment, or some combination, is worth investigation (male wolverines seem not to always entirely encompass a female’s territory, leaving her open to potentially overlap with other males, which raises questions about the accepted narrative of males “controlling” access to females). Both males and females disperse over long distances, although the longest movements have been observed in males like M56.

Habitat availability for females is the limiting factor on wolverine population growth and range expansion in the US Rockies. I’d hypothesize that keeping a certain number of territories occupied is critical to the long-term persistence of wolverines in the lower 48, and that there is some distinction to be made between female population numbers, strictly speaking; the percentage of habitat that’s occupied; and where that habitat is located in relation to other habitat. There’s been an enormous focus on “connectivity”– concurrent with the fashion for corridors among conservationists –  but a surprising lack of attention paid to the population nodes themselves. For example, the question that Gregersen raises about recolonization of vacant territories is interesting and important, especially given the observed disappearance of wolverines from places like the Tetons. Presumably this disappearance represents some kind of natural cycle of die-off for a relatively isolated population node, but how long does it take before those territories are reoccupied? And how is time-to-recolonization related to population density and occupancy of the next-nearest population nodes? Questions about functional connectivity among wolverine population nodes are important, but connectivity as a single conservation strategy for wolverines seems like an odd allocation of resources; wolverines don’t migrate, they disperse, and their dispersal patterns are unique and erratic. They are likely to benefit from the broad and intense focus on connectivity and road-crossing structures for other species, but trying to preserve wolverine-specific corridors seems like a good recipe for driving oneself nuts. As one of my Mongolian interviewees once said when discussing wolverines, “One day it’s here, the next day it’s 50 kilometers away. It could turn up anywhere!” I hope to see a greater focus on what’s going on within habitat in the future, including investigation of questions about what drives territoriality and territory size, and how territorial turnover works in a meta-population.

That’s it for now. If any of you have any thoughts about the function of territoriality in female or male wolverines, if you want to point out an obvious resource on this question that I overlooked, or if you just want to say hi, please feel free to comment.

References (with apologies for lack of consistent style formatting and for referring to multiple authors as “et al” instead of writing them out. Time constraints!)

Aronsson, M. 2017. ‘O Neighbour, Where Art Thou?’ Spatial and social dynamics in wolverine and lynx from individual use to population distribution. Doctoral dissertation. Swedish University of Agricultural Sciences. Uppsala. ISBN (electronic version) 978-91-576-8822-4

Chadwick, D. 2010. The Wolverine Way. Patagonia press.

Copeland, J. P., Landa, A., Heinemeyer, K., Aubry, K. B., van Dijk, J., May, R., Persson, J., Squires, J., and Yates, R. 2017. Social ethology of the wolverine. In: Biology and Conservation of Musteloids. Edited by David W. Macdonald, Christopher Newman, and Lauren A. Harrington: Oxford University Press. DOI 10.1093/oso/9780198759805.003.0018

Gregersen, E. 2014. Assessing territoriality in wolverines (Gulo gulo) using non-invasive genetic sampling. Master’s thesis. Norwegian University of Life Sciences.

Hornocker, M and H Hash. 1981. Ecology of the wolverine in northwestern Montana. Canadian Journal of Zoology. V. 59. pp. 1286-1301.

Inman et al. 2007. Wolverine space use in greater Yellowstone. In Greater Yellowstone Wolverine Program Cumulative Report. Wildlife Conservation Society.

Inman R. et al. 2012. The wolverine’s niche: linking reproductive chronology, caching, competition, and climate. Journal of Mammalogy 93(3):634-644.

Inman, R. et al. 2012. Spatial Ecology of Wolverines at the Southern Periphery of Distribution. Journal of Wildlife Management. 76(4). 778–792. DOI: 10.1002/jwmg.289

Persson, J. 2003. Population Ecology of Scandinavian Wolverines. Doctoral dissertation. Swedish University of Agricultural Sciences. Uppsala.

 

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Wolverines in Norway

Scandlynx, the program that monitors lynx and wolverine in Scandinavia, just issued a report on the status of the Norwegian animals that they tracked over the past year. The report is in Norwegian; readers are welcome to have fun with google translate, which will inform you that in Norway, wolverines are lured into “cubicles” to be fitted with GPS “necklaces” – maybe only the linguistically-obsessed among us find this amusing, however, so I’ll summarize the report here, in English, with a bit of background information. Caveat: I don’t speak Norwegian, and internet translation is always subject to oddities, so don’t hold me to the specifics.

Wolverine and lynx in Norway and Sweden present a unique problem that we fortunately don’t have to deal with here in the US; large herds of domestic reindeer are a major food source for these predators, generating correspondingly high levels of animosity from reindeer herders. Herders receive compensation for losses in both Sweden and Norway, but the compensation takes different forms; in Sweden, herders are paid a flat (and very high, somewhere around $20,000) rate for each successful wolverine den within their herding territory. In Norway, the compensation scheme is similar to our own wolf depredation funds in the western US; herders are given payments per animal on the basis of a confirmed kill. In an argument that precisely parallels arguments from ranchers here in Montana and Wyoming, reindeer herders claim that for each confirmed kill, several others go undetected, and that they are therefore being under-compensated for their losses.

In 2011, the report says, the Norwegian Environment Directorate commissioned a study on the rate of depredation (“murder rate,” according to google, which probably accurately summarizes how the herders feel about it) to try to clarify the actual loss of reindeer to lynx and wolverine. The study covered provinces in central and northern Norway, and will continue for five years. There’s also a component that looks at territoriality of lynx family groups, and a component that tracks collared female lynx and wolverine. The project is tied explicitly to management outcomes, and also considers the effectiveness of current monitoring protocols. The current report is preliminary, not a final report.

Researchers dart animals from helicopters, collar them, and then, with the input of knowledgeable locals, set up a series of stations or traps (the aforementioned “cubicles”) which the animals visit. During the first year of the study, four lynx and 11 wolverine were collared and tracked. The GPS transmitters were set in an intensive mode for a particular period of time, with fixes taken at one hour intervals, in order to determine predation by looking at places where the animals lingered.

The lynx had huge home ranges – 2800 km2 for one of the females, 4200 km2 for one of the males – and seem to be making a living primarily off of roe deer and domestic reindeer, supplemented with the occasional moose calf and some small game (hare, the prey species we most commonly associate with lynx here, comprised only a small part of the diet of these four animals.) The lynx killed six adult and nine calf reindeer, as well as three reindeer classified as “unknown.” These kills occurred throughout the year.

Seven of the 11 wolverine were monitored with the intensive GPS, six wolverine (all male) during the winter, and three (one female, two males) during the summer. The six male wolverine killed between four and six adult reindeer (there seems to be some uncertainty about the status of two of the kills) and three calves during the winter. No depredations were recorded during the summer. Wolverines relied much more heavily on scavenged and previously stored carcasses. The female was recorded killing a roe deer and a bird during the summer, while one of the males added bird and hare to his diet. The researchers specify that they are unsure whether some of the carcasses visited by wolverines in the winter might have been wolverine kills stored away before the study began.

This work is interesting because the researchers are considering some intriguing, landscape-scale systems questions – predator interactions, human landuse, social tolerance, and management and policy issues. It will be interesting to follow the work as it progresses, to see how management and compensation rates change, and to look at how this does or does not increase tolerance for carnivores on the landscape.

If you’re truly obsessed and/or have a lot of time on your hands, you can allegedly track some of the animals in this study here. There is an English version of the page as well. You will have to register, but the process seems to be open to anyone. I still haven’t been able to find the wolverines, however, even after selecting the wolverine project and selecting specific wolverines. So have fun, and if you figure it out, let me know.

South of 54: The Bioclimatic Constraints of Wolverines on the Edge

(For months, the question floating around the office was, “Has Jeff Copeland’s snow paper been published yet?” It came out in March of 2010. This is a summary of what that paper found. I’ve been working on this for weeks and, like some Sysiphian task, I’ve found that the closer I’ve come to something workable, the busier life becomes, and the further from completion the piece gets. So I’m posting it as is, because I have to leave for Canada tomorrow and then Mongolia the week after that. Forgive any imperfections or lack of cohesion.)

The Magic Line

Find a map of the northern hemisphere, and locate the 54th parallel. Trace its arc across the globe – in Siberia, 54N cuts across the southern tip of Kamchatka and the northern tip of Lake Baikal, then passes just south of Moscow. It brushes past the southern edge of Sweden, decapitates the Danish peninsula, and slices England in half. Across the Atlantic, it comes ashore on Newfoundland, sweeps across northern Quebec, and passes through the top of James Bay. In Ontario, it demarcates the southern edge of Polar Bear Provincial Park, and then cuts across a sweep of sparsely inhabited, lake-studded Manitoba, Saskatchewan, Alberta, and British Columbia, reaching the Pacific at Hecate Strait just south of Alaska. North of 54, the world stretches, colder and ever more severe, in a mix of boreal forest and tundra, towards the Arctic Circle at the 80th parallel. The region is underlain by permafrost, discontinuous but widespread closer to 54N, deeper and more continuous to the north. The landscape, though ridged with several mountain ranges in both hemispheres, is primarily flat, and remains covered in snow until late spring. North of 54, the Pleistocene lingers.

54 North is the wolverine’s Magic Line. In the flat, snowbound tundra to the north of this parallel, everything is, essentially, wolverine habitat, and the species is spread in a continuous distribution across much of the landscape. South of 54, boreal habitat grows sparser, migrating uphill as you go further south, until, by the time you reach the US Rockies, it is pinched into the upper reaches of the highest mountains. Wolverine distribution migrates uphill with the habitat, until a creature designed for cruising the vast and primarily flat North becomes, of necessity, a mountaineer. Restricted to these islands of boreal habitat in a sea of sagebrush desert, the wolverine faces a unique set of challenges that center around access to suitable places to live.

Parameter 1: Persistent Spring Snow

Wolverine biologist Jeff Copeland and his colleagues in the wolverine biology world were attempting to determine the parameters of these challenges when they began to look at variables that might influence wolverine habitat selection. What, exactly, did a wolverine need in order to consider a place livable? Working with telemetry locations and confirmed wolverine den sites from research projects in North America and Scandinavia, they began sorting through a list of possibilities – elevation, aspect, vegetation, prey, weather conditions, proximity to human development. Habitat selection models for wildlife can become complicated as factors interact with each other, but in the case of the wolverine, a single overriding variable seemed to be the best predictor of wolverine distribution: persistent spring snow.

At an intuitive level, this made sense. Wolverines den in the snow, so of course they would tend to adhere  to regions with adequate snow to protect their babies until the kits were capable of limited independence, in early to mid-May. And the logical and intuitive conclusion would be that wolverines, being dependent on snow through May, are vulnerable to reduced spring snowpack predicted in climate models for the next century. All of this would seem to suggest that the wolverine requires protection and careful management to ensure that it remains on the landscape as declining snow levels and rising temperatures limit reproductive habitat. But in science, intuition and logic are never enough. Someone has to prove that the hypothesis is sound, and proof, in science, involves running the gantlet of the peer-review process, and publishing your results.

Jeff Copeland and his colleagues at the Rocky Mountain Research Station took on the task, and proposed an obligate relationship between wolverines and snow. Obligate relationships, in ecology, are relationships of dependence that are restrictive for the organism in question and that in many cases serve to define the environment on which it relies. So an obligate wolverine-snow relationship implies that you will never find a wolverine living in a place without persistent spring snow and, conversely, that if you see a mother wolverine traveling with two kits sometime in late spring, you know you’re in an area where snow persists until at least mid-May.

To test the model, Copeland and fellow researchers used remote-sensing data to construct a map of late spring snow spanning 7 years, from 2000-2006. To qualify as having persistent spring snowpack, a location (represented as a pixel on the map) had to remain snow covered between April 24 and May 15 – the period in which wolverine kits emerge from the den to begin the freewheeling life of a juvenile gulo – without a single day of bare ground.

Norway and Sweden maintain national wolverine den monitoring programs, which allowed Copeland to access precise den locations during the years for which the snow model was constructed. In North America, den location data are more scattered, so the authors of the paper drew on information spanning 1981- 2007 to obtain an adequate sample. In total, they compiled locations of 562 dens (327 of which were in Norway, 160 of which were in Sweden, 10 of which were in Finland, and 65 of which were in North America, which illustrates how little we know about wolverine reproduction in the US and Canada.) To look at year-round habitat use, Copeland compiled winter and summer telemetry locations of instrumented wolverines from 10 studies in North America and Norway. When the den locations were placed on the snow map, 98% of the dens were located on pixels that were classified as having persistent spring snow cover for at least one of the years, and 69% of the time, female wolverines were selecting for sites with snow cover for six or seven out of seven years. In the 2% of cases in which the den locations fell outside the snow map, the sites were investigated and determined to be snow dens; in these cases, the dens were located in expanses of snow too small to register by way of remote-sensing. Here was conclusive, statistically significant evidence that the relationship between wolverines and snow is, indeed, obligate.

The telemetry points reinforced the hypothesis. During summer, 95% of the telemetry locations adhered to the snow map, and in winter, 86% of the telemetry locations stayed within the bounds of persistent spring snowpack. The discrepancy makes sense if wolverines prefer snow; during winter, a greater portion of the landscape is snow covered and wolverines are therefore better able to travel outside the bounds of spring or summer snowpack. (Even then, however, Copeland determined that the wolverines traveling outside the snow map were primarily males – females maintained a higher fidelity to the snow map in all seasons.) Taken together, this meant that no matter how much snow was actually on the ground in a given season, wolverines predominantly operate in places where there is snow in late May.

Predominantly – but not exclusively. In one study, in the Omineca Mountains of British Columbia, wolverines actually appeared to be avoiding the snow locations during most of the year, but occupying those locations during summer when temperatures were highest. This suggested that there was another factor influencing wolverine habitat selection. To expand the test of whether wolverine distribution is limited by climate, Copeland and his co-authors decided to look at the next logical variable: temperature.

Parameter 2: Upper Thermal Limits

Wolverines are designed for the cold and snow, and early investigations into wolverine metabolism focused on the impressive insulating qualities of wolverine fur.  Scientists suggested that a wolverine in a winter coat could tolerate temperatures down to -40º C. But no one had ever tested the upper limits of a wolverine’s thermal tolerance.

For Copeland and his colleagues, there was an easy way to construct a test, without ever handling a wolverine or its pelt. They modeled 50 years of temperature data in the locations where wolverines were hanging out in the telemetry studies, and established that the average maximum August temperature  in these areas was 22° C. They then made a global map of 22°C maximum August temperature and laid it over the snow layer map. The temperature layer painted a cool swath across the boreal north and then, further south, splintered into peninsulas and islands that corresponded to mountains or cool maritime regions. In these insular and peninsular southern regions, the August temperature layer and the snow layer corresponded, but further to the north, where the entire landscape had maximum August temperatures of 22°C,  there were regions with no snow in mid-May. The point of divergence between the snow layer and the August temperature layer was just south of the Omineca study area, at approximately 54N latitude.

The wolverine studies that Copeland included in the analysis crossed 24 degrees of latitude, with average temperature variations of about 10°C between the southernmost and northernmost locations. To the north, where average August temperatures are consistently below 22°C, wolverines operated across the landscape in a more general way. To the south, where average August temperature varies with latitude, they were selecting for locations with lower temperatures – generally, at higher elevations. Year-round, wolverines adhere to swaths or stretches or, further south, scraps and slivers, of boreal habitat.

‘Whales in the Desert’

The paper that Copeland and his colleagues wrote wields the weighty title The bioclimatic envelope of the wolverine (Gulo gulo): do climatic constraints limit its geographic distribution?, and was published in the Canadian Journal of Zoology in March, 2010. The paper answers the question posed in its title with a decided ‘yes.’ In a statistically verified way, wolverines have been shown to select for cold and snow, and to avoid areas that possess neither of these characteristics.

The paper and the science behind it are simple and elegant, and Copeland, in a phone conversation this morning, mentioned that the paper was ‘fun to write,’ because it dealt with three simple factors: areas of known wolverine presence, areas with persistent spring snowpack, and areas with a limited maximum August temperature. The first factor defined the geographic limits of the study, and this, in turn, increases the confidence that the results are valid. Referring to what he and his co-author Kevin McKelvey call a ‘whales in the desert effect,’ Copeland pointed out that in creating a habitat selection model for whales, you will find that whales are avoiding selecting deserts if you include deserts in your analysis. The problem with many habitat selection models is the fact that they are so broad and encompass so much territory that, by default, the ‘deserts’ are included, and you end up showing avoidance of areas or factors that might not actually be relevant to the needs of the species, but that might appear statistically significant in an analysis.

People have speculated about whether wolverines might have been widespread in the past, and were driven up into their current  mountain habitat by expanding human activity.  Copeland says that if he and his co-authors had included all of North America in their analysis, the results would have shown a statistically significant avoidance of human development and lowland areas. Because they confined their analysis to the known current habitat of the wolverine, and were able to show that within that habitat wolverines are making fine-scale selections for specific factors – snow, and low August temperatures – they were able to elucidate something truly significant for the gulo-curious and for managers: wolverines need cold and snow, and they can’t live in places that don’t have it. Post-Pleistocene, wolverines in the Lower 48 have probably always been confined to the mountains.

As Copeland explained the analogy over the phone, the idea of whales in the desert fused neatly with the vision that I hold in my head of the fragmented peninsulas and outlying islands of boreal habitat in the Rockies. I’ve always thought of it in terms of a land mammal having to swim across non-habitat between those islands – exhausting, but possible. But something about the idea of a whale trying to cross dry land seems apt, too. North of 54, the wolverine is swimming in the ocean for which it evolved. South of 54, the wolverine is a whale in puddle in a desert, contemplating survival in a landscape in which it survives by the grace of a cold and snowy climate, but to which it otherwise does not belong.

Wolverines in Motion

I’m headed off to a ranch that’s located in what we suspect is a wolverine travel corridor, a spot that’s almost dead center between two high mountain ranges. It’s a beautiful and remote place, accessible in winter only on skis or snowmobile. I’ll be up there for three days and hope to do a longer ski tour up into the high country behind the ranch, checking out a few drainages just to see if I cross any interesting tracks.

I doubt I’ll be lucky enough to actually see a wolverine, but there’s always a slim chance. For those who don’t have access to potential wolverine habitat, here’s a collection of videos that allow a vicarious wolverine-viewing experience. These brief clips highlight wolverines doing everything from being gratuitously adorable (kits near den) to scent marking to gnawing on ungulates five times their size. The clips illustrate the unique gait and oddly bear-like look of the animal, and if you’ve never seen a wolverine in motion before, you should check them out. Enjoy!

As a postscript, for anyone who must have all things wolverine, Norway is issuing a 2010 wolverine stamp. The site offers a brief profile of the species and an outline of its conservation history in that country.

Waffles, anyone?

My friend is in Trondheim, Norway, for a biodiversity conference. In 2002, the UNEP and the Convention on Biological Diversity agreed that 2010 was the target year for halting, or at least substantially slowing, biodiversity decline. The conference in Trondheim will address what to do now that we’ve reached 2010, and biodiversity decline continues at a shameful rate. Gloomy stuff.

On the more humorous side, Trondheim apparently has a resident wolverine. After my dramatic assertions about how the wolverine is the last representative of true wilderness, you will share my sense of entertainment – and slight chagrin – to learn that this wolverine hangs out in town and, apparently, has a taste for….waffles.

The waffle-loving Trondheim wolverine