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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Sweden Incidents in Wolverine Research

I’m the first to admit that I give Scandinavian wolverine research less attention than it deserves. I’ve never been there, I don’t know the researchers or the research or the policy situation as well as I know the North American and Mongolian situations, and it’s easy to put aside detailed write-ups when I can’t draw on background expertise to pull together a quick post. Things that take more background work on my part tend to get written about less, because writing this blog is uncompensated intellectual labor (or escapist fun; it varies by the day) and the greater the investment, the less likely something is to get written.

Something apparently happened recently in Sweden, however, and no one can figure out quite what the so-called “Sweden Incident” entailed. Since the question remains open, the Sweden Incident may or may not involve wolverines. So I thought this would be a good chance to run down a few of the Swedish wolverine resources, to remind all you wolverine-interested people that there is, in fact, some pretty cool stuff happening in Sweden. And this stuff is verifiably and objectively true. So enjoy.

Scandlynx is a joint project with participation by both Sweden and Norway. They monitor lynx and wolverine and put out some interesting research. The website is available in Swedish, Norwegian, and English.

The Swedish Wolverine Project has its own page, where you can learn more about the specifics of wolverine research, check out a good bibliography with links to research publications, and enjoy a brief synopses of wolverine ecology and life history.

In less savory news, Swedish wolverines regularly depredate on domestic reindeer. This creates a lot of conflict with the reindeer herders, but has resulted in some innovative experimental compensation schemes. Here in the US, of course, our biggest conflict is between livestock producers and wolves, and we tend to compensate after a wolf pack depredates. Ranchers aren’t keen on this way of doing things, because they have to prove that the depredation happened, and they claim that far more animals are killed by wolves than those that they can find and verify. Consequently, they claim, the rate of compensation is far too low to actually pay for their losses.

Sweden solved this problem by instituting a preemptive payment scheme for wolverine dens on Sami herder territories. Rather than compensating after a depredation happens, the Swedish government pays for “conservation performance” when a den is successful. While this compensation scheme seems to be better at addressing herder concerns, poaching remains a problem; as of 2009, at least, up to 60% of wolverine mortalities in Scandinavia were due to poaching. And there’s also evidence that national parks in northern Sweden, rather than serving as places of protection for wildlife, are actually the loci of greater illegal hunting. This may be due to the fact that these parks are large and remote, and therefore difficult to patrol.

The intensive monitoring program in Sweden allows researchers there to look at predator interactions in a way that is difficult elsewhere. Ecology as a field has spent a lot of time studying predator-prey dynamics, but less time understanding how suites of carnivores interact with each other. The Swedish wolverine project is able to monitor both lynx and wolverines on reindeer herding territories, and has produced some interesting work on how these two species share resources. It appears that the resource-sharing strategy in Sweden primarily involves avoidance; although wolverines scavenge off lynx kills, the two species don’t interact that often. The most detailed work on this topic is Jenny Mattisson’s 2011 dissertation. Follow-up work on wolverine-lynx interactions is also available.

Perhaps the most intriguing Sweden Incident in the wolverine world, however, is the recent establishment of a population in a previously uninhabited boreal forest region of southern Sweden. In a paper published last year, the authors contend that Sweden’s monitoring and management programs need to adapt to account for this new population, since most monitoring protocols and management objectives are related to populations in northern alpine herding regions. They also mention that the presence of this wolverine population in an area where the snow melts earlier in the spring is evidence that wolverines may not be as dependent on late spring snow as the Copeland et al 2010 snow model paper suggests. I have a lot of thoughts on that topic, but for now I’ll just confine myself to two points. First, the original snow model paper pointed out that north of 54 latitude, the relationship between spring snow and wolverine presence breaks down, and this site is substantially farther to the north, somewhere around 60 latitude. So in fact, the original snow model paper accurately predicted this exact situation. Second, the framing of questions about wolverine snow-dependency has becoming depressingly binary, because of the management implications for listing in the US, with a tendency towards papers that highlight apparent exceptions to very specific features of the model to make claims that apply to the particular US policy questions, without taking the larger picture into account. This population in southern Sweden, similar to populations north of 54 latitude in Canada, offers us a chance to ask a much bigger set of questions about wolverine habitat requirements and the relationship between cold climates and wolverine persistence. I’m looking forward to learning more about this.

Finally, though, if certain individuals are implying that there is a Swedish Incident or Incidents involving terrorizing of the good and upstanding people of Sweden, our best avenue of analysis would probably focus on moose. As documented here, rowdy drunken moose are apparently a perennial issue in Sweden, destroying property, breaking into stores, holding public orgies, and even occasionally killing people. The establishment of that southern wolverine population followed the implementation of a moose hunting season in the region where the wolverines have taken up residence – perhaps that season was instituted in response to aggressive moose issues? In any case, there’s a theory that gut piles left by moose hunters might actually be responsible for supplementing wolverines in that region and giving them a foothold in a place where they wouldn’t otherwise be able to persist, proving once again that ecosystems are complex, that humans are embedded within them, and that thinking about complexity, in complex and nuanced ways, is necessary to understanding the world we live in.

Here’s hoping that whatever the Sweden Incident actually was, it doesn’t impede the ability of Swedish wolverine researchers to keep up their good and interesting work on the species.

 

 

 

 

 

 

 

The End of the World As We Know It

Since the Mayan apocalypse is due to hit on Thursday (or is it Friday?), I figure I should get at least one more post out before we are all (possibly) wiped out. Just in case we aren’t, and you are interested in volunteering on a post-apocalypse wolverine project, the Friends of Scotchman Peaks Wilderness in Idaho are looking for people to help them run live traps and cameras this winter. Information can be found here.

The first recorded case of rabies in a wolverine has been documented in Alaska, and an article on the Alaska Fish and Game website provides a thorough explanation of the circumstances. The female wolverine was carrying a strain of Arctic fox rabies and apparently fought with and infected a wolf shortly before she died (she also had a goose egg in her stomach, proving once again that wolverines are indeed versatile in their eating habits.) The incident is notable because it represents a first instance of recorded infection in a species, but the article is worth reading for its deeper exploration of rabies epidemiology in Alaskan fox species, the relationship between rabies outbreaks and ecological processes, and the possible connection between climate change, displacement of Arctic foxes by red foxes, and a potential related change in rabies prevalence.

Several interesting reports and papers have come out over the past few weeks – the 2012 report for the North Cascades Project was released, as well as the most recent update to the Idaho Recreation study. Both are available at the Wolverine Foundation website. And a new paper from Sweden looks at habitat selection in areas where lynx and wolverine overlap. I have not yet had a chance to read through all of these in detail but will report back once I do – provided, of course, that we have not met with fiery doom in the meantime.

 

 

 

 

Colorado Wolverine Presentations This Week

Just a reminder for Colorado folks – Jason Wilmot of the Northern Rockies Conservation Cooperative and the Absaroka-Beartooth Wolverine Project will be giving four talks this week.

Edwards
January 26th, 6-8pm
New Battle Mountain High School

Golden
January 27th, 6-8pm
American Mountaineering Center

Denver
January 28th, 7-9pm
The Denver Zoo

Boulder
January 29th, 6-8pm
REI

Also in wolverine news, a short film of wolverines in the Stockholm Zoo offers another glimpse of wolverines for those who think they might have seen one and are looking for material on ID-ing the species, and/or people who just like an excuse to watch wolverines. This film was apparently picked up by the AP, and I find the caption particularly entertaining: “Officials at the Stockholm zoo put extra snow into the wolverine enclosure after they discovered last year that the animals enjoyed digging tunnels, hiding food and playing.” No offense to the Stockholm Zoo, but did they really not realize prior to this that wolverines like snow? If not, I’m glad they figured it out, because the wolverines look like they’re enjoying themselves – especially the one in the beginning of the film, who seems to be contemplating starting a snowball fight.

About halfway through the film, look for a shot of a wolverine rubbing its belly across the snow – perhaps scent-marking the spot. Wolverines have a pale line of fur stretching down their lower abdomens; I’ve asked about this during collaring operations and no one is clear on exactly why that bit of fur is lighter. In this video, it looks like the wolverine is rubbing that precise spot across the snow, so maybe it has something to do with the chemicals used in scent-marking. We also get a few shots of wolverines digging, so you can see what a wolverine-excavated hole might look like, and, of course, the obligatory glimpse of a gulo hauling a gigantic piece of raw meat.

Another media piece that might prove interesting to gulo fans appeared in the New York Times this weekend. Wolverines are not mentioned, but a suite of other climate-sensitive species are. The article suggests that the effects of climate change on biodiversity are already observable in countries around the world, and are likely to get worse in coming years. It’s a depressing reminder that wolverines are in multitudinous company – up to 30% of the world’s species might be lost as the climate shifts. I dislike hopeless-narratives-of-environmental-apocalypse, but sometimes it feels like there’s no other way to tell it.

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.

Tricksters, Depredation, and One Funny Dog

The wolverine’s reputation as a tricky devil is probably outweighed by that of another, much more common North American carnivore: the coyote, who features in the lore of dozens of Native American tribes as a sly conman, a buffoon, or a serious troublemaker, depending on the occasion.

So it seems fitting that Coyote and Wolverine have teamed up: the Cascades Carnivore Project’s sole gulo scat sample, which the project was having analyzed to determine its sex and genetics, turned out to belong not to a wolverine, but to a member of the Canis latrans family. Apparently the coyote was following the wolverine’s tracks (I’ve seen this as well; a coyote walked in the wolverine tracks I found in Death Canyon in February, confusing the heck out of me for a good mile) and left a deposit.

Determining the sex of the Cascades wolverine was a prerequisite of a contest to give a name to the critter, but in light of the continuing mystery, they settled on the androgynous name of Wildy – certainly among the more fitting names for a wolverine, regardless of sex – and are determined to collect another sample sometime soon. I’m eager to find out, so hopefully Wildy will be good enough to enlighten us soon.

In other gulo news, I was talking to a wildlife biologist who recently returned from Sweden, and he mentioned his surprise at the ability of Swedish wolverines to survive in an intensively managed landscape. All of this is anecdotal, and wolverines are not his species of focus, but he mentioned that in Sweden, no tree is more than 500 meters from a road – a design of the government to enable easy forestry activities – and that despite logging and traffic, wolverines negotiate this landscape successfully. Like most of us, he was surprised that an animal so emblematic of wilderness – to the American mind, anyway – was capable of living in such proximity to industry.

Swedish wolverines also run the risk of retaliatory killings by Sami, whose domestic reindeer herds are vulnerable to depredation. In its commitment to maintaining wolverines, the Swedish government came up with a payment-for-ecosystem-services model to help prevent herders from killing wolverines. If a wolverine dens in a herder’s territory, the herder is paid $30,000 up front, which seems to work better among the Sami than post-depredation compensation programs for wolf kills have worked among American ranchers (the Sami also receive payment for a wolf den on their land – $70,000 – but even this rate of payment isn’t enough to build tolerance for wolves, which scatter reindeer across the landscape – a more grievous difficulty, evidently, than the loss of a few animals to depredation, since it makes herding extremely challenging.) There are apparently some problems with the payment system for wolverine depredations, but the level of payment at least represents a serious commitment to conservation on the part of the Swedes. Fortunately we don’t have to deal with the question of livestock depredation by wolverines here in the US, but several of my herder friends in Mongolia mentioned that wolverines do sometimes prey on sheep and goats. One of the objectives of my project in Mongolia this summer is to determine exactly how prevalent depredation is, and how Mongolians respond to wolverine depredations, as opposed to wolf, lynx, or snow leopard depredations.

Wolverine skull

Finally, we are counting down to Wolverine Night festivities on Thursday! The office is overflowing with gulo-related materials, including a skull and pelt that Wyoming Game and Fish was good enough to lend to us. All of us were excited (but a little sad) at the opportunity to handle a pelt and skull, with the exception of Targhee, Lydia’s dog, who growls and barks every time he sees the pelt. Lydia says that Targhee dislikes ‘animal parts that are not attached to animals,’ but maybe, since he’s a herd dog, it’s simply his livestock-defending heritage kicking in.