Resource-related interactions between wolverines and brown bears in Scandinavia

1. What they did

Intense persecution in the 1900s confined Sweden’s wolverines to the northern mountains. Now they are recolonizing boreal forest to the south and east, into the core of the brown bear range, where bear density is high (about 60 bears per 1,000 km²). Both species scavenge moose, so the researchers asked whether the two meet over food.

They monitored five GPS-collared wolverines: two males and three females, all three females with cubs. The collars recorded a location every hour for about three weeks in late spring of 2019 and 2020; one animal was tracked for only 9 days because the collar failed. Whenever a wolverine’s locations clustered, a field team visited within three days. They recorded what had been eaten, the likely cause of death, and whether bears had been there.

In 2020, they also set trail cameras at four sites. Three were “rendezvous sites,” where mothers leave cubs hidden with cached food while they forage. The fourth was an adult moose that a bear appeared to have killed.

2. What they found

Feeding sites

• The team checked 165 clusters and found 115 wolverine feeding sites and 46 bed sites.
• Moose remains were the most common food, at 80 feeding sites: calves at 35, adults at 24 and unknown age at 21. Forest grouse came next, at 30.
• Of 16 clusters with adult moose remains, five appeared to be bear kills, three were hunters’ leftovers (sawn-off bones) and eight had no clear cause.
• Bears had been at 25 of the 115 feeding sites (22%).

Fresh bear kills

• Wolverines made five visits to the area around the two fresh bear kills (less than a week old), lasting from 2 hours to about 7 days.
• In one case, a wolverine stayed about 7 days within 3 km of the carcass, using several bed sites, and returned three days later for a 2-hour visit.

Moose carcass camera

• At least three wolverines visited: the collared male, an uncollared adult the authors think was a female, and a smaller uncollared animal, presumably her cub.
• The male and the female were seen together without aggression, but the cub was never there when the male was present. Males are known to kill cubs that are not their own.
• A bear arrived 2.5 hours after a wolverine visit, fed for about half an hour and carried off most of the carcass.
• An uncollared wolverine arrived 34 minutes after the bear left and fed on the scraps.

Rendezvous site cameras

• Mothers alone made brief visits (under 3 minutes), mostly at night. Visits with cubs were longer (about 15 minutes on average) and mostly by day.
• At every site, the mother ate or carried off the cached food.
• At one site, the cubs were left alone for about 2 hours. One cub spent 16 minutes outside and then went into a cavity under the rocks.
• Bears investigated two of the three rendezvous sites, once just 1.5 hours after the wolverine family left. It could not be confirmed whether the bears took any cached food.
• No direct encounters between wolverines and bears were recorded.

3. What they concluded

• Food provider: bears may be an extra source of carrion for wolverines where the two overlap, especially in spring. Wolves play this role further south in Scandinavia, and lynx in the north. Bear kills may help females raise cubs.
• Risk: bears are drawn to the same carcasses and caches. This could put cubs at risk, especially when they are left alone at rendezvous sites. The authors temper this: the cubs stayed mostly hidden under rocks, and mothers brought cubs to bear-killed carcasses, which would be counterintuitive if bears were a major threat.
• Overall: the relationship likely involves both competition and facilitation.
• Next steps: the interactions need to be studied in other seasons. The authors expect the two species also meet at hunting slaughter piles, since bears commonly scavenge hunting remains.

4. How tracking was or was not used

No trailing was reported. The researchers used track and sign identification to interpret what had happened at GPS clusters:

• Prey species: identified from hair, bones, hooves and feathers.
• Cause of death: judged from species-specific marks such as tracks, hair and claw marks (carnivore kills), or sawn-off bones (hunter kills).
• Fresh kills: the two bear kills were judged to be less than a week old, for example from signs of struggle in the vegetation.
• Bear presence: recorded from scat, hair, claw marks, and bear behaviour such as covering a carcass with vegetation.
• Camera-site moose: judged to be a bear kill from removed vegetation and trampled ground.
• Cache sites: body parts hidden under boulders or logs, or dug into holes. There were 22 cache sites.
• Bed sites and rendezvous sites: recognized by bed impressions, wolverine hair in cavities, and cached food.

The cluster-investigation method is cited from earlier Scandinavian predation studies. Beyond those citations, the paper gives no criteria, confidence levels or details on how sign age was judged, even though the bear-kill and bear-presence findings depend on this interpretation.

5. Tracker or observer certification

Not reported. No tracker qualification or evaluation is described, although the bear-kill and bear-presence findings rest on sign interpretation. E. Sveinsen is thanked for “proficient work in the field,” and T. Jonsson and B.-E. Göransson for capturing and collaring the wolverines.

6. Other technologies, skills or innovations

• GPS collars (Vectronic Vertex Lite GSM) taking hourly locations
• Standardized cluster analysis in R (ClusterApp)
• Motion-triggered cameras (Reconyx HyperFire PC800) at knee height, taking three-photo bursts. They produced 15,112 photos: 2,049 of wolverines and 1,999 of bears.
• Live capture with log-box traps and chemical immobilization (ketamine–medetomidine)
• Collared animals identified by their collars; cubs identified by their smaller size
• A limitation: the camera setup could not identify uncollared wolverines from their chest patterns.

7. Implications for conservation (from the paper)

• Recovering carnivores are bringing back interactions between species that had disappeared.
• Bear kills may help wolverine mothers feed cubs. At the same time, bears visiting cache and rendezvous sites may put cubs at risk.
• More research is needed in other seasons and on the underlying mechanisms.