Wolf Pack Territory
By studying territory size of wolf packs in Montana, Dr. Sarah Sells and colleagues figured out that wolves carve out territories “economically” (4). In other words, wolves balance the costs and benefits of different size territories.
The benefits of a territory are the sole access to the prey in a territory (i.e., no other wolf packs can access or kill that prey) while the cost is continually defending a territory from other wolves. To balance the cost and benefits, wolves choose the smallest suitable territory possible with a suitable territory being one with enough prey for the pack to survive.
By Dr. Thomas Gable, Voyageurs Wolf Project
Like many ecological questions, this one seems deceptively simple: how do wolf packs figure out how big their territory should be? Reaching an answer is anything but simple. However, by combining fieldwork, advanced GPS-tracking technology, large datasets, and some snazzy statistics, recent research published in 2021 provides one of the best answers to this question so far.
Biologists have known for decades that wolf pack territories vary considerably in size depending on the ecosystem. For example, wolves in southern boreal forest ecosystems can have territories as small as 7.5 mi2 whereas wolves in northern Arctic ecosystems can have territories that surpass 1,000 mi2 (1,2).
These large differences in territory size are largely a result of how far north or south wolf packs are living. Ecosystems in the far north, like the arctic, are less productive because of shorter growing seasons, meaning those ecosystems cannot support as many large prey as more southern ecosystems. And when there are less prey, wolves must maintain bigger territories to ensure they have sufficient food.
More interestingly, however, is the variation in territory size even within the same ecosystem. Wolf packs in Minnesota, for example, can have territories that range from 7.5 mi2 to >214 mi2— a 28 fold difference in territory size (1,3)! Naturally, biologists have wondered for decades why there is such variation in territory size within the same ecosystems.
Economical Wolves
By studying territory size of wolf packs in Montana, Dr. Sarah Sells and colleagues figured out that wolves carve out territories “economically” (4). In other words, wolves balance the costs and benefits of different size territories.
The benefits of a territory are the sole access to the prey in a territory (i.e., no other wolf packs can access or kill that prey) while the cost is continually defending a territory from other wolves. To balance the cost and benefits, wolves choose the smallest suitable territory possible with a suitable territory being one with enough prey for the pack to survive.
Glacier National Park, Montana
Specifically, researchers found a strong connection between wolf territory size and the density of prey in territories. On average, smaller wolf territories had a higher density of prey (more food per area) than larger wolf territories, providing strong evidence that wolves adjust their territory size to ensure there is enough food to eat. This finding aligns with recent research from northern Ontario that identified a similar trend in territory size and prey density (5).
Interestingly, the amount of “low-use” roads was also connected to territory size. Territory size generally got smaller as the density of low-use roads increased. Low-use roads make it easier for wolves to travel, and an increased ability to travel means wolves can find vulnerable prey and defend their territory more readily. And the easier wolves can find prey, the smaller the territory they need to have enough food. It is possible that roads might help wolves defend their territory more efficiently.
And then there are the neighboring wolves—the competitors. As the number of neighboring wolves increase, defending a territory becomes more arduous and challenging for a pack. The researchers of this study found that wolf territories generally get smaller when there are more competitors around because a smaller territory is easier for a wolf pack to defend.
Wolves appear to recognize that there is little reason to have a larger territory than necessary because the effort or “cost” of defending that territory will only increase as the territory gets bigger. And the cost of defending a territory will get higher as the number of neighboring wolves (i.e., competitors) increases.
The authors also noted that larger packs in their study generally had smaller territories, which is perplexing. Why would territories get smaller as the number of wolves in a pack gets bigger? Wouldn’t a larger pack need more food and, in turn, a bigger territory?
The researchers agreed that the finding appears counterintuitive; however, they suggest that larger packs might not need bigger territories. Instead, they suspect that larger packs might be able to kill prey more efficiently and frequently than smaller packs. This increase in efficiency would increase the amount of food for each wolf in a pack.
Regardless, there is not consensus around the connection between the number of wolves in a pack and territory size. For example, previous research found that wolf territories increased in size as pack size increased—which is the opposite of this most recent research—while another study found no connection between the number of wolves in a pack and territory size (5,6).
Contrasting data such as this is not uncommon when studying wildlife like wolves that occupy a variety of ecosystems such as deserts, boreal forests, and the arctic. Perhaps the relationship between the number of wolves in a pack and pack territory size is dependent on the ecosystem or certain environmental and ecological characteristics. Or perhaps one or two of these studies reached incorrect conclusions. Whatever the reason, this highlights that there is much we do not fully understand about the ecology of wolves—and more excitingly, that there is still much to be studied and learned!
This article was written by Dr. Thomas Gable of the Voyageurs Wolf Project as part of an educational partnership with the International Wolf Center.
Sources:
- Mech, L. D. and S. Tracy. 2004. Record high wolf, Canis lupus, pack density. Canadian Field Naturalist 118:127-129.
- Lake et al. 2015. Wolf (Canis lupus) winter density and territory size in a low biomass moose (Alces alces) system. Arctic 68:62-68.
- Fritts, S.H. and L. D. Mech. 1981. Dynamics, movements, and feeding ecology of a newly protected wolf population in Northwestern Minnesota. Wildlife Monographs 80: 3-79.
- Sells et al. 2021. Evidence of economical territory selection in a cooperative carnivore. Proceedings of the Royal Society B 288: 20210108.
- Kittle et al. 2015. Wolves adapt territory size, not pack size to local habitat quality. Journal of Animal Ecology 84: 1177-1186.
- MacDonald, D. W. 1983. Science. The ecology of carnivore social behavior. Nature 301: 379-384.
