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Why are Some Wolf Packs Large and Others Small? What Factors Influence Pack Size?

Lisa Matthews / Wolf Song of Alaska / Volunteer

Almost all predators fall into one of two groups: either they are similar in size to their prey and can challenge it one-on-one, or they are smaller than their prey and therefore must hunt in groups. Wolves fit into the second category. They are a group hunter preying primarily on animals much larger than themselves. Although a single wolf may be able to kill even an adult moose, caribou or elk, it is much safer, easier, and more reliable for several animals to do the job.

The number of wolves in a pack varies greatly and it is interesting to contemplate the reasons for such variability. A pack may have between 2 to 30 wolves (the average is about 10), but may even reach 36 with mostly related wolves. Red wolves are less social than gray wolves and usually form packs of just three or more (the mated pair and offspring). Packs are formed when two wolves of the opposite sex (who may have dispersed from their natal packs) develop a bond, breed, and produce a litter of pups. These pups, particularly with gray wolves, will become the future aunts and uncles to any further offspring of their parents. Packs, or families, are comprised of mostly related wolves.

Within any area, some variations in pack size are due to differences in birth and death rates. Of course, there is more to the story. There appear to be four factors affecting pack size, all of which stem from an ecological basis and have resulted in various behavioral adaptations in the wolf. These are: (1) the smallest number of wolves required to locate and kill prey safely and efficiently, (2) the largest number of wolves that could feed effectively on any one particular prey, (3) the number of other pack members with which each wolf could form social bonds, and (4) the amount of social competition that each wolf in the pack could accept.

The fact that wolves do live in packs and kill prey much larger than themselves suggests that the first factor is operating. During the evolution of pack formation, those ancestors of wolves who hunted together tended to survive longer than nonsocial wolves and, consequently, produced more pups with similar social tendencies than the nonsocial wolves (as behavior is partly genetic and partly what is learned in the environment). However, it appears that this factor operates only generally because packs vary greatly in size, and because large packs may not operate as efficiently as possible. Within a larger pack, rarely is the whole pack in on the kill and, usually, only few animals actually come in contact with the prey while other individuals may only be important in helping to locate, chase, or harass the prey. Larger packs might also split up temporarily and hunt in smaller groups.

The second factor that seems to affect pack size is the amount of prey available to feed the all the wolves in the pack. A pack might be so large that, after all the highest ranking members had finished eating, there wouldn't be anything left for the subordinates. In such a situation, hungry ones would go off to hunt again. This factor would limit pack size if other factors did not. Even so, other factors do operate as well, for most packs contain fewer members than would seem necessary. In other words, most of the time there is enough food left over from a kill which could allow for bigger pack size than what usually occurs.

So, why aren't packs bigger? It appears that the above two factors act only as secondary controls. Actual pack size appears more regulated by the third and fourth factors, which are the two social factors. Wolves possess a high potential for forming social bonds, and the social-attachment factor would tend to increase the number of wolves in a pack. It probably accounts for the large packs that contain more members than necessary. However, the larger the pack the greater the competition is for important resources, such as food and mates. There is also more competition for leadership and dominance the larger the group gets. When competition becomes too tense, pack organization can be detrimentally disrupted. In these cases the pack cannot function efficiently and some members are forced to leave. Apparently, it is this social-competition factor that limits the number of wolves in a pack to less than what could feed effectively on a prey animal. Dispersing wolves go off, find a mate and form their own packs.

There have been two other hypotheses proposed regarding the number of members in a wolf pack. One of these hypotheses indicates that variation in pack size is linked to the type of habitat the wolves reside in. All of the wolf species living in larger groups are found in open vegetation. This makes sense because in order to hunt cooperatively the animals must fan out, maintain contact with other individuals, and adjust positions during pursuits. Such behaviors could not occur in a dense habitat. This provides one reason why red wolves would form smaller packs than the gray wolves, as red wolves tend to occupy denser habitats than gray wolves. The second hypothesis states that among predatory species, population group size increases with prey size. The more a pack depends on large ungulates (such as moose, caribou, and deer), the larger the pack becomes, and the more cooperation between members is required. Unlike gray wolves who hunt mostly large prey, red wolves depend mostly on smaller animals and, therefore, do not require large groupings to hunt down large prey. Because of differing hunting habitats and kinds of prey available, the red wolf does not need to learn to hunt cooperatively to the extent gray wolves do. What a beautiful example of how ecology has affected the evolution of social behavior and cooperation!

Both of the above hypotheses are probably occurring together, just as the four factors mentioned earlier are playing a part in wolf pack size. Proposing one hypothesis, or factor, does not exclude the other. There may even be more reasons yet to be discovered. The ecological pressures placed on wolves, indeed all living things including humans, are very complex and are always changing and unfolding. An attempt to understand certain aspects of behavior, such as why wolf packs vary in size, must always include an account of a multitude of ecological factors, for it is in this way that we can only hope to understand why animals behave the way they do.