The interactions between apex predators and their prey are some of the most awesome and meaningful in nature—displays of strength, endurance, and a deep coevolutionary history. And there is perhaps no apex predator more impressive and important in its hunting—or more infamous, more misjudged—than the wolf. Because of wolves’ habitat, speed, and general success at evading humans, researchers have faced great obstacles in studying their natural hunting behaviors. The first book to focus explicitly on wolf hunting of wild prey, Wolves on the Hunt seeks to fill these gaps in our knowledge and understanding.
Combining behavioral data, thousands of hours of original field observations, research in the literature, a wealth of illustrations, and—in the e-book edition and online—video segments from cinematographer Robert K. Landis, the authors create a compelling and complex picture of these hunters. The wolf is indeed an adept killer, able to take down prey much larger than itself. While adapted to hunt primarily hoofed animals, a wolf—or especially a pack of wolves—can kill individuals of just about any species. But even as wolves help drive the underlying rhythms of the ecosystems they inhabit, their evolutionary prowess comes at a cost: wolves spend one-third of their time hunting—the most time consuming of all wolf activities—and success at the hunt only comes through traveling long distances, persisting in the face of regular failure, detecting and taking advantage of deficiencies in the physical condition of individual prey, and through ceaseless trial and error, all while risking injury or death.
By describing and analyzing the behaviors wolves use to hunt and kill various wild prey—including deer, moose, caribou, elk, Dall sheep, mountain goats, bison, musk oxen, arctic hares, beavers, and others—Wolves on the Hunt provides a revelatory portrait of one of nature’s greatest hunters.
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Wolves on the Hunt
The Behavior of Wolves Hunting Wild Prey
By L. David Mech, Douglas W. Smith, Daniel R. MacNulty
The University of Chicago PressCopyright © 2015 University of Chicago
All rights reserved.
To the public, and even to some wildlife biologists, the white-tailed deer is seen as easy prey for wolves. This view probably results from the fact that deer represent one of the smallest of wolf prey and because wolves are large and live in packs. Compared superficially with moose or bison, deer would seem to be much easier prey. However, to capture and kill deer, wolves must first find them, catch up to them, and confront them. At each step in this process deer possess effective antipredator strategies. These traits help explain why deer and wolves can continually coexist without either one going extinct (Olson 1938; Stenlund 1955; Mech 2009), except under unusual circumstances (Mech and Karns 1977; Nelson and Mech 2006).
Science may know more about the interactions between wolves and white-tailed deer than it does about wolf relations with any of its other prey, except moose and elk. This is the case for two main reasons. First, some of the earliest studies of wolves were conducted where white-tailed deer were the wolf's main prey (Olson 1938; Stenlund 1955; Mech 1966b; Rutter and Pimlott 1968; Pimlott et al. 1969; Mech and Frenzel 1971). Second, much of the information results from the only long-term, intensive study that has employed radio tracking of both wolves and their prey. That study of wolves began in 1968 in the Superior National Forest of northeastern Minnesota, and incorporated radio tracking of deer starting in 1973. Through 2006, this study had livetrapped 712 wolves and radio-collared most of them, and through 2007, radio-collared 347 deer (Mech 2009). Thus numerous publications have synthesized and discussed wolf behavior hunting deer (Mech 1970, 1984; Mech and Frenzel 1971; Nelson and Mech 1981, 1993), and many others have synthesized information about wolf-deer interactions in general (Kolenosky 1972; Fritts and Mech 1981; Kunkel and Pletscher 1999; Theberge and Theberge 2004; DelGiudice et al. 2009; Ballard 2011).
Here we highlight the major findings from the above studies of wolf hunting behavior and update them with information published since and with additional information gleaned from the many unpublished accounts and observations below. Unless otherwise mentioned, information in this chapter applies only to white-tailed deer.
In current wolf range, white-tailed deer are distributed throughout the Southwest, Midwest, and West of the United States and throughout southern and northwest Canada, with densities lower in extreme northwest Canada. Thus wolves prey on deer in many areas. That deer have been able to extend their range farther and farther north in the face of wolf predation (Heffelfinger 2011) is evidence of the species' considerably effective antipredator traits and behavior.
Adult female deer that live in wolf range generally weigh 70–80 kg (Mech and McRoberts 1990), and males weigh up to 180 kg (Sauer 1984). Only males possess antlers, which are fully hardened in fall and retained for a few months of fall and winter. All deer except newborns sport small, sharp, pointed hooves, which can kill a wolf (Frijlink 1977; Nelson and Mech 1990) or can alert other deer to danger (Caro et al. 1995), and all possess a conspicuous fanlike tail that is white underneath and that the deer "flags" or raises when disturbed.
Most deer occupy home ranges of about 0.4–7.0 km2, with most females remaining close to where they are born while males disperse (Nelson and Mech 1981). Females generally produce their fawns relatively close to where they themselves were born, and a matriarchal society develops around them with offspring home ranges often overlapping those of their mothers (Nelson and Mech 1981). Thus many deer home ranges fit within a single wolf-pack territory (fig. 1.1). Like most northern ungulates, deer begin gaining weight in spring and reach their peak weight in midautumn and their nadir in late winter or early spring (DelGiudice et al. 1992).
The weight and nutritional condition that deer lose throughout winter is directly related to snow depth and temperature (Verme 1963). Further, if winter conditions are extremely poor (deep snow primarily) or favorable, there is evidence that the extreme effect (positive or negative) on an individual deer's nutritional condition can accumulate over consecutive winters (Mech et al. 1987). Thus a series of severe winters could result in deer of poor condition, or vice versa. (This finding has since been both challenged [Messier 1991, 1995; Garroway and Broders 2005] and supported [Feldhamer et al. 1989; McRoberts et al. 1995; Post and Stenseth 1998; Patterson and Power 2002]).
There is also evidence that the extreme effects of snow depths can even persist across generations of deer, such that deer whose grandmothers lived through a winter of deep snow while gravid with offspring produced fawns whose own offspring (the F generation) were in poorer condition regardless of the condition of the mothers (i.e., the F generation [Mech et al. 1991]). Although this finding seems surprising, it was confirmed by Monteith et al. (2009) for deer and by Lumey and Stein (1997) and Bygren et al. (2001) for humans. (A possible mechanism is epigenetics [Pembrey 1996]). Of course, a prey animal's condition is extremely important to escaping predators.
In much of their northern range, many deer migrate between their summer ranges and winter "yards" or areas where deer from as far as 40 km away (Nelson and Mech 1981) concentrate. Usually by late winter most deer have migrated to winter yarding areas if snow is deep enough and/or temperatures low enough (Nelson and Mech 1981). In spring, deer migrate back to their summer ranges as snow melts and temperatures rise.
At least one important function of this yarding is for each individual to reduce its risk of predation. Nelson and Mech (1981, 36) explained this functioning as follows:
Yarding behavior provides several significant antipredator benefits: (1) The mere congregation of many animals creates a system of trails which can become escape routes during chases by predators. (2) Herding provides greater sensory capability and makes gregarious animals less vulnerable than solitary individuals to undetected predator approach (Galton 1871, Dimond and Lazarus 1974, Treisman 1975, and references therein). (3) Social grouping may confuse the search image of predators (McCullough 1969). One way such confusion may operate is suggested by observations of 15 deer groups in our study area chased by wolves in winter (Mech unpublished). The groups tended to split up when closely pursued, just as moose do (Mech 1966[a]). Such a maneuver could give group members an added chance of survival as the wolves tried to choose which individual to chase. (4) Grouping probably would expose the more vulnerable members when predators test the group. This would tend to place the burden of predation on older animals (Pimlott et al. 1969, Mech and Frenzel 1971, Mech and Karns 1977) that already have contributed genetically to the population, thereby increasing the chances of their own offspring to survive and breed. (5) Congregating would increase the ratio of deer to wolves in and near the yard, thus decreasing relative predation level through a sheer mathematical effect (Brock and Riffenburgh 1960). However this also has an important biological effect. Deer that summer in several wolf-pack territories may assemble in 1 yard. For example, deer that wintered in [one yard] summered around the edges of at least 4 wolf-pack territories. Although the wolves in whose territories the deer summer may also try to concentrate in or near the yard, their territorial nature tends to minimize the number of packs that can frequent a given yard. Competing packs will seek each other out and fight (Mech unpublished), often leading to the deaths of alpha animals [breeders] (Mech 1977d) [Mech 1977c]. (6) Individual herd members may have to spend less time alert and thus may be able to spend more time eating or ruminating than solitary individuals, as has been found in several other species (summarized in Hoogland 1979).
Besides their grouping in winter, and "spacing away" (i.e., spreading out into individual home ranges) in summer when fawns are most vulnerable, deer possess many other antipredator defenses. Newborn fawns, which are vulnerable if discovered by even small predators like foxes or fishers, are cryptically colored, thus matching the forest floor. They freeze in place motionless for their first week or so except when with their dam nursing. They also seem either to be odorless (which seems improbable) or to have their odor masked such that predators have great difficulty finding them. For example, dogs have been observed passing right by them (Severinghaus and Cheatum 1956). When fawns reach 1–2 weeks of age, they no longer freeze in place but rather spring up when disturbed and bolt off at least fast enough that humans cannot catch them. They also bleat noisily, which no doubt serves to call the doe to their defense. Although no one seems to have observed a doe defending her young fawn against a wolf or wolves, a single deer has been known to stand off three wolves (Nelson and Mech 1994), and a doe was seen following a black bear that was carrying off her fawn (L. L. Rogers, pers. comm.). These observations suggest that does are very protective of their fawns. Does are also known to attempt to lure predators away from their fawns through a kind of "broken leg" act (Severinghaus and Cheatum 1956; Mech 1984).
Adult deer also possess many effective antiwolf traits, contrary to Stenlund (1955). Any deer hunter knows how keen deer vision, hearing, and scenting abilities are. These senses evolved in the face of wolves and other predators. And deer employ their senses well, usually by either running off as soon as they detect wolves or waiting until they confirm that wolves are approaching and then bolting off. Deer in dense cover are more wary and tend to flee sooner than those in more open cover, presumably because it is easier to detect an approaching enemy in the latter (LaGory 1986, 1987). As they flee, they "flag" their white tails. This flagging may demonstrate to wolves that the deer are fleet enough to escape, a potentially honest signal of condition that may inform wolves that it is useless to continue the chase (Caro et al. 1995).
When deer flee, they can speed away at 56 km/hr (Young and Goldman 1944; Severinghaus and Cheatum 1956), about the same speed as wolves (Stenlund 1955; Mech 1970). If wolves are close enough when the deer bolts away, they try to follow. However, most often, the deer bounds away, leaving the wolves behind. Usually the deer quickly senses that it is outrunning the wolves and then stops and watches its backtrail. If the wolves do persist, the deer bursts off again. It appears that the deer keeps trying to save its energy for when it really needs it.
Only rarely do wolves persist in chasing a deer for very long. However, if they do, as in one case of a single 2.5-year-old female wolf pursuing a deer for at least 20.8 km over a 2-hr period (Mech and Korb 1978), the deer was still able to stay ahead of the wolf. Unfortunately, the observation ended without the observers determining the outcome. The pursuing wolf and its pack were seen the next day feeding on a deer kill some 4.8 km from where the observation ended (Mech unpublished). Whether the kill was the same deer is unknown, but this incident is evidence of the deer's running endurance.
In some instances when a wolf or wolves do catch up to or confront a deer, then the deer employ other defenses. Bucks in autumn or early winter can kill wolves with their antlers (Nelson and Mech 1985), and adult deer at any time of year can kill with their hooves (Frijlink 1977; Nelson and Mech 1990). Thus some deer when confronted by wolves merely stand their ground and fight off the wolves (Mech 1984; Nelson and Mech 1994). These observations contrast to those of a sample of 16 white-tailed deer being approached by coyotes; those deer all fled (Lingle and Pellis 2002).
Another approach deer use to escape an attack by wolves is to flee into water (Joslin 1966; Pimlott et al. 1969; Mech 1970). Although a wolf has been observed killing a deer while swimming after it (account 47), fleeing to water is usually effective. The safety of large waterways probably explains why deer tend more to inhabit shores of lakes, points, and other edges of waterways during summer (Hoskinson and Mech 1976; Nelson and Mech 1999).
Besides the tendency for deer to live close to lakes, rivers, and other waterways for protection from wolves, another major way that white-tailed deer reduce their chances of predation by wolves, at least in Minnesota's Superior National Forest, is through their occupation of buffer zones between wolf-pack territories (Mech 1977a, 1977b). These areas around the periphery of wolf-pack territories constitute regions about 2–6 km wide where deer survive better than in the territory centers (Hoskinson and Mech 1976; Mech 1977a, 1977b; Rogers et al. 1980; Nelson and Mech 1981). The theory is that in these overlap zones, wolves have the best chance of encountering members of neighboring packs and thus are subject to altercation and death (Mech 1994a; Hayes 1995; Mech et al. 1998). Thus wolves spend less time in these buffer zones than in their territory center (Mech and Harper 2002). Each wolf visited a given deer home range along the edges of their territory an average of only about once per 20 d (Demma et al. 2007; Demma and Mech 2009b). Wolves also spent at least 1 hr/visit during five of eight recorded visits to these deer home ranges (Demma et al. 2007). Although comparable data for deer deep inside a wolf-pack territory have not been available, wolf visits there no doubt would be much more frequent.
The deer summer spatial organization is based on a matriarchy in which female offspring remain near their mothers as they mature and begin breeding, forming a close-knit cluster of mothers and offspring home ranges (Nelson and Mech 1984, 1999). Apparently fawn survival is greater in these buffer-zone clusters than in wolf-pack territory centers, so these deer demes (Nelson and Mech 1987) persist there, whereas fawn survival is less in wolf-pack territory centers (Kunkel and Mech 1994), so deer demes are fewer, smaller, or nonexistent there. The end result is that deer tend to be found more along edges of wolf-pack territories than in centers. Evidence of similar wolf-deer dynamics within buffer zones was also found at the opposite end of Minnesota, where deer density was higher and wolf density lower (Fritts and Mech 1981). There is also theoretical evidence that buffer zones may be prey refuges (Lewis and Murray 1993) and that territorial stability in such zones requires interpack aggression by wolves (Taylor and Pekins 1991) as has been found (Mech 1994a). Thus, this type of deer refuge appears to be somewhat general. The extent to which it might apply to other prey species has not yet been tested.
Given the many antipredator traits, behavior, and other defenses that help deer survive in the face of wolf densities that can reach 182/1,000 km2 (Mech and Tracy 2004) and pack sizes as high as 23 (Mech 2000), it is not surprising that deer do not often seem to allow wolf-predation risk to interfere with where they choose to forage (Kittle et al. 2008), contrary to Lima and Dill (1990) and Brown et al. (1999). Some deer even live as close as 800 m to active wolf homesites (Nelson and Mech 2000).
As deer in wolf range go about their daily lives, then, wolves are never far away and at any time may threaten any deer. The result is a constant tension between the two species, with each trying to survive by outdoing the other. This vital game is exemplified in each of the following hunting accounts based primarily on observations during winter from light, fixed-wing aircraft circling over the unfolding dramas in the Superior National Forest of northeastern Minnesota (unless otherwise indicated). Many of these observations resulted from Mech or his assistants homing in on radio-tagged wolves and finding them hunting deer. Those observations not made by Mech are attributed at the beginning of each to those who made them or reported them. The wolf identification numbers are given in each account, but the age and sex of the radioed wolf is given only where that animal is the only one involved in the attack. Where there are additional pack members, the identities of the accompanying wolves are unknown. Unless mentioned otherwise, the sex and age of the deer were unknown. The word "we," when used in the aerial observations, usually includes the pilot.
Excerpted from Wolves on the Hunt by L. David Mech, Douglas W. Smith, Daniel R. MacNulty. Copyright © 2015 University of Chicago. Excerpted by permission of The University of Chicago Press.
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Table of ContentsForeword
Introduction: The Wolf as a Killing Machine
Chapter 1—White-Tailed Deer
Chapter 5—Mountain Sheep and Goats
Chapter 7—Musk Oxen
Chapter 8—Miscellaneous Prey
Appendix: List of Scientific Names of Birds and Mammals Mentioned
A Note on Accompanying Videos by Robert K. Landis
Videos of wolf-prey interactions, by Robert K. Landis, are available to readers of the print book at the following URL and with these password credentials:
User name: wolves
Readers of the ebook will find the videos embedded in the text.