Wolves: Behavior, Ecology, and Conservation / Edition 1 available in Paperback
Wolves are some of the world's most charismatic and controversial animals, capturing the imaginations of their friends and foes alike. Highly intelligent and adaptable, they hunt and play together in close-knit packs, sometimes roaming over hundreds of square miles in search of food. Once teetering on the brink of extinction across much of the United States and Europe, wolves have made a tremendous comeback in recent years, thanks to legal protection, changing human attitudes, and efforts to reintroduce them to suitable habitats in North America.
As wolf populations have rebounded, scientific studies of them have also flourished. But there hasn't been a systematic, comprehensive overview of wolf biology since 1970. In Wolves, many of the world's leading wolf experts provide state-of-the-art coverage of just about everything you could want to know about these fascinating creatures. Individual chapters cover wolf social ecology, behavior, communication, feeding habits and hunting techniques, population dynamics, physiology and pathology, molecular genetics, evolution and taxonomy, interactions with nonhuman animals such as bears and coyotes, reintroduction, interactions with humans, and conservation and recovery efforts. The book discusses both gray and red wolves in detail and includes information about wolves around the world, from the United States and Canada to Italy, Romania, Saudi Arabia, Israel, India, and Mongolia. Wolves is also extensively illustrated with black and white photos, line drawings, maps, and fifty color plates.
Unrivalled in scope and comprehensiveness, Wolves will become the definitive resource on these extraordinary animals for scientists and amateurs alike.
“An excellent compilation of current knowledge, with contributions from all the main players in wolf research. . . . It is designed for a wide readership, and certainly the language and style will appeal to both scientists and lucophiles alike. . . . This is an excellent summary of current knowledge and will remain the standard reference work for a long time to come.”—Stephen Harris, New Scientist
“This is the place to find almost any fact you want about wolves.”—Stephen Mills, BBC Wildlife Magazine
|Publisher:||University of Chicago Press|
|Edition description:||New Edition|
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About the Author
L. David Mech is a senior research scientist with the Biological Resources Division, U.S. Geological Survey and adjunct professor in the Department of Fisheries, Wildlife, and Conservation Biology and the Department of Ecology and Behavioral Biology at the University of Minnesota. He is the author of The Wolf: The Ecology and Behavior of an Endangered Species, The Way of the Wolf, and The Arctic Wolf, among other books, and is coauthor of The Wolves of Denali. Luigi Boitani is a professor of vertebrate zoology and animal ecology at the University of Rome. He is the author of Dalla parte del lupo, coauthor of Simon and Schuster’s Guide to Mammals, and coeditor of Research Techniques in Animal Ecology.
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WolvesBehavior, Ecology, and Conservation
By L. David Mech
University of Chicago PressCopyright © 2007 L. David Mech
All right reserved.
L. David Mech and Luigi Boitani
THE FIRST REAL BEGINNING to our understanding of wolf social ecology came from wolf 2204 on 23 May 1972. State depredation control trapper Lawrence Waino, of Duluth, Minnesota, had caught this female wolf 112 km (67 mi) south of where L. D. Mech had radio-collared her in the Superior National Forest 2 years earlier. A young lone wolf, nomadic over 100 km2 (40 mi2) during the 9 months Mech had been able to keep track of her, she had then disappeared until Waino caught her. From her nipples it was apparent that she had just been nursing pups.
"This was the puzzle piece I needed," stated Mech. "I had already radio-tracked lone wolves long distances, and I had observed pack members splitting off and dispersing. My hunch was that the next step was for loners to find a new area and a mate, settle down, produce pups, and start their own pack. Wolf 2204 had done just that."
During the decades since, we have seen this process many times, and it represents one of the primary ways in which wolves become breeders (Rothman and Mech 1979). However, there are several other ways, and it is only now, after 25years of study and the wedding of wolf radio-tracking with biochemical analyses of wolf genetics (see Wayne and Vila, chap. 8 in this volume), that we seem to have a reasonably complete picture of wolf social ecology (Meier et al. 1995; D. Smith et al. 1997; Mech et al. 1998).The basic social unit of a wolf population is the mated pair. Known variations include a mature male and two mature females; a mature male, his yearling son from a previous mating, and a new mate; and a mature female with a new mate and his younger brother (Mech and Nelson 1990b). There is no reason to believe that other similar combinations of a mated pair with various relatives of one or both members are not also possible.
There are two reports of packs of males, but these packs are not well documented or understood, and presumably are temporary until a mate is found. Ballard et al. (1987) reported without documentation that a pack of three males occupied a 3,077 km2 (1,200 mi2) area of Alaska for over a year. Two radio-collared males split off from a Montana pack and lived together from June to September before being joined by a third animal of unknown age and sex (Ream et al. 1991).
The most unusual type of pack ever recorded formed in Yellowstone National Park 7 years after wolf reintroduction (D. W. Smith, unpublished data). During winter 20012002, three packs were formed of various assortments of at least twelve dispersers from four packs. Each new pack included a Druid Peak pack female born in 1997. Individuals moved among these packs, sometimes daily. By late spring, one pack contained two males from the Chief Joseph pack and four Druid Peak females. These wolves produced two litters in separate dens, merged in midsummer into six adults and four pups, and remained such at least into winter. Less is known about the other two new packs.
Mech also once recorded an adult male, his yearling son, and his three pups remaining together for 10 weeks after his mate (wolf 5091) was killed by other wolves (Rothman and Mech 1979). This situation can be considered a temporary exception; a new mature female (5079) joined the pack after 10 weeks and remained with it, producing pups the next spring.
The natural extension of the mated wolf pair is the pair with its collection of offspring, or family, as earlier workers surmised (Olson 1938; Murie 1944; Young and Goldman 1944) and numerous radio-tracking studies have documented. In a thriving population, a wolf pair produces pups every year (Fritts and Mech 1981; Mech and Hertel 1983; Peterson, Woolington, and Bailey 1984; but cf. Mech 1995d). The offspring usually remain with their parents for 1054 months, but except under special circumstances, all offspring disperse (Gese and Mech 1991; Mech et al. 1998). Packs therefore may include the offspring of as many as 4 years. A wolf pack, then, is some variation on a mated pair, and packs have contained as many as forty-two members, although most include far fewer (see table 1.1).
One poorly understood exception to the above basic rule is that strange wolves sometimes join packs already containing a breeding pair, at least temporarily (Fritts and Mech 1981; Peterson, Woolington, and Bailey 1984; Messier 1985b; Ballard et al. 1987; Mech 1991b; Boyd et al. 1995; Meier et al. 1995). We will refer to these animals as "adoptees" (Meier et al. 1995) to distinguish them from wolves that enter a pack to replace a lost breeder (see below). Most adoptees are males, and most adoptions take place from February through May (Messier 1985b; Meier et al. 1995).
One of the main mysteries of this behavior is why strange wolves are sometimes allowed to join packs, whereas in so many other cases they are chased, attacked, or killed (Mech 1993a, 1994a; Mech et al. 1998). A clue may be the fact that most adoptees are 13 years old (Messier 1985b; Meier et al. 1995), whereas a high percentage of wolves killed by other wolves are adults (Mech 1994a; Mech et al. 1998). Tests with captive wolves confirm that degree of aggressiveness depends on the rank, age, and residency status of the wolves involved (Fox et al. 1974).
The incidence of packs adopting strange wolves would be very difficult to measure without sampling each wolf in every pack of a population and resampling over time. Based on genetic determinations, nine of twenty-seven packs from three study areas included apparent adoptees (Lehman et al. 1992). However, most members of most packs were not sampled, and the sampling was done over several years. In an Alaskan population subject to harvesting by humans, over 21% of the wolves that dispersed over a 7-year period were accepted into other packs (Ballard et al. 1987). These diverse sampling schemes, plus the fact that adoptees remain in packs for periods of only a few days to over a year, preclude an estimate of the proportion of adoptees at any given moment. A rough guess might be 1020%, and this proportion could well vary by time and place. (Additional information about adoptees can be found in the discussion of multiple breeding below.)
As in the case of wolf 2204, described above, one of the main methods of pair formation is for dispersing wolves of the opposite sex to find each other. However, there are several other methods ("strategies") of pair formation.
To understand the various breeding strategies wolves use, we must first make it clear that every wolf is a potential breeder, and as each begins to mature (see Kreeger, chap. 7 in this volume), its tendency will be to try to breed. This idea is contrary to earlier views that some wolves relinquish breeding "for the good of the species" (Rabb et al. 1967; Woolpy 1968; Mech 1970; Van Ballenberghe et al. 1975; Haber 1977).
Detailed studies of captive (Packard and Mech 1980; Packard et al. 1983, 1985) and wild wolves (Mech 1979a; Fritts and Mech 1981) show that many young wolves merely defer reproduction while still in their natal packs. In the basic social life of the wolf, this strategy can now be seen as merely a natural result of breeding competition, much like the failure to breed of many young male ungulates that lose in their competition with mature bulls.
The wolf population is comprised of tight, territorial social groups. To breed successfully, individual wolves must find a mate and a territory with sufficient food resources (Rothman and Mech 1979). In a saturated population, all territories are occupied, so the only local breeding possibilities will be to (1) wait until the established breeding position opens (A) in the natal pack or (B) in a neighboring pack, (2) become an extra breeder within the pack, (3) carve out a new territory from the established mosaic, or (4) usurp an active breeder.
Local Breeding Strategies
Wolves attempt all the above strategies and more. In Minnesota, a 2-year-old female bred with her stepfather after her mother was shot (Fritts and Mech 1981), illustrating strategy 1A above. The immigration of neighboring wolf 5079 into the pack described above after its breeding female (wolf 5091) was killed by other wolves illustrates strategy 1B; in this case, 5079 had produced pups in a neighboring pack the year before and apparently had lost them (L. D. Mech, unpublished data). Other cases of outside lone wolves joining existing packs to replace lost breeders have been documented by Fritts and Mech (1981), Mech and Hertel (1983), Peterson, Woolington, and Bailey (1984), and Stahler et al. (2002).
In some cases, wolves leave their pack but remain in the pack territory as "biders," presumably waiting for a chance to breed (Packard and Mech 1980). Such wolves have solved one of the two parts of their breeding problem, finding a territory with resources. However, they may have to wait for a parent to perish before they can breed. Lindstrom (1986) believed that in red foxes, biding might be the only type of breeding option for a weak individual.
Rather than replacing a pack breeder, some maturing wolves breed in addition to the pack's established breeders while remaining in their natal pack. Such multiple breeding is favored by close genetic relatedness among the pack members (see below). Although some pertinent details about this behavior are still lacking, the behavior itself is well documented (Murie 1944; Rausch 1967; Clark 1971; Haber 1977; Harrington et al. 1982; Van Ballenberghe 1983b; Packard et al. 1983; Peterson, Woolington, and Bailey 1984; Ballard et al. 1987; Meier et al. 1995). In no case was the relationship between or among the multiple-breeding females known, but one suspects that the breeding females were mother and daughter, because the known structure of wolf packs (see above) suggests that strange females are adopted into packs only rarely (Meier et al. 1995) unless the breeding female is lost (see above).
The important unanswered question when more than one female in a pack breeds is, which male bred the extra (nondominant) female? The likely suspect would be the dominant male, even if the extra female were his daughter, since close inbreeding is well known in captive wolves (Medjo and Mech 1976; Packard et al. 1983; Laikre and Ryman 1991) and has long been considered common for wild wolves (Haber 1977; Woolpy and Eckstrand 1979; Theberge 1983; Shields 1983; Peterson, Woolington, and Bailey 1984). However, recent genetic studies of mated wolf pairs from the Superior National Forest (Minnesota) and Denali National Park (Alaska) populations indicated that inbred pairings were probably rare (D. Smith et al. 1997). This means chances are good that extra matings in a pack may be with immigrants from other packs, or even with outsiders through temporary liaisons. Or, if daughters of the dominant female breed, this could explain the role of adoptees (see above) and why most adoptees are males (Peterson, Woolington, and Bailey 1984; Messier 1985b; Meier et al. 1995).
Adoptee males may become interested in maturing females, which would explain their attraction to new packs. Sometimes such adoptees remain in their new pack from days (Peterson, Woolington, and Bailey 1984) to months (Fritts and Mech 1981) to over a year (Meier et al. 1995; M. E. McNay, personal communication). In Denali, an adoptee left his new pack after a year and was observed just outside the pack's territory with another wolf (a maturing female from the pack?); the adoptee and his mate produced pups in an adjacent territory the next year (Meier et al. 1995).
On the other hand, interest in a maturing female is not always an apparent motive for adoptees joining a pack, or for breeding pairs allowing them to do so. A male wolf radio-collared as a 10-month-old in Alaska during 1995 remained with his natal pack until June, then joined a breeding pair and their pups 58 km (36 mi) away in July, and remained with them at least through January and in their territory until the next July (M. E. McNay, personal communication). The pack had no maturing female when the adoptee joined, and when the pack's pups began maturing, the adoptee left.
Another situation in which multiple females in a pack could breed without inbreeding is when the father of maturing females is lost and replaced by a new male. This stepfather is then unrelated to any pack female and could breed any of them without inbreeding (Stahler et al. 2002).
For two reasons, it seems logical to suggest that multiple breeding is possible only when food supplies are flush (Mech et al. 1998), a hypothesis similar to the suggestion that multiple breeding is fostered by heavy exploitation (Ballard et al. 1987). First, ample food would be required for more than one female to gain sufficient nutrition to produce pups; young pack members receive less food when it is scarce (Mech 1988a; Mech et al. 1998). Second, as will be discussed, maturing members are more likely to remain with the pack when food is more plentiful, whereas aggression increases when food is scarce.
Previous workers have emphasized the importance of social and behavioral factors in prompting dispersal (Haber 1977; Harrington et al. 1982). While these factors may be involved, we believe that nutrition stress underlies them, as social competition is very much a function of food abundance (see below).
Regardless of the uncertainties about various aspects of extra litters per pack, multiple breeding represents a viable strategy by which some wolves succeed in the breeding arena.
Budding and Splitting
Another breeding strategy is for a dispersed wolf and its new mate to try to set up a territory along the edges of its natal pack territory; this approach can involve either a male or a female from the natal pack. The animal frequents one end of the territory, presumably pairs with a floater (see below) or a similar member from a neighboring pack, and forms a territory adjacent to, or overlapping with, its natal territory, a process known as "budding" (Fritts and Mech 1981; Fuller 1989b; Meier et al. 1995; L. Boitani, unpublished data). Budding conforms to the territory inheritance hypothesis, which attempts to explain why group living in carnivores is a stable strategy (Lindstrom 1986).
A variation on this strategy is pack splitting. Pack splitting differs from budding in that, rather than a single wolf budding off a pack with a mate, a group of wolves splits off and assumes a new territory. Pack splitting in this sense is not the same as the temporary splitting of large packs during winter (Mech 1966b, 1970; Haber 1977; Carbyn et al. 1993). Rather, pack splitting as a form of budding is a permanent phenomenon.
Several cases of permanent pack splitting have been reported, all involving larger-than-average packs during or around the breeding season (Mech 1986; Meier et al. 1995; Hayes et al. 2000). In Denali, a pack of twenty split into two packs of eleven and nine and split the territory; at least one of the new packs produced pups that year (Meier et al. 1995; Mech et al. 1998). In one recolonizing population, packs split when they averaged twelve ( 1.5) wolves, and after 4 years of recolonization, nine of twenty-eight (32%) packs were the products of pack splitting (Hayes and Harestad 2000a; Hayes et al. 2000).
It is probably when two related breeding pairs are present that packs split, perhaps after an immigrant male breeds a pack daughter. Presumably the additional members of the subunits are the previous offspring of each pair. Because breeders control the feeding of their offspring (see Packard, chap. 2 in this volume), they may compete too aggressively with other pack breeders as food needs peak in winter because of maximal pup weights. A solution that circumvents mortal competition among kin is to split the territory and resources (Mech 1970). This may be necessary only when food is scarce, thus explaining why large packs do not split every year.
Carving Out New Territories
Dispersers can also breed locally by carving new territories out of the existing pack territorial mosaic. Dispersers using such a strategy wander around the population ("floaters"), frequent areas along the interstices among territories (Mech and Frenzel 1971a; Rothman and Mech 1979; Fritts and Mech 1981; Meier et al. 1995), meet members of the opposite sex, mate, and attempt to set up a new territory (Rothman and Mech 1979). In some areas, however, such as parts of Quebec (Messier 1985b) and Denali (Mech et al. 1998), lone wolves do not seem to frequent pack territory edges and interstices. In any case, lone floaters may circulate over areas of 10,500 km2 (4,100 mi2) or more, many times the size of local pack territories (Mech and Frenzel 1971a; Fritts and Mech 1981; Berg and Kuehn 1982; Merrill and Mech 2000; Wabakken et al. 2001).
Often loners frequent two or three areas along various pack territory edges and float long distances among them until they meet a mate in one of them; then they settle (Mech and Frenzel 1971a; L. D. Mech, unpublished data). In a recolonizing population in northwestern Minnesota, three floaters that were monitored for more than 4 months all paired, and at least two of the pairs produced pups; in the same population, seven of eight dispersers paired, usually within 20 days of dispersal (Fritts and Mech 1981). Although most lone wolves float independently, three pairs in this recolonizing population formed and then floated together, exploring areas until they found one to settle in (Fritts and Mech 1981). The only other area where this strategy seems to have been reported was Scandinavia (Wabakken et al. 2001).
Whether any of the pairs that attempt to carve out territories in an established population succeed depends in part on food abundance in the population. In the recolonizing population of northwestern Minnesota, these pairs tended to succeed (Fritts and Mech 1981), whereas 250 km eastward in the saturated, food-stressed Superior National Forest (SNF) population in the early 1970s (Mech 1977b), they tended to fail (L. D. Mech, unpublished data).
In the SNF during 19691989, a time that included periods both of food stress and of improved conditions, 65% of those wolves that dispersed as adults, 26% as yearlings, and 8% as pups succeeded in pairing and denning (Gese and Mech 1991). In an increasing wolf population in Denali National Park during 19861991, nine (56%) of sixteen new pairs succeeded in founding new packs that lasted a year or more (Meier et al. 1995).
Usurping a Breeder
A last way in which maturing wolves can breed in their own population is to usurp an established breeding position. An example of this approach was seen in the SNF, where a 3-year-old female bred with her stepfather a year after her mother bred with him and left (Mech and Hertel 1983); whether the mother was ousted or left voluntarily is unknown. On Ellesmere Island, a 3-year-old daughter took her mother's breeding role while the mother remained in the pack as a helper (Mech 1995d). In this case, the male had been the mother's mate for 2 years; he could have been the daughter's older sibling or an unrelated wolf, but probably was not the daughter's father.
No doubt the most dangerous strategy for gaining a breeding position would be to challenge an established breeder. Such challenges have been observed in captive situations, where yearling sons challenged their fathers and bred with their mothers (Zimen 1976; Packard et al. 1985). However, such fights that could become mortal in captivity might never take place in a wild pack, where a beaten contender can escape; furthermore, the best evidence so far is that close inbreeding does not occur where outbreeding is possible (D. Smith et al. 1997).
Nevertheless, wolves do often fight to the death in the wild (see below), and the losers are usually wolves encountered near a territory edge or inside a neighbor's territory (Mech 1994a; Mech et al. 1998). A disproportionate number of the dead wolves are adult breeders, but subordinate, maturing animals are also killed. There is a strong possibility that some of these fights results from potential breeders challenging established breeders. The best such record was Messier's (1985b) observation in Quebec that a presumed breeding male was killed one March at the time his pack adopted a young immigrant male.
An incident that L. D. Mech (unpublished data) observed in the SNF during the breeding season (Mech and Knick 1978) also suggests such a challenge. The SNF Greenstone pack (four members) trespassed south of its southern neighbor, the Pagami Lake pack (five members), on 15 February 1972, then returned to its territory. The next day, Mech watched as the Greenstone pack entered the Pagami pack's territory from the south and attacked the sleeping five. At least one wolf from the Pagami pack was wounded, and the Greenstone pack returned to its territory. The only radio-collared Pagami wolf was alone the next eight times it was seen during the next month, and then dispersed. The one radio-collared Greenstone wolf was not seen with more than two others during the next twelve observations through 13 March; then her signal was lost. By fall, however, a newly radio-collared pup was part of a pack of six living in the former territories of both packs. Did the neighboring breeders form one pair after the fight, oust the others, and usurp both territories?
In Denali, the McKinley River pack (ten members) invaded the territory of the Bearpaw pack (also ten members) and, between January and March 1988, killed all three radio-collared members of the Bearpaw pack, wounded at least one other member, and may have killed two others (Meier et al. 1995). Two McKinley River wolves and two new wolves (former Bearpaw members?) then usurped the Bearpaw pack territory, even using the Bearpaw pack den.
Besides the several strategies described above for obtaining a breeding position in the local population, wolves also use a strategy that takes them into a new population or to the very edge of the species' range. This strategy, called directional dispersal (Mech and Frenzel 1971a; Mech 1987a), is a tendency to move a long distance in more or less a single direction. Wolves of both sexes have dispersed to areas up to 886 km (531 mi) away (Fritts 1983; Ballard et al. 1987; Boyd et al. 1995), and some have crossed four-lane highways and open areas and circumvented large lakes and cities (Mech, Fritts, and Wagner 1995; Merrill and Mech 2000; Wabakken et al. 2001; L. Boitani, unpublished data). When long-distance dispersers settle, they may attempt to squeeze into the territorial mosaic of a distant population, join an existing pack, or pair with a member of the opposite sex in an area uninhabited by breeding wolves (Rothman and Mech 1979; Fritts and Mech 1981; Berg and Kuehn 1982; Peterson, Woolington, and Bailey 1984; Messier 1985b; Ballard et al. 1987; Fuller 1989b; Meier et al. 1995; L. D. Mech, unpublished data).
Frequency of Various Strategies
The relative proportions of potential breeders that use these various breeding strategies have not been measured (but see below). Those proportions must vary over space and time and depend a great deal on food supply and whether the population is increasing, decreasing, or stable (see Fuller et al., chap. 6 in this volume). However, a general idea of those proportions can be obtained from the proportions of wolves of various ages that disperse and the distances they move. Near-dispersers would include those wolves that attempt to breed with neighbors through biding, budding, or replacing established breeders. Distant-dispersers would be those that chance finding or founding new populations.
Some information on proportions of breeding strategies can be gleaned from both the SNF and Denali studies. In the SNF population, which between 1969 and 1989 declined, stabilized at a low level, and then increased, the pairing success of some seventy-five wolves that dispersed from their packs was examined (Gese and Mech 1991). A significantly greater proportion of maturing animals dispersed during the declining and increasing phases than during the stable phase, probably reflecting the least competition during the stable phase. Most of the wolves dispersing at less than 1 year of age traveled more than four territories away, whereas most yearlings and adults remained within a radius of three territories. (More details about dispersal are presented below.)
In the increasing Denali population, sixteen new pairs formed in 19861991 (Meier et al. 1995). Two of these pairs died out without producing pups; five produced pups, but failed to hold their territory beyond a year, in most cases because the adults were killed by other wolves; and nine produced pups and held territories for a year or more. Of the nine successful pairs, it is significant that at least seven succeeded through "budding," or carving out a territory partly inside or just adjacent to their natal territory.
The Breeding Flux
Competing with maturing wolves for new breeding positions are lone adults that have left or lost their mates or breeding positions. Individuals such as wolf 5079 in the SNF, mentioned above, as well as examples recorded by Fritts and Mech (1981), Peterson, Woolington, and Bailey (1984), Mech (1987a), L. D. Mech (unpublished data), Ream et al. (1991), and Meier et al. (1995), indicate that many adults join the floating members of the wolf population to compete with the maturing members. These adults tend to remain within 50 km of the area they leave, at least in Minnesota (Gese and Mech 1991).
Given all the above breeding strategies, a wolf population can be viewed as a highly dynamic system in which breeding pairs hold territories and pump out numerous offspring that travel about, criss-crossing the population and striving to gain their own breeding positions. In this flux, each pack tries to hold its position while competing with neighbors that try to expand their territories (see below) as well as with new breeding pairs, local lone wolves, and immigrants that are all trying to leverage themselves into the population structure.
The flexibility in the sizes of wolf packs and territories helps buffer the constant fluctuations in social and ecological factors that wolves face. Wolf populations are constantly churning, and a high proportion of their members are temporary. In the Denali National Park population, which is one of the least human-disturbed wolf populations anywhere, only 15% of wolves under 3 years of age remained in the population for more than 5 years (Mech et al. 1998). Thus at least some of the population's long-term breeders must be immigrants, another indication of the constant genetic mixing of the population.
Why Do Wolves Live in Packs?
The wolf and the wolf pack are as closely linked in the human mind as a child is linked to a family, and rightly so. The human family is a good analogy for the wolf pack. The basic pack consists of a breeding pair and its offspring, which function in a tight-knit unit year-round. As with humans, male wolves generally are larger than their mates, about 20% heavier in general (Mech 1970).
The offspring of the breeding pair often include members of more than one litter. Wolf pups reach adult size by winter, so the presence of pups then gives the pack the appearance of a group of adults. Because at least some young often remain with the pack for a year or more, when new pups are born, the social group constantly appears to contain more than a pair of adults.
Why do wolves remain with their parents for as much as 1054 months while many other mammals leave sooner? At least some wolf pups can survive without their parents when as young as 4 months of age (Fritts et al. 1984, 1985). Their permanent canine teeth are in place by 7 months (Van Ballenberghe and Mech 1975), their long bones cease growth by 12 months (Rausch 1967), and at least some males and females are capable of breeding at 10 months (Medjo and Mech 1976).
The Pack as Nursery
One answer might be that there is great variation in wolf maturation. Some wolves are not reproductively capable even at 3 years of age (Mech and Seal 1987). Physiologically, wolves may not be completely "mature" until about 5 years of age. U. S. Seal et al. (unpublished data) found that wolf androgen and estrogen levels increased until this age. Thus the continued association of young wolves with their natal pack may simply be a way for the young to mature while still being subsidized by their parents. From the parents' standpoint, caring for young until they are mature may be the best way to ensure their original investment. In addition, long association with parents would increase the opportunity for offspring to learn the more subtle components of hunting and foraging behavior that are not innate (Leyhausen 1965, cited in Eaton 1970).
Pack Size and Prey Size
On the other hand, there is some evidence that wolf pack sizes may be influenced by other factors. There has been much theoretical discussion of carnivore group sizes (Murie 1944; Mech 1970; Kleiman and Eisenberg 1973; Zimen 1976; Bekoff and Wells 1980; Rodman 1981; Bowen 1981; Lamprecht 1981; Brown 1982; D. W. Macdonald 1983; Packer and Ruttan 1988; and others). Theory holds that pack size should vary with prey size up to some optimum number; this optimum should be that which allows predation with the least energy expenditure and the most energy return (D. W. Macdonald 1983).
Wolf pack sizes tend to be largest where wolves prey on the largest ungulates. Despite records of hundreds of wolf packs from many areas, however, the relationship of pack size to prey size is not definitive (see Fuller et al., chap. 6 in this volume). This is partly because of the extreme variation in pack size within each area and because in many of the areas studied the wolves were subject to harvesting or control.
Pack size data are available for relatively unexploited wolf populations in Minnesota, Denali National Park, Alaska, Wood Buffalo National Park, Alberta, and Yellowstone National Park. Other data from exploited populations tend to support these data, but are less definitive because of the possible effect of exploitation. The smallest packs tend to feed on garbage and small animals, and the largest on moose and bison (table 1.1).
However, this pattern is only a very general tendency (Mech 1970). For example, in 19711991, the mean pack size for Isle Royale, in Lake Superior, Michigan, where moose are the only ungulate prey, was 7.5, whereas for north-central Minnesota, where white-tailed deer were the exclusive prey, pack size averaged 7.3 (see table 1.1). Average pack sizes for wolves feeding on deer and moose are significantly smaller than for those feeding on elk and caribou (see Fuller et al., chap. 6 in this volume). Nevertheless, the largest packs where moose and bison were preyed on were twice as large as the largest packs from deer areas (see table 1.1).
As discussed above, it is reasonable to try linking group size to prey size. Some of the earliest wolf biologists assumed that wolf packs exist because they may promote greater hunting efficiency (Murie 1944), and this conclusion seems logical (Mech 1966b, 1970; Zimen 1976; Peterson 1977; Nudds 1978; Carbyn et al. 1993). Several important factors, however, complicate the picture.
If large numbers of wolves were necessary to prey on large ungulates, it would be difficult for lone wolves and pairs to survive and produce the offspring that enlarge the pack. In fact, large numbers of wolves are not necessary to kill large prey. Single wolves have been recorded to kill even the largest of the wolf 's major prey species, including adult moose (Cowan 1947; A. Bjarvall and E. Isakson, personal communication; Thurber and Peterson 1993; Mech et al. 1998), muskox (Gray 1970), and bison (D. Dragon, cited in Carbyn et al. 1993).
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Table of Contents
Chapter 1 - Wolf Social Ecology
L. David Mech and Luigi Boitani
Chapter 2 - Wolf Behavior: Reproductive, Social, and Intelligent
Jane M. Packard
Chapter 3 - Wolf Communication
Fred H. Harrington and Cheryl S. Asa
Chapter 4 - The Wolf as a Carnivore
Rolf O. Peterson and Paolo Ciucci
Chapter 5 - Wolf-Prey Relations
L. David Mech and Rolf O. Peterson
Chapter 6 - Wolf Population Dynamics
Todd K. Fuller, L. David Mech, and Jean Fitts Cochrane
Chapter 7 - The Internal Wolf: Physiology, Pathology, and Pharmacology
Terry J. Kreeger
Chapter 8 - Molecular Genetic Studies of Wolves
Robert K. Wayne and Carles Vilá
Chapter 9 - Wolf Evolution and Taxonomy
Ronald M. Nowak
Chapter 10 - Wolf Interactions with Non-prey
Warren B. Ballard, Ludwig N. Carbyn, and Douglas W. Smith
Chapter 11 - Restoration of the Red Wolf
Michael K. Phillips, V. Gary Henry, and Brian T. Kelly
Chapter 12 - Wolves and Humans
Steven H. Fritts, Robert O. Stephenson, Robert D. Hayes, and Luigi Boitani
Chapter 13 - Wolf Conservation and Recovery
Appendix: Species Names Used in the Text
List of Contributors