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Is Incest Common In Gray Wolf Packs?

Introduction


Gray wolves (Canis lupus) live in packs that generally contain a breeding pair and their offspring of one or more litters (Mech, 1970; Murie, 1944). Additionally, packs in the wild may also include siblings or earlier offspring of one of the breeding pair (Mech and Nelson, 1990). Some packs may at least temporarily contain unrelated individuals (Mech, 1991; Meier et al., 1995; Messier, 1985; Peterson et al., 1984; Van Ballenberghe, 1983). Recently, a molecular genetic study showed that 8% and 44% of Minnesota and Denali wolf packs, respectively, included individuals unrelated to the breeding pair and their offspring (Lehman et al., 1992; Meier et al., 1995). However, the common elements of all long established wolf packs are the breeding pair and their offspring.

The origin and genetic relationships of the breeding pair have been the subject of conjecture. Many pairs are formed from individuals that have dispersed from different packs, met, and pair-bonded (Rothman and Mech, 1979). Additional ways in which a breeding pair can develop include (1) an unattached lone wolf replaces one of the breeding pair that had dispersed or died (Fritts and Mech, 1981; Mech and Hertel, 1983); (2) an offspring replaces one of the parents (Mech, 1995; Mech and Hertel, 1983); (3) parents breed with offspring; and (4) siblings breed with each other. Incestuous matings between parent and offspring or siblings have been recorded in captive wolves (Medjo and Mech, 1976; Packard et al., 1983) and on Isle Royale, Michigan, where wolves have no other choice than to mate with close relatives because of a lack of immigration from the mainland (Wayne et al., 1991).

Although there has been some speculation, the frequency of incestuous matings in the wild is unknown. Haber (1977:246) believed that "there is a high degree of genetic isolation between unexploited wolf packs in the wild, that there is intense inbreeding and hence increased homozygosity within packs." Peterson et al. (1984), Shields (1983), and Theberge (1983) assumed that inbreeding was common in wolves, although they disagreed on its significance or the degree to which it would be detrimental. Mech (1987) held that the high frequency of wolf dispersal would help ensure a high level of outbreeding in wolf packs, but that occasional dispersal to nearby packs would result in some cousin-cousin matings. In captive wolves, incestuous mating can lead to inbreeding depression (Laikre and Ryman, 1991), but it does not always (U. S. Fish and Wildlife Service, 1982). In the wild, inbreeding can persist for decades without population extinction, although some researchers believe that it may be the reason small populations do not increase in size (Peterson and Page, 1988; Wayne et al., 1991).

Nonreproductive, maturing wolves generally help provision and protect young (Haber, 1977; Mech, 1988; Murie, 1944) and as reproductive tenure in wolves is often short (Meier et al., 1995), helper wolves have a significant chance to reproduce, possibly within their natal pack. Therefore, because of the uncertainty about the origin of breeding wolf pairs and to better understand the role of inbreeding in wolf social behavior, we assessed the genetic relatedness of mated pairs in wolf populations whose mortality is minimally affected by humans. To do this, we used hypervariable simple repeat loci, or microsatellites (see reviews in Bruford and Wayne, 1993; Queller et al., 1993). Microsatellite loci have been used to assess paternity (e.g. Amos et al., 1995; Hagelberg et al., 1991; Morin et al., 1994; Schlotterer et al., 1992), to measure population differentiation (Paetkau and Strobeck, 1994; Roy et al., 1994) and to assess relatedness of individuals within social groups (Macdonald et al., 1994).

Because microsatellites are abundant in the mammalian genome, many loci can be surveyed and used to accurately measure relatedness (Chakraborty et al., 1988). In this study, we survey 20 microsatellite loci in two wolf populations and calculate relatedness between parents and offspring, among siblings and between mated pairs. We predicted that if avoidance of close inbreeding is an important constraint on wolf behavior, then incestuous matings should be uncommon and few mated pairs should be as closely related as parent and offspring or siblings.


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