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Is Incest Common In Gray Wolf Packs?

Discussion


Because wolves live in packs that are primarily family units, there is considerable opportunity for incestuous matings and the possibility for reproductive succession by helpers. Most adolescent wolves disperse from their natal packs when ≤3 years old (Gese and Mech, 1991; Mech, 1987), but some remain longer or disperse only a short distance to nearby packs (Lehman et al., 1992; Mech, 1987; Meier et al., 1995). Consequently, incestuous matings are possible, especially with the death of one of the mated pair. Instead of dispersing, a young wolf could attempt to challenge a parent for breeding rights. In fact, in other carnivores, subdominants that are excluded by the dominant male from copulation or whose reproduction is hormonally suppressed may produce occasional offspring through sequestered matings following the death of a dominant individual or a change in the dominance hierarchy (reviewed in Gompper and Wayne, 1996). A viable reproductive strategy in wolves might involve subdominant helpers forgoing dispersal for the possibility of direct reproduction within their natal pack ("biders"; Packard and Mech, 1980). However, observed incestuous matings in wolves occur primarily where wolves are prevented from outbreeding, such as in captivity or on Isle Royale (Medjo and Mech, 1976; Packard et al., 1983; Wayne et al., 1991). These observations suggest that wolves might breed incestuously only when dispersal opportunities are limited.

We find no evidence in two natural wolf populations that mated pairs are related as parent and offspring or siblings. None of the R values between members of 16 mated pairs overlapped those of sibling or mother-offspring dyads, and the mean value of relatedness R for mated wolves was >2 SDs below the mean R of sibling and mother-offspring dyads (Figures 2 to 4). In fact, wolf 75 from SNF had three different mates during the period of the study; each time he paired with an unrelated individual rather than related packmates (Table 1). However, a larger sampling of mated pairs might reveal that some are highly related. The binomial probability that a sample of 16 mated pairs would yield no highly related pairs if their frequency in the population was 20% is only 0.03, but it is 0.19 if their frequency was only 10% in the population. Therefore, the formation of highly related pairs must be relatively rare, but we cannot exclude its possibility or the possibility of incestuous matings of more distantly related individuals.

These results imply an aversion to incestuous matings because wolves have far more opportunities to breed with a sibling or a parent than with an unrelated individual. Such opportunities include replacement of one of the breeding pair or the establishment of new packs by siblings. Breeding tenure is short; a preliminary estimate of mean tenure of breeding wolves in the Denali population is 4 years (Meier et al., 1995 in preparation). Our results suggest that adult offspring rarely replace a parent when the opposite-sexed parent is present. Presumably, the negative fitness consequences of incestuous matings are not offset by the direct reproductive benefits of mating with a parent or sibling.

We cannot exclude other possible means by which inbred offspring may be produced in wolf packs. For example, inbreeding could result from multiple paternity due to the union of the breeding female and her mate and a son, from sequestered matings between parents and their offspring, or from matings between siblings. Such incestuous matings would be difficult to detect if they involved the female parent and her son. However, the insemination by one male of both his mate and daughters or matings between siblings would result in multiple litters. In the SNF, multiple litters were observed only rarely, and pack size is small (Mech, 1986), suggesting this is an uncommon source of inbred offspring. In Denali, packs are larger, and we have observed multiple litters in some packs that were of uncertain paternity (Meier et al., 1995), leaving open the possibility of father-daughter or sibling matings in large wolf packs. However, in a preliminary study, none of 10 adult mated dyads from large packs appears to be related as siblings or as parent-offspring (Meier et al., in preparation).

In sum, our results show that within wolf packs, mated wolves are rarely related as siblings or as parent-offspring. This observation suggests that in general, wolf packs are established by unrelated or more distantly related wolves. Offspring do not often, if ever, replace either parent unless the opposite-sexed parent is first replaced by an unrelated wolf, nor do full siblings often become the breeding pair. Despite frequent opportunity, incestuous reproductive succession is not a common means to attain reproductive success.

Inbreeding avoidance may be one of the primary motivations for individuals to disperse (Pusey, 1987, 1996), although ecological and kinship factors critically influence the probability of dispersal (e.g., Creel and Waser, 1994; Koenig et al., 1992). In Minnesota gray wolves, interpack aggression is the largest source of mortality aside from that caused by humans (Mech, 1991). Consequently, the risks of dispersing and defending a new territory near hostile wolves might be sufficient cause for maturing wolves to remain in their natal pack where they have a chance to reproduce with a close relative. Over many generations, wolf packs would become inbred and the alpha pair would be genetically more similar than individuals known to be unrelated. In naked mole rats, no immigration is tolerated into colonies, and they are entirely inbred (Reeve et al., 1990). The frequent pairing of unrelated wolves that we have observed ensures genetic heterogeneity within wolf packs and suggests inbreeding avoidance may be one of the primary reasons for dispersal from natal packs.


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