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Is Incest Common In Gray Wolf Packs?

Results


Rarefaction analysis showed that estimates of relatedness varied little after about 10 loci were sampled (Figure 1). For example, values differed on average by less than 4% if 10 rather than 11 loci were used to calculate R. Consequently, as few as 10 loci provide consistent measures of relatedness. Wolf dyads scored for this study averaged 16 loci out of a possible twenty compared. Only 4 of 500 dyads were compared at fewer than 10 loci.

To determine the correspondence of known and estimated relatedness, we first analyzed wolves of known genetic relationships from the two captive wolf populations. In the Julian population, all comparisons were between parents and offspring or between siblings (r = .5), except for the two breeding adults, which are presumably unrelated. In the Forest Lake colony, comparisons included parents and offspring and siblings (r = .5), first and second cousins (mean r = .21), and unrelated individuals (r = .0). The average Queller and Goodnight estimator, R, for each of these relatedness categories, .50 .09, .20 .27, and -.09 .09, respectively, are within about 1 SD of the corresponding actual mean r value (Figure 2). The mean values of unrelated (r = 0) and sibling or parent-offspring dyads (r = .5) are significantly different, as in none of 1000 random permutations did the difference in means equal or exceed the observed difference. The range of R values for parent-offspring or sibling dyads is limited; only 2 of 65 dyads have R values < .25 (Figure 3). However, the presence of a few unrelated dyads with large R values was unexpected and may reflect mistakes in the genealogy or the labeling of DNA samples.

Figure 2
Figure 2.  Mean relatedness (R) and SDs for different relationship categories in captive wolves. (Upper panel) R for mother-offspring, father-offspring, siblings, and first-and second-cousin dyads. (Lower panel) R for three categories of relatedness based on a known geneology. The number of dyads for each category is indicated next to the mean value. Error bars indicate 1 SD above or below the mean value.

Figure 3
Figure 3.  Frequency distribution of R values for unrelated (r = .0, N = 392) and highly related (r = .5, N = 65) captive wolves.

In the SNF population, we identified five mother-offspring dyads that fulfilled the specified behavioral and genetic criteria for a parent-offspring relationship. Similarly, we identified 10 sibling dyads as found in the same litters and with alleles that did not exclude either putative parent. All other sampled individuals were excluded as parents. The average exclusion probability in both the SNF and the Denali population was greater than .999 and hence the likelihood of drawing at random another individual from the entire population that was consistent as mother for a given offspring was less than 1 in a 1000. Finally, in the SNF population, we identified six mated pairs based on behavioral data and the absence of excluding alleles (Table 1). In the Denali population, we identified 5 mother-offspring dyads, 1 sibling dyad, and 10 mated pairs using behavioral data and the presence of excluding alleles (Table 2).

Table 1.  Histories and relatedness of bonded wolf pairs in the Superior National Forest, Minnesota, USA.
Male no. Female no. Ra Durationb Pack Pups? Together (%)c Fate
75 6753 .12 Aug '89-Nov '89 BL No 11/13 (85) 6753 shot
75 257 .11 Oct '90-Mar '92 BL Yes 69/100 (69) 75 left
75 313 -.23 Mar '92-May '92 LL No 27/34 (79) 313 signal lost
93 313 -.08 Dec '91-Mar '92 KL No 14/17 (82) 93 killed by wolf
253 273 -.17 Sep '90-Apr '95 PL Yes 87/120 (73) 273 signal lost
453 451 -.08 July '93-July '94 FR Yes 49/81 (60) 453 signal lost
a See Queller and goodnight (1989).
b Period when wolves were together and radio-collared.
c Percentage of radio locations when pair was together.

Table 2.  Histories and relatedness of bonded wolf pairs in Denali National Park, Alaska, USA.
Male no. Female no. Ra Durationb Pack Pups? Together (%)c Fate
511 501 .12 Mar '93-Oct '94 SV Yes 12/15 (80) 511 shot
223 227 .03 July '86-Jan '89 CW Yes 137/187 (73) 223 killed by wolf
363 361 .19 Mar '89-Dec '89 CW Yes 21/42 (50) 363 died, cause unknown
4520 529 .00 Mar '93-Oct '93 TU No 15/27 (56) 529 killed by wolf
513 467 .12 Mar '93-Jan '95 EF Yes 14/20 (70) 513 died, cause unknown
441 495 .19 Mar '92-Sep '92 FO Yes 4/6 (67) 441 died, cause unknown
251 307 -.01 Feb '88-Nov '93 HQ Yes 182/206 (88) 251 capture mortality
351 349 -.01 Oct '88-Feb '90 ST Yes 59/64 (92) Both killed by avalanche
515 499 -.24 Mar '93-Jan '94 TF Yes 16/17 (94) 515 killed by avalanche
455 475 .12 Mar '92-present ST Yes 13/13 (100) Active
a See Queller and goodnight (1989).
b Period when wolves were together and radio-collared.
c Percentage of radio locations when pair was together.

The Queller and Goodnight R values for the mother-offspring and sibling dyads that we identified in wild wolves were close to the predicted value of r = .5 (Figure 4). Mated pairs had R values close to zero, the value expected for unrelated dyads. In the Denali population, the mean R of mother-offspring and sibling dyads was .57 .04 (range, .51 to .63) and .54, respectively. These values were slightly higher than the corresponding values of .50 .10 (range, .40 to .55) and .45 .08 (range, .22 to .72) in the SNF population. To determine if R values for these related categories differed between the two populations, we randomly selected dyads from the pooled data to create samples of the same size as actually observed. The simulated populations had mother-offspring and sibling mean R values that differed by an amount equal to or greater than that observed in 117 and 366 of 1000 random permutations, respectively. Consequently, values of R are not significantly different in the two populations.

Figure 4
Figure 4.  Mean relatedness (R) and SDs for different relationships categories in wild wolves. The number of dyads examined for each category is indicated next to the mean value. Error bars indicate 1 SD above or below the mean value.

The mean value R value of 6 mated pairs in SNF was -.054 .14 and of 10 Denali mated pairs was .05 .11 (Tables 1 and 2). These mean values are not significantly different because mean values of R between mated pairs in simulated populations differed by a amount equal to or greater than that observed in 134 of 1000 random permutations. R values of mated pairs are within 1 SD of the observed value in unrelated, captive wolves and are more than 2SDs below the mean for wolves related as mother-offspring or siblings (Figures 2 and 4). None of the R values for mated pairs in Denali or SNF overlap those of mother-offspring or sibling dyads in either population. However, some alpha pairs may be slightly related considering the large variance in Queller and Goodnight relatedness values of captive wolves having known r values of .2 (Figure 2).


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