Average number of pairwise amino acid differences at 3 MHC loci between the members of the 26 breeding pairs compared to that expected between all the possible random pairs of breeders (after 10×5000 permutations in Perm; upper part). Average number of pairwise amino acid differences between 10 breeders and their actual mates, versus the same 10 breeders and 10 alternative mates, defined as unrelated individuals present in their same pack, but not reproducing (t-test, Bonferroni corrected; lower part). Significant values are indicated with an asterisk.