*Abstract*

Some marine mammals are so rarely seen that their life history and social structure remain a mystery. Around New Zealand, Gray’s beaked whales (Mesoplodon grayi) are almost never seen alive, yet they are a commonly stranded species. Gray’s are unique among the beaked whales in that they frequently strand in groups, providing an opportunity to investigate their social organization. We examined group composition and genetic kinship in 113 Gray’s beaked whales with samples collected over a 20-year period. Fifty-six individuals stranded in 19 groups (2 or more individuals), and 57 whales stranded individually. Mitochondrial control region haplotypes and microsatellite genotypes (16 loci) were obtained for 103 whales. We estimated pairwise relatedness between all pairs of individuals and average relatedness within, and between, groups. We identified 6 mother–calf pairs and 2 half-siblings, including 2 whales in different strandings 17 years and 1500 km apart. Surprisingly, none of the adults stranding together were related suggesting that groups are not formed through the retention of kin. These data suggest that both sexes may disperse from their mothers, and groups consisting of unrelated subadults are common. We also found no instances of paternity within the groups. Our results provide the first insights into dispersal, social organization, and the mating system in this rarely sighted species. Why whales strand is still unknown but, in Gray’s beaked whales, the dead can tell us much about the living.