Assortative mating in two populations of Short-tailed Albatross Phoebastria albatrus on Torishima

Assortative mating is an important pre-mating isolation mechanism that has been observed in some wild populations of seabirds. The Short-tailed Albatross Phoebastria albatrus is a globally Vulnerable seabird that breeds mainly on Torishima and the Senkaku Islands in the north-western Pacific Ocean. Our previous studies suggested that two genetically distinct populations exist, one on Torishima and the other on the Senkaku Islands. Recently, however, several un-ringed birds in subadult plumage have been observed breeding on Torishima in the Hatsunezaki colony. As almost all birds hatched on Torishima since 197(have been ringed, the natal site of the un-ringed birds was suspected to be the Senkaku Islands. Genetic differences between the two populations would reveal the natal sites of un-ringed birds. By observing the ring status ( ringed or un-ringed) of mating pairs and analysing the mitochondrial DNA ( mtDNA) control region 2 of un-ringed birds, we assessed whether birds that originated from Torishima and the Senkaku Islands achieved pre-mating isolation. There was a small number of pairs on Torishima that consisted of one ringed and one un-ringed bird, but the observed number was significantly lower than that expected if ringed and un-ringed birds mated randomly. Furthermore, mtDNA analyses of nine un-ringed birds demonstrated that all belonged to a particular haplotype clade from the Senkaku Islands. These results show that birds from Torishima and the Senkaku Islands mate assortatively but that there is incomplete pre-mating isolation between birds from the two island groups. The premating isolation of these two populations of Short-tailed Albatross could arise from differences in the timing of breeding and incompatibility in mating displays. As the divergence between the two populations is unlikely to be sufficient to achieve complete post-mating isolation, the two groups are likely to be hybridizing. Further studies using molecular and/or behavioural analyses would be required to reveal the evolutionary significance of hybridization between these two populations.