Relatedness within and among sperm whale (Physeter macrocephalus) social units assessed using single nucleotide polymorphisms

Abstract

Altruistic behavior, such as caring for the offspring of others, is an intriguing phenomenon observed in several mammalian species. One of the most successful theories for explaining how this behavior could have evolved is kin selection. According to this theory, altruism can be adaptive if the individuals being helped are related to the helper. Female sperm whales and their calves live in stable matrilineal families called “units” within which there is extensive altruism, notably in the form of alloparental care. Furthermore, two or more units will often temporarily associate into larger “groups” and hunt for prey together. Because units are largely composed of related individuals, it is likely that this unusual social system evolved by kin selection. To investigate this, the detailed relatedness patterns within and among units must be known. Sperm whales naturally shed pieces of skin that can be used to extract DNA for this purpose. However, the DNA within these samples is often heavily degraded. Microsatellites are informative DNA markers that are commonly used for such genetic analyses. However, microsatellite analysis is difficult with degraded DNA, because they tend to be relatively long sequences. Single nucleotide polymorphisms (SNPs) are another, increasingly popular DNA marker that are only one nucleotide in length, so they can be more easily analyzed in degraded DNA. The purpose of this study was to use SNPs to investigate kin selection in sperm whales from highly degraded DNA in sloughed skin. A total of 12 SNPs, eight of which were linked, were genotyped in 72 individuals, and a network analysis was performed. Results were unable to resolve any genetic structuring, owing to the poor informativeness of individual SNPs. It was concluded that microsatellites are still more useful for studying relatedness, and that biopsy sampling of whales for higher quality DNA may be required.