Fidelity to natal social groups and mating within and between social groups in an endangered false killer whale population

Most mammals exhibit natal dispersal of one or both sexes, a behavior that likely evolved in part to reduce the chances of breeding with close relatives. When natal social group fidelity of both sexes has been documented, the risk of inbreeding is reduced by breeding among rather than within social groups. We investigated mating patterns in an endangered population of false killer whales Pseudorca crassidens from the main Hawaiian Islands (USA) using both genetic and photo-identification data. We tested the presence of the 2 most commonly observed inbreeding avoidance behaviors, i.e. natal dispersal and exogamy (mating occurring primarily among individuals from different social groups). Because not all mother-offspring pairs or individual ages were known prior to this study, we used re-sighting histories to determine plausible ranges of birth year for individuals, thereby limiting the pool of candidate parents and increasing analytical power. We identified 32 parent-offspring pairs, revealing strong natal social group fidelity for both sexes. Our results indicate that between 36 and 64% of matings involved individuals from the same social group. Because the population declined from over 400 to around 150 individuals between the 1980s and early 2000s, the intra-group matings may be the result of reduced opportunities for inter-group mating since the decline. Prior to the decline, social groups may have been sufficiently large that selective pressure to develop inbreeding avoidance mechanisms was low, or the population may have evolved alternate inbreeding avoidance mechanisms such as kin recognition.