Multiple paternity in leopard shark (Triakis semifasciata) litters sampled from a predominantly female aggregation in La Jolla, California, USA

• 8 of 22 Triakis semifasciata litters showed multiple paternity using microsatellites.
• No more than two sires were detected per litter.
• The frequency of multiple paternity showed significant interannual variability.
• Skew of male reproductive success was high in some multiply sired litters.
• The most likely explanation for multiple mating appears to be “convenience polyandry”.

The number of sires per litter was determined for the leopard shark (Triakidae: Triakis semifasciata) to investigate the potential effect of female-biased aggregation behavior on the frequency of multiple paternity (FMP). Four highly polymorphic microsatellite markers were developed and used to genotype 449 pups from 22 litters (20.4 ± 7.0 pups per litter) sampled from pregnant females collected from a female-dominated leopard shark aggregation in La Jolla, California, USA. Multiple paternity was detected in 8 of 22 litters (FMP = 36.4%), each having two sires per litter. The relatively low FMP (compared to other shark species) is generally consistent with the hypothesis that female aggregation behavior reduces mating attempts by males and thus limits genetic polyandry. Significant interannual variability in FMP observed between two years of the study (2010: FMP = 20.0%, n = 10, and 2011: FMP = 83.3%, n = 6) appears to be correlated with the frequency of males in the aggregation. Although females may benefit indirectly from mating with multiple males by promoting sperm competition and hedging against nonviable sperm, the most probable explanation for genetic polyandry in the leopard shark appears to be “convenience polyandry,” where females acquiesce to superfluous mating attempts if the costs of resistance outweigh the costs of capitulation. Thus, FMP is expected to increase as the male-to-female ratio increases and as capacity of females to resist coercive males decreases at the time and place of mating.