Genetic analysis of sneaking and egg-thievery in a natural population of the three-spined stickleback (Gasterosteus aculeatus L.).

Among the fish species that show exclusive male parental care, the three-spined stickleback represents one of the most intensively studied species with regard to reproductive behaviour. In this species, the most common 'parasitic' male tactics in relation to male reproductive behaviour are sneaking and egg thievery, which are often collectively referred to as nest-raiding. However, little is known about the genetic consequences of sneaking and egg thievery in natural populations. Here we assessed the frequency of sneaking and egg-stealing in a natural population, male traits that are associated with the victims of sneaking, and the impact of sneaking and egg-stealing on the reproductive success of nesting males as deduced from the number of offspring in their nests. Fourteen nest-guarding males and a random sample of about 100 eggs/fry of each nest from a natural freshwater population of three-spined sticklebacks were analysed at three microsatellite loci. The analysis revealed a high frequency of genetically successful nest raiding (sneaking or egg thievery), i.e. more than half (57%) of the 14 nests contained offspring (1-94%) which were unrelated to the guardian male. Three of the 14 nests (21%) contained progeny of sneaking males and four of the nests (28%) contained offspring which were unrelated to the guardian male and which probably originated from egg-stealing events. Victims of sneaking were significantly smaller than other guardian males. Moreover, reproductive success correlated positively with male body size.