Does paternity or paternal investment determine the level of paternal care and does female choice explain egg stealing in the fifteen-spined stickleback?

An earlier field study on the fifteen-spined stickleback (Spinachia spinachia) showed that frequent male-male interactions result in high frequencies of sneaking and egg stealing. Moreover, sneaking behavior was performed not only by males adopting alternative mating strategies, but also by males with their own nests. The advantage of sneaking is easily understood, but it is more difficult to explain the evolutionary benefit of stealing eggs from other males. I investigated whether males suffering from sneaking adjust their paternal effort in relation to their degree of paternity. I also examined whether females prefer males that have more eggs in their nests, as this could explain egg stealing. There was no relationship between the degree of paternity and fanning activity, hatching success, or nest defense. However, the older the eggs become, the more the males increase their attack rate toward potential egg predators (goldsinny wrasse and shore crabs). Thus, males adjusted their level of defense to the amount of energy and time already invested in the clutch. Females did not prefer males with more eggs in their nests. On the contrary, females preferred males with reduced clutches over males with enlarged clutches. Therefore, female choice is unlikely to be a driving force behind egg stealing in this species.