Gravidity-Associated Shoaling Decisions in Three-Spined Sticklebacks (Gasterosteus aculeatus)

Group living has evolved in many animal species as an antipredator behavior, an evolutionary effect that might be augmented by grouping with similarly looking individuals. Consequently, groups are often composed according to species, size, or coloration. During egg ripening or embryo growth, the outer appearance of females often changes drastically within days, which makes them more prone to predation. Thus, a female's group preference should change according to her reproductive state, an issue that has seldom been investigated. To test this, we gave gravid and non-gravid three-spined sticklebacks (Gasterosteus aculeatus) the choice to shoal either with a gravid or a non-gravid conspecific. The results showed that shoaling preferences of gravid and non-gravid individuals differed significantly. Non-gravid females preferred gravid fish as shoaling partners, which might rely on the fact that gravid sticklebacks show reduced escape performance, which in turn might increase a non-gravid female's chance of survival. However, in contrast to our predictions, gravid fish did not show any significant preference. A reason for this pattern might be competition between gravid females for mating partners, which might overrule benefits of shoaling with similar looking individuals. Hence, gravidity influences social preferences in a shoaling fish, which might pose a largely overlooked form of the cost of reproduction.