Despite the central role female mate choice plays in the production of biological diversity, controversy remains concerning its evolution and maintenance. This is particularly true in systems where males do not offer females obvious direct benefits, such as nuptial gifts, that increase female survival and fecundity. In the absence of obvious direct benefits, indirect benefits (i.e. the production of superior offspring) are often invoked to explain the evolution of mate choice. In many taxa, indirect benefits have proven to be common, but they are often assumed to be weak and incapable of compensating for the direct costs females incur for investing in choice. Alternatively, females may receive “cryptic” direct benefits for being choosy, and such benefits may be magnified in species with prolonged pre-mating interactions (e.g. courtship and precopulatory mate guarding).
We assessed the direct benefits of female choice for large male size in a freshwater amphipod species complex in the genus Hyalella. We focused on two undescribed species (hereafter Species A and Species B) that naturally encounter different predators. Species A is found in deeper water where it experiences predation by positive size-selective fish. Species B occupies shallow water habitats where it experiences predation by negative size-selective dragonfly nymphs. In Hyalella, males guard females for 1 to 4 days before mating. We used a series of experiments that assessed the benefits females paired with large males receive compared to females paired with small males. For both species, we predicted that single males would cause less harassment of females paired with large males than females paired with small males. We also predicted that females paired with large males would face increased predation by fish but decreased predation by dragonflies compared to females paired with small males.
Results/Conclusions
In both species, we found that females paired with larger males were harassed less often by single males. In Species B, females paired with larger males had decreased predation by dragonflies. In both species, females paired with larger males had decreased predation by fish. Our results demonstrate that female choice for large male size provides direct benefits to females in a system where males do not offer traditional direct benefits (e.g., nuptial gifts). We suggest that such “cryptic” direct benefits may be common, especially in species that have prolonged mating interactions, and are important for understanding the evolution of mate choice.