Although positive assortative mating has been the subject of much theoretical analysis, it causes are still poorly understood. Results of a study of the blister beetle Lytta magister suggest that assortative mating by size occurs because both males and females prefer large mates. Males were less likely to terminate courtships with larger females, resulting in a mating advantage for larger females in the field. In captivity, males provided with a size-matched and a larger female courted and mated more frequently with the larger of the two females. However, in experiments with a female and two males, small males courted and mated with relatively small females more often than did large males. Even though size-related male-male aggression was absent, large males showed a mating advantage in the field. This finding suggests that large males are preferred by females. Because large size is the universally preferred phenotype, these individuals are more able to express their preference and tend to pair off. Small beetles are then left no choice but to mate among themselves. Preference for large mates has presumably evolved because large females produce more eggs per clutch and large males are more capable of producing the large spermatophore passed during mating. © 1990 The Association for the Study of Animal Behaviour.
