Coercive mating, fluctuating asymmetry and male mating success in the dung fly Sepsis cynipsea

Males of the dung flySepsis cynipsea (L.) (Diptera: Sepsidae) mate guard before copulation by mounting the female's dorsum upon her arrival at the dung. After completing oviposition, females walk off the dung and vigorously shake to attempt to dislodge the guarding male. Only males that succeed in remaining on the female's dorsum will establish genital contact and copulate. Males are smaller than females, and engage wing clamps (specially modified areas on the femur and tibia of each foreleg) to grip the female's wing bases during guarding. The foretibiae of males and females collected at three phases of pairing (prior to struggling, during struggling and in copula) were measured. No differences in the foretibia size of either males or females across the three phases were found. However, the absolute fluctuating asymmetry of the foretibiae of males differed significantly between phases. On average, males paired in copula were the most symmetrical whilst those paired prior to struggling were the least symmetrical. It was concluded that males with more symmetrical foretibiae are indirectly selected as mates by females because of their better competitive ability to grip the female's wing bases and thereby remain on the dorsum of a struggling female during guarding.