Low but genetically variable male mating ability in a tropical Drosophila despite fitness benefits of remating

Male mating success is a key source of variation in fitness, particularly in organisms where males can mate multiply and there is potential for high variance in reproductive success. Males should therefore produce large numbers of gametes to capitalise on all opportunities for mating. In addition, strong selection on male mating success should reduce genetic variation in male mating traits relative to other traits. Despite this, males of the tropical Australian fruitfly Drosophila birchii show significant variation in their remating potential and the resulting number of offspring. We quantified mating latency, mating duration and productivity in D. birchii males, in 30 isofemale lines collected from across two elevational gradients (20 - 1100 m), when they were given opportunities to mate with up to four females consecutively. Male remating rates were low compared to other Drosophila species (only 14 - 27% of males achieved a fourth mating in ~1 day). Mean mating duration also approximately doubled across successive copulations. However, although successive remating produced progressively fewer offspring than a male9s first mating, it consistently increased overall male reproductive success. Critically, we found no reduction in the productivity of the (male) offspring derived from these later matings, indicating a sustained cumulative fitness benefit to remating. Heritable variation was observed for all traits (H2 = 0.035 - 0.292) except mating latency, although no evidence was found for divergence of trait means across elevation. The surprisingly restricted remating ability of male D. birchii may be explained by a low female encounter rate due to this species9 low densities in the field, possibly combined with a high cost of sperm (or ejaculate) production.