Decline in offspring quantity but not quality from successive matings in male rainforest Drosophila, with no evidence for genetic divergence in male mating behaviour along elevational and density gradients

Fertilisation success is a major component of male fitness, meaning males should capitalise on all opportunities for mating. However, other sources of variation in fitness mean that males often evolve life histories that limit their ability to mate frequently. We quantified mating latency, mating duration and offspring production in males of the tropical fly Drosophila birchii when presented with up to four females consecutively. Males were sourced from isofemale lines from the extremes of two elevational gradients (20–1100 m), that show substantial differences in population density, temperature and humidity. Total offspring sired increased with number of matings achieved, demonstrating substantial benefits of multiple mating. However, mean numbers of offspring declined with each successive mating, and mean mating durations increased, while mating latencies remained consistent. We saw no reduced fitness in male offspring from later matings, suggesting that declining offspring production is not associated with decreasing quality. Although differences between gradients were observed in total offspring production, reductions in offspring number were as great for males from high density sites as those from low density sites, despite expectations that males from high density sites would show higher mating investment. We also detected no divergence between high and low elevation sites for other traits, suggesting little adaptive divergence in mating strategies across this species’ entire elevational range. The steep decline in offspring production over successive matings may reflect low encounter rates, or mating opportunities with females in natural populations of this species, even in high density environments, reducing relative investment in sperm or ejaculates.