Drosophila melanogaster are a polygamous species: males and females repeat copulation with multiple mates. When more than one male’s sperm have to compete to fertilise the same female’s eggs, sperm competition occurs. This has driven individuals to develop physiological and behavioural plastic responses to enable them to outcompete others and increase their chances of siring successful offspring. But what effect do variable environmental conditions have on these responses? Plasticity comes with a cost, often causing reproductive-survival trade-offs to occur. Previous experiments have failed to show how stress impacts reproductive terminal investment, and other plastic responses, so this study aims to contribute knowledge to this field.
The experiments were completed using a combination of stresses to weaken Drosophila: a two-day starvation period and either age (three, 14 and 28 days) or varying sperm competition levels (solitary males, pairs, groups of eight and solitary males in the presence of pheromones). Evidence of trade-offs and plastic reproductive terminal investment was investigated by monitoring behavioural responses (courtship behaviours, rejection responses and copulatory durations), physiological responses (spermatogenesis, egg production and fat reserve levels), as well as activity and starvation resistance as a proxy for longevity. Aged and starved flies, and flies that were not placed under sperm competition, were less active and had a shorter lifespan. These weaker males also tended to exert less energy into courtship behaviours; however they also copulated for longer. This suggests that they were investing in reproductive terminal investment. When males were placed with females that had been continuously fed, copulation lasted significantly longer. This could be to enable him to transfer more sperm, or as a method of mate guarding. In addition to this, older females spent less time rejecting males, suggesting that they were more willing to succumb to copulation. Despite efforts, both males and females produced significantly fewer offspring when placed in stressful environments.
