Exposure time to rivals and sensory cues affect how quickly males respond to changes in sperm competition threat

Phenotypic plasticity can increase fitness in rapidly changeable environments, but may be limited if the underlying mechanisms cause a lag between environmental change and individual response or if the information individuals receive is unreliable. Hence to understand the evolution of plasticity we need to assess whether individuals respond to fine-scale variation in environmental cues. In this study we used a Drosophila melanogaster fruit fly model to investigate factors that determine how quickly males alter their behaviour in response to changes in sperm competition cues. Male D. melanogaster respond to exposure to rival males prior to mating by extending mating duration and increasing ejaculate investment. We have previously shown that to build-up the response, males need about 24 h exposure to a rival. We reasoned that this time lag was necessary to increase ejaculate production, but this physiological limitation should not apply when moving from high- to low-competition environments; hence we predicted that males should immediately decrease their investment when competition is removed. Here we tested this by measuring how long rival-exposed males maintained an extended mating duration after removal of the rival. We assessed how exposure time and sensory information affected the speed of change in behavioural state. Males maintained extended mating duration for hours after a rival was removed, but this was dependent on time of exposure to a rival. Furthermore, although sensory-impaired males were able to respond to rivals, the time required for the response to build and diminish depended on males possessing their full sensory repertoire. Our results suggest that males use exposure time and multiple sensory cues to assess whether the threat of sperm competition is transient (so unlikely to translate into realized competition) or sustained (requiring a response). Therefore, time lags between environmental changes and responses may buffer animals against making hasty decisions in fluctuating environments.