Variation in intraspecific sperm translocation behaviour in a damselfly and its consequences for sperm viability

Abstract

Sperm quality and viability affect both male and female fitness. Most dragonfly and damselfly males translocate sperm from the testis to the seminal vesicle before each copulation, a behaviour known as intramale sperm translocation (ST). However, some published observations indicate that odonate males can occasionally skip ST prior to copulation. Our aim was to determine the circumstances under which males skip ST and how this might affect sperm viability. We allowed males of the damselfly Ischnura graellsii to perform ST (interrupting the copulation at this stage) and we studied ST behaviour during subsequent copulation. Males were randomly assigned to four treatments, which consisted of allowing the experimental male to copulate again 15 min or 1–3 days after his last ST. Fertility of females mated with the experimental males was analysed as a proxy for sperm viability. All males used the sperm that they translocated previously when the second mating took place 15 min after the manipulation, while the proportion of males that repeated ST increased steadily over time. Both treatment (time elapsed since last ST) and the interaction between treatment and ST (yes/no) had a significant effect on fertility, which decreased only in males that did not perform ST immediately before copulation. Additional experiments with damselflies of the genus Calopteryx showed also that males did not repeat ST when the time to the next copulation was less than a day. Our results suggest that sperm quality decays over time in odonates, and that males can choose whether to keep and reuse the sperm in the seminal vesicle or to discard it. We conclude that the special anatomical disposition of odonate males might increase selective pressures to maximize sperm viability and/or repeated intramale ST behaviour.