Prolonged tandem formation in firebugs (Pyrrhocoris apterus) serves mate-guarding

When copulating, firebugs (Pyrrhocoris apterus) form tandems for prolonged periods. Half of the copulations of marked individuals in the field lasted longer than 12 h, and some lasted up to 7 days. We found that: (1) females mate usually with several males prior to an oviposition bout, and (2) they store sperm from each mating for a large proportion of their reproductive lives. This causes a high potential for sperm competition to occur within female firebugs. We studied whether prolonged tandem formation is a male adaptation to this situation by testing five alternative hypotheses: (1) mateguarding, (2) sperm-loading, (3) mate monopolization for future clutches, (4) prevention of ejaculate removal, and (5) mechanical sperm displacement. The sperm-utilization pattern was determined using a genetic marker. The second male to mate had a slight but significant fertilization advantage (P-2=0.59). In laboratory experiments, copulation duration varied systematically with the operational sex ratio, from a median duration of 7.3 h with a female-biased sex ratio to 15.3 h with a male-biased sex ratio. Sperm transfer commenced from the beginning of copulation, but the number of sperm in the female spermatheca reached an asymptote after 3-4 h. Smaller males had longer copulation durations than large ones, while there was no relationship between female size and copula duration. From our results, we exclude hypotheses 2-5 as possible explanations for prolonged tandem formation. Rather, males prolong copulations as a form of ejaculate-guarding under high competition with other males. Sperm displacement by prolonged sperm transfer may act in addition to this function, although this was not tested in this study.