Male density influences mate searching speed and copulation duration in millipedes (Polydesmida: Gigantowales chisholmi)

Abstract

Abstract: Density effects can have a strong influence over both the mating system of a species and the reproductive strategies of individuals. The way in which males respond to an increase in the density of other male competitors is generally explained by sperm competition theory. When the perceived risk of sperm competition is high, males increase reproductive effort to aspects of mate searching, copulation and mate-guarding that will ensure reproductive success. In species with little likelihood of female defence, male competition is largely restricted to a scramble for access to females and sperm competition. In such species, the ability to search quickly and locate females will be under strong selection. Millipedes (Arthropoda: Myriapoda: Diplopoda) are classic scramble competitors, although their utility in the study of scramble competition has been generally overlooked. Here we investigate the Australian Polydesmidan millipede Gigantowales chisholmi and describe their mating behaviour. We manipulated male density, exposing individual males to either high or low male density treatments, and compared aspects of mate searching and copulation. We found that males from high-density treatments searched at faster speeds and copulated for shorter durations, than those kept alone. We also found that larger males achieved higher mating success but copulated for shorter durations. Our data support the idea that for scramble competitors, males who are more likely to achieve mating success (in this case large males) will use mating tactics that emphasise increased efforts towards mate acquisition rather than investment in their current mates. Significance statement: In an extraordinary number of animal species, the males who are most successful are those who can efficiently locate females, mate with them and move on in search of additional mates. This is scramble competition and is relatively unstudied compared with other animal mating systems such as those where males aggressively fight for mates. Millipedes, also relatively unstudied, represent great systems for the study of scramble competition. Here we show that males of the Australian millipede, G. chisholmi, search for females at higher speeds and mate more quickly when they are kept at higher density. Larger males are more successful but mate for shorter durations. This suggests that for scramble competitors, those more likely to achieve mating success (in this case large males) use tactics that increase mate acquisition rather than investment in their current mates.