Male body size and parental relatedness but not nuptial colouration influence paternity success during scramble competition in Rana arvalis

Female mate choice promotes the development of male secondary sexual traits such as nuptial colouration, whereas scramble competition favours male traits which enhance their ability for access to females. In the explosively breeding moor frog (Rana arvalis), males express a conspicuous blue colouration during a short reproductive period characterised by scramble competition. In the present study we used controlled mating experiments and genetic markers to disentangle the effects of colouration, body size and pairwise genetic relatedness in determining paternity success. Males observed in amplexus with a female prior to spawning were larger than their competitors but did not differ from them in colouration. Polyandry occurred in 67% of the 18 analysed egg clutches, and amplecting males did not always achieve the highest siring success, most likely due to stray sperm. Successful mating pairs were characterised by higher genetic divergence between them than expected by chance. We confirm previous evidence that male nuptial colouration is not a trait selected by females, and provide evidence that male reproductive success is influenced by male size as well as genetic attributes.