Sperm competition in fluctuating populations of common voles in southern Europe: reversed relationship between relative testes size and density

Abstract

Investment in sperm numbers is a key predictor of success in sperm competition and there is usually a positive relationship between sperm competition level and male reproductive effort on ejaculates, which is often measured using relative testes size (RTS). Demographic processes can drasti- cally alter levels of sperm competition and we should expect males to respond to increasing competition risk (RTS increase with density). Here we investigate whether RTS of common voles respond to fluctuations in population density. We evaluated variation of RTS depending on current and recent densities of con- specifics, also taking into account the condition of individual males (fat levels and parasite loads). Contrary to our prediction and in line with recent findings reporting ambiguous results among other vole species, we found a reversed relationship between RTS and density. Our data show that demographic factors can strongly affect RTS and we discuss possible mechanisms to explain these findings.