Correlates of reproductive success within alternative mating tactics of the common shrew

Abstract

Male common shrews (Sorex araneus) adopt two discrete mating tactics. The most successful males, in terms of number of offspring fathered, are those that establish large overlapping home ranges in areas of high female density early in the breeding season. An alternative, less successful mating tactic is to travel long distances in search of mating opportunities. This study is an investigation of correlates of reproductive success for males adopting these different mating tactics. Reproductive success under natural conditions was assessed using DNA fingerprinting, and survival of offspring was monitored in the field. The number of offspring fathered by males with overlapping home ranges was positively correlated with the number of female ranges overlapped during the breeding season and widi testes mass. The number of offspring fathered by males diat made long-distance movements was positively correlated with their epididymal sperm counts. It is argued that competitively superior (overlapping) males achieve high reproductive success by competing to maximize the number of females inseminated, whereas those adopting an alternative mating tactic instead compete largely via sperm competition, aiming to maximize insemination success with any particular female. There was no significant difference in the fluctuating asymmetry (FA) of males adopting different mating tactics (FA was measured as the difference in length of the paired lateral scent glands). Fluctuating asymmetry was not related to die number of offspring fathered by males adopting either mating tactic, but was significantly correlated with die proportion of male offspring fathered that survived to sexual maturity. Although apparendy not correlated with mating success in diis species, FA may reflect some aspect of genetic quality that affects offspring survival.