Indeed, on the basis of the similar

way in which sperm are stored and utilized in all birds, it seems likely that passive sperm loss is ubiquitous in this taxon. It is important to recognize that second or last male sperm precedence is not the rule in birds, especially in the wild. The experimental studies demonstrating the existence of last male sperm precedence in birds were conducted under very restrictive conditions, notably with equal numbers of equally competitive sperm in two inseminations. This is an unlikely scenario in the wild. Moreover, it is now known that both sperm numbers and sperm quality, which RO4929097 research buy can vary substantially between males, have a marked

influence on the outcome of sperm competition (Birkhead et al., 1999). In some species, males can allocate sperm number strategically (Cornwallis & Birkhead, 2006). It is also known that females can influence the uptake of a male’s sperm, and so the outcome of sperm competition in the wild is likely to be a due to a combination of factors that can obscure or override the influence of insemination order. Sperm competition mechanisms in mammals seem to be simpler than in birds or insects, probably because in most species, there is little or no sperm storage by the female and as a result, the interval between insemination and fertilization is usually much shorter, and sometimes just a few hours. An early, prescient model LEE011 mw of sperm competition Ergoloid in mammals by Ginsberg & Huck (1989) proposed that the timing of insemination relative to sperm capacitation and that in turn relative to when the female ovulated would be crucial for the outcome of sperm competition. There is now good

evidence for this and that the timing of capacitation varies between species (Gomendio et al., 2006). A particularly striking adaptation to sperm competition in rodents is ‘sperm trains’– groups of sperm operating as a unit. The woodmouse Apodemus sylvaticus, for example, is a species with relatively large testes and high levels of multiple paternity (Baker, Makova & Chesser, 1999) in which sperm trains are typical. Moore et al. (2002) found that the curiously extended hook on the sperm head allowed sperm to grasp each others’ flagella and swim as a ‘train’. They also showed that trains swan faster than individual sperm, because their flagella beat in unison, and speculated that this sperm cooperation was an adaptation to sperm competition, allowing sperm to rapidly traverse the hostile vagina and enter the cervix, before moving individually to the site of fertilization. Later, in a comparative study, Immler et al.