Sperm competition: linking form to function

Abstract

BackgroundUsing information from physics, biomechanics and evolutionary biology, we explore the implications of physical constraints on sperm performance, and review empirical evidence for links between sperm length and sperm competition (where two or more males compete to fertilise a female's eggs). A common theme in the literature on sperm competition is that selection for increased sperm performance in polyandrous species will favour the evolution of longer, and therefore faster swimming, sperm. This argument is based on the common assumption that sperm swimming velocity is directly related to sperm length, due to the increased thrust produced by longer flagella.ResultsWe critically evaluate the evidence for links between sperm morphology and swimming speed, and draw on cross-disciplinary studies to show that the assumption that velocity is directly related to sperm length will rarely be satisfied in the microscopic world in which sperm operate.ConclusionWe show that increased sperm length is unlikely to be driven by selection for increased swimming speed, and that the relative lengths of a sperm's constituent parts, rather than their absolute lengths, are likely to be the target of selection. All else being equal, we suggest that a simple measure of the ratio of head to tail length should be used to assess the possible link between morphology and speed. However, this is most likely to be the case for external fertilizers in which females have relatively limited opportunity to influence a sperm's motility.

Patterns of genetic variation and covariation in ejaculate traits reveal potential evolutionary constraints in guppies

“…In some species, sperm length is positively associated with sperm's swimming velocity (for example, Mossman et al, 2009;Fitzpatrick et al, 2010), which may explain macroevolutionary patterns of sperm evolution, which suggest that species with relatively high levels of sperm competition produce longer sperm (Fitzpatrick et al, 2009;Tourmente et al, 2009). Despite these latter observations, however, the relationships between sperm length and sperm performance at the intraspecific level can be positive and negative (see Humphries et al, 2008), and some studies have shown that relatively short sperm can have a competitive advantage in sperm competition (Gage and Morrow, 2003;García-González and Simmons, 2007;Firman and Simmons, 2008). …”

Light microscopic features and morphometry of sperm in the emu (Dromaius novaehollandiae)

“…Studies on sperm dimensions have mainly focused on the benefits of increased length as a way of adapting to variations in the female reproductive tract. For instance, longer heads have been proven to positively influence fertility [61] while the general assumption is that longer sperm tails will increase the swimming potential of sperm [66,67]. Humphries et al [67], working on various animal species, came to the conclusion that, although there are a number of factors which may influence the swimming speed of sperm, the head:tail ratio provides a simple measurement to assess potential speed.…”

“…The primitive filiform sperm is 'worm-like' in appearance and is also referred to as 'sauropsid' since it resembles reptilian sperm [37]. A degree of overlap between the two basic morphological sperm types has been observed, based mainly on the tendency towards spiralization seen in some non-passerine birds [67]. Sub-oscine species are also reported to display morphological features of both passerine and nonpasserine bird sperm [52].…”

Sperm competition differentially affects swimming velocity and size of spermatozoa from closely related muroid rodents: head first

“…Swimming velocity may also increase as a result of changes in the proportions between sperm components. A proportionately longer flagellum (in relation to sperm head size) may better overcome the drag of the sperm head (Humphries et al 2008, Tourmente et al 2011a.…”