Maintenance of Sperm Variation in a Highly Promiscuous Wild Bird

Abstract

Postcopulatory sexual selection is an important force in the evolution of reproductive traits, including sperm morphology. In birds, sperm morphology is known to be highly heritable and largely condition-independent. Theory predicts, and recent comparative work corroborates, that strong selection in such traits reduces intraspecific phenotypic variation. Here we show that some variation can be maintained despite extreme promiscuity, as a result of opposing, copulation-role-specific selection forces. After controlling for known correlates of siring success in the superb fairy-wren (Malurus cyaneus), we found that (a) lifetime extra-pair paternity success was associated with sperm with a shorter flagellum and relatively large head, and (b) males whose sperm had a longer flagellum and a relatively smaller head achieved higher within-pair paternity. In this species extrapair copulations occur in the same morning, but preceding, pair copulations during a female's fertile period, suggesting that shorter and relatively larger-headed sperm are most successful in securing storage (defense), whereas the opposite phenotype might be better at outcompeting stored sperm (offense). Furthermore, since cuckolding ability is a major contributor to differential male reproductive output, stronger selection on defense sperm competition traits might explain the short sperm of malurids relative to other promiscuous passerines.

Sperm storage reflects within- and extra-pair mating opportunities in a cooperatively breeding bird

“…Possibly, since the unrelated (to the female) subordinate is not only a reproductive competitor, but often also a son of the dominant male (Kingma et al 2009), there might be reduced fitness incentive for investment in sperm competition. Alternatively, our results could suggest that defensive sperm competition strategies require different investment compared to offensive sperm competition strategies, as has been shown for sperm morphology in M. cyaneus (Calhim et al 2011).…”

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

“…Avian sperm can be divided into two basic types according to their structure, namely, the more 'primitive' filiform sperm typical of non-passerine birds and the 'complex' helical sperm of passerine species [66]. The primitive filiform sperm is 'worm-like' in appearance and is also referred to as 'sauropsid' since it resembles reptilian sperm [37].…”

“…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.…”

Evolutionary Morphology of the Primary Male Reproductive System and Spermatozoa of Goblin Spiders (Oonopidae; Araneae)

“…We identified certain morphological characteristics that are likely influencing specific functions of the sperm, e.g., sperm movement. Furthermore, the relative size of the nucleus compared to the axoneme, for example, was proposed to be a good predictor of sperm velocity (Calhim et al, 2011). It has been shown in mammals that proportions between individual sperm cell components, i.e., the ratio of the nucleus to the flagellum, influence the swimming velocity (e.g., Gage, 1998;Malo et al, 2006;Humphries et al, 2008;Lü pold et al, 2009).…”