Sperm competition in tropical versus temperate zone birds

Abstract

Sperm competition represents an important component of post-copulatory sexual selection. It has been argued that the level of sperm competition declines in birds towards the equator. However, to date, sperm competition estimates have been available mainly for avian species inhabiting the northern temperate zone. Here we apply a novel approach, using the coefficient of between-male variation (CV bm ) in sperm size as an index for sperm competition risk, in a comparative analysis of 31 Afrotropical and 99 northern temperate zone passerine species. We found no difference in sperm competition risk between the two groups, nor any relationship with migration distance. However, a multivariate model indicated that sperm competition risk was highest in species with a combination of low body mass and few eggs per clutch. The effect of clutch size was most pronounced in tropical species, which indicates that sperm competition risk in tropical and temperate species is differently associated with particular life-history traits. Although tropical species had lower sperm competition risk than temperate zone species for overlapping clutch sizes, the idea of a generally reduced risk of sperm competition in tropical birds was not supported by our analysis.

Variation in Apical Hook Length Reflects the Intensity of Sperm Competition in Murine Rodents

“…Sperm length variation has been found as a good indicator of extrapair paternity (i.e. the risk of sperm competition) in passerine birds [16] and is recently used as a proxy measure for sperm competition in comparative studies [21].…”

Small and variable sperm sizes suggest low sperm competition despite multiple paternity in a lekking suboscine bird

“…Furthermore, sperm length was short and testis size small in relation to the majority of species studied to date. This suggests low levels of sperm competition in the study population, despite some evidence of multiple mating, small clutch size, and small body size, which predict high sperm competition risk (Albrecht et al 2013). One explanation for apparently low sperm competition could be that males of this species reproduce at a low rate over multiple years and, therefore, may be expected to invest more in advertisement and less in reproductive physiology.…”

“…and Zebra Finches (Taeniopygia guttata) (CV bm ¼ 5.6-9.6%), species in which sperm competition is low (Birkhead et al 2005. By comparison, in 5 recent comparative studies of both temperate and tropical passerines, mean CV bm between males in sperm head length, tail length, and/or total length ranged between 0.5% and 6.2%, with many species having values 3.5% (Immler et al 2008, Kleven et al 2008, Lüpold et al 2009, Lifjeld et al 2010, Albrecht et al 2013.…”

“…Although 80% of variation in total sperm length could be explained by variation among males, mean variability in sperm measurements within alpha male Lance-tailed Manakins was also relatively high (CV wm ¼ 6.01% for head length, CV wm ¼ 4.60% for tail length, and CV wm ¼ 2.58% for total length). Within males of other passerines, CV wm ranged between 0.8% and 6.7% (Immler et al 2008, Kleven et al 2008, Lifjeld et al 2010, Albrecht et al 2013) but was again markedly higher in species with low sperm competition (6.1-7.7% within males; Birkhead et al 2005. By contrast, in Superb Fairywren (Malurus cyaneus), a species with extreme promiscuity, the coefficients of variation in head, tail, and total lengths were 1.1-2.3% within males and 2.1-4.1% between males, with repeatability of 0.65-0.68 .…”

“…However, more studies from species with differing levels of sperm competition are needed to assess whether this is a general trend (Snook 2005, Kleven et al 2008, Rowe and Pruett-Jones 2011. For example, although it is commonly predicted that tropical species experience lower sperm competition than temperate species-because of differences in life history traits such as life span, clutch size, breeding synchrony, migration, and promiscuity rates (Ricklefs and Wikelski 2002)-data on sperm variation in tropical passerines are rare, and a recent review found little evidence for such a distinction (Albrecht et al 2013). Furthermore, although the level of sperm competition is often predicted to be relatively low in lekking systems, given that no pair bonds restrict females from all mating with the most attractive male, evidence for multiple mating and multiple paternity in lekking species indicates that postcopulatory sperm-competition levels may be higher than previously thought (Petrie et al 1992, Lank et al 2002, Hess et al 2012.…”