Sperm morphology and evidence for sperm competition among parrots

Abstract

Sperm competition is an important component of post-copulatory sexual selection that has shaped the evolution of sperm morphology. Previous studies have reported that sperm competition has a concurrently directional and stabilizing effect on sperm size. For example, bird species that show higher levels of extrapair paternity and larger testes (proxies for the intensity of sperm competition) have longer sperm and lower coefficients of variation in sperm length, both within and between males. For this reason, these sperm traits have been proposed as indexes to estimate the level of sperm competition in species for which other measures are not available. The relationship between sperm competition and sperm morphology has been explored mostly for bird species that breed in temperate zones, with the main focus on passerine birds. We measured sperm morphology in 62 parrot species that breed mainly in the tropics and related variation in sperm length to life-history traits potentially indicative of the level of sperm competition. We showed that sperm length negatively correlated with the within-male coefficient of variation in sperm length and positively with testes mass. We also showed that sperm is longer in sexually dichromatic and in gregarious species. Our results support the general validity of the hypothesis that sperm competition drives variation in sperm morphology. Our analyses suggest that post-copulatory sexual selection is also important in tropical species, with more intense sperm competition among sexually dichromatic species and among species that breed at higher densities.

Keywords: parrots; post-copulatory sexual selection; sperm competition; sperm morphology.

Figure 1

Relationship between mean sperm length and the within‐male coefficient of variation in sperm length (CV_wm) for 62 parrot species (Pearson's r = −0.43, p < 0.001; no control for phylogeny). Plotted points represent the mean values per species. The line and the 95% CI (grey) are based on a linear model. The only two parrot species described as polygynandrous (1: Vasa parrot; 2: Eclectus parrot) and the one species described as lekking (3: Kākāpō) are highlighted

Figure 2

Mean total sperm length in relation to gregariousness and sexual dichromatism for 61 and 62 parrot species, respectively. Shown are estimates (dots) and their 95% CI (error bars) from the univariate models shown in Table 2. Numbers above the X‐axis show sample sizes (number of species in each group)

Figure 3

Relationship between residual testes mass (log₁₀‐transformed) and (a) mean total sperm length and (b) the within‐male coefficient of variation in sperm length (CV_wm) for the 10 parrot species for which testes mass data were available in the literature. The line and 95% CI (grey) are based on the model shown in Table 2. The only two parrot species described as polygynandrous (1: Vasa parrot; 2: Eclectus parrot) are highlighted

Figure 4

(a) Mean sperm length for 62 parrot species (Psittaciformes), 16 shorebird species (Charadriiformes) and 55 passerine species (Passeriformes); scale on Y‐axis is log₁₀ transformed. (b) Within‐male coefficient of variation in total sperm length (CV_wm) only for the parrot and passerine species; scale on Y‐axis is log₁₀ transformed. (c) Relationship between mean sperm length and the within‐male coefficient of variation for the parrot and passerine species; the lines and 95% CI (grey) are based on a linear model without controlling for phylogeny; scale on Y‐axis and X‐axis is log₁₀ transformed. The data for passerines are from Lifjeld et al., 2010 and those for shorebirds from Johnson & Briskie, 1999