Abstract
Sperm cells vary tremendously in size and shape across the animal kingdom. In songbirds (Aves: Passeri), sperm have a characteristic helical form but vary considerably in size. Most of our knowledge about sperm morphology in this group stems from studies of species in the Northern temperate zone, while little is known about the numerous species in the tropics. Here we examined sperm size in 125 Afrotropical songbird species with emphasis on the length of the major structural components (head, midpiece, flagellum), and total sperm length measured using light microscopy. Mean total sperm length varied from 51 μm to 212 μm across species. Those belonging to the Corvoidea superfamily had relatively short sperm with a small midpiece, while those of the three major Passeridan superfamilies Passeroidea, Muscicapoidea and Sylvioidea showed large interspecific variation in total sperm length and associated variation in midpiece length. These patterns are consistent with previous findings for temperate species in the same major clades. A comparative analysis with songbird species from the Northern temperate zone (N = 139) showed large overlap in sperm length ranges although certain temperate families (e.g. Parulidae, Emberizidae) typically have long sperm and certain Afrotropical families (e.g. Cisticolidae, Estrildidae) have relatively short sperm. Afrotropical and temperate species belonging to the same families showed no consistent contrasts in sperm length. Sperm length variation among Afrotropical and Northern temperate songbirds exhibits a strong phylogenetic signal with little or no evidence for any directional latitudinal effect among closely related taxa.
Highlights
Sperm show extraordinary morphological diversity across the animal kingdom (Pitnick et al, 2009)
We report on a study of sperm length variation among Afrotropical songbirds (Aves: Passeriformes: Passeri)
Most knowledge of variation in sperm traits across the global avifauna originates from studies of birds in the Northern temperate zone with little known from the tropics
Summary
Sperm show extraordinary morphological diversity across the animal kingdom (Pitnick et al, 2009). Sperm competition accelerates the rate of sperm length evolution, but changes in sperm length can go either way (Rowe et al, 2015) This means that sperm competition may select for both sperm number and larger or smaller sperm, and that the trade-off between numbers and size may vary considerably among taxa (Immler et al, 2011). Most knowledge of variation in sperm traits across the global avifauna originates from studies of birds in the Northern temperate zone with little known from the tropics This might imply a biased picture as ecological requirements and adaptation may affect trait evolution differently in the two regions, especially the evolution of various reproductive traits and mating systems (Stutchbury and Morton, 2008), and the evolution of sperm. We present the first comparative analysis of this kind based on 125 Afrotropical species, and combined this with data on 139 temperate species of songbird to investigate whether they follow the general patterns known for temperate species of the same clade or whether there are specific patterns to Afrotropical species
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