Sperm head morphology is associated with sperm swimming speed: a comparative study of songbirds using electron microscopy

Sperm exhibit extraordinary levels of morphological diversification across the animal kingdom. In songbirds, sperm have a helically shaped head incorporating a distinct acrosomal membrane or ‘helical keel’, the form and extent of which varies across species. The functional significance of this helical shape, however, remains unknown. Using scanning electron microscopy, we quantified inter- and intra-specific variation in sperm head morphology across 36 songbird species (Passeriformes: Passerida). Using phylogenetic comparative methods, we investigated the relationship between sperm head morphology and both sperm swimming speed and the frequency of extra-pair young (EPY). We found that species whose sperm had a relatively more pronounced helical form (i.e. long acrosome, short nucleus, wide helical membrane, and a more pronounced waveform along the sperm head ‘core’) had faster-swimming sperm. We found no evidence of a relationship between inter-specific variation in sperm head morpholog...