Data from: 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 morphology and
EPY, although we did find that among- and within-male variation in sperm
head traits were negatively correlated with EPY. Applying principles of
fluid mechanics, we discuss how the helical form of the sperm head may
influence swimming speed, and suggest that further studies considering
aspects of sperm morphology beyond sperm length are needed to improve our
understanding of sperm structure-function relationships.