Data from: Sperm head abnormalities are more frequent in songbirds with more helical sperm: A possible trade-off in sperm evolution

Sperm morphology varies enormously across the animal kingdom. While
knowledge of the factors that drive the evolution of interspecific
variation in sperm morphology is accumulating, we currently have little
understanding of factors that may constrain evolutionary change in sperm
traits. We investigated whether susceptibility to sperm abnormalities
could represent such a constraint in songbirds, a group characterised by a
distinctive helical sperm head shape. Specifically, using 36 songbird
species and data from light- and scanning electron microscopy, we examined
among-species correlations between the occurrence of sperm head
abnormalities and sperm morphology, as well as the correlation between
sperm head abnormalities and two indicators of sperm competition. We found
that species with more helically shaped sperm heads (i.e. a wider helical
membrane and more pronounced cell waveform) had a higher percentage of
abnormal sperm heads than species with less helical sperm (i.e. relatively
straight sperm), and that sperm head traits were better predictors of head
abnormalities than total sperm length. In contrast, there was no
correlation between sperm abnormalities and the level of sperm
competition. Given that songbird species with more pronounced helical
sperm have higher average sperm swimming speed, our results suggest an
evolutionary trade-off between sperm performance and the structural
integrity of the sperm head. As such, susceptibility to morphological
abnormalities may constrain the evolution of helical sperm morphology in
songbirds.