Data from: On sperm length mean-variance relationships

Sperm length is highly variable within ejaculates, between males,
populations and across species. While theory makes strong predictions
about expected mean sperm size, there is less clarity on variation in
sperm, although studies have reported sperm-length variation consistent
with some theoretical expectations. Typically, the coefficient of
variation (CV) is used in these investigations to control for
mean-variance scaling. However, a key assumption for this metric to be
appropriate in controlling for mean sperm size is that the standard
deviation in size scales linearly with the mean. Unfortunately,
sperm-length mean-variation relationships are rarely reported making it
hard to assess the validity of using CV as a way to compare mean-corrected
sperm variation. Here we investigate mean-variation relationships using
19,873 sperm length measures from 54 species and find little evidence of a
consistent relationship between mean sperm-length and sperm-length
variation among males within species, meaning CV is not appropriate for
comparing relative (mean corrected) variation in sperm size at this level.
We also find significant scaling of sperm-length variation with mean
sperm-length across species, but the scaling exponent is consistently less
than one, the exponent required by analyses using CV to control for sperm
size. Our assessment shows sperm mean-variation scaling-relationships are
rare within species and strong across species, but that neither supports
the uncritical use of CV in studies of relative variation in sperm length.