Data from: Testes size increases with sperm competition risk and intensity in bony fish and sharks

Female multiple mating provides the opportunity for sexual selection to
continue after gamete release, generating strong selection on male
reproductive traits. In particular, in species where female multiple
mating is common, males are expected to invest more in testicular tissue
to afford them a numerical advantage during sperm competition. However,
although testes size (correcting for body size) is a commonly used proxy
of the strength of sperm competition, there is surprisingly scant direct
evidence linking male investment in testes with genetic estimates of
multiple paternity across species. Here we test the hypothesis that testes
size is influenced by genetic estimates of sperm competition risk
(multiple paternity percentage) and intensity (number of sires per brood)
in fishes, the most diverse and specious vertebrate group. We provide
conclusive evidence that testes size is larger in species experiencing a
higher risk and intensity of sperm competition, a finding that remains
consistent among sharks and bony fishes (including in separate analyses
focused only on cichlids). These findings shed new light on evolutionary
processes governing sperm competition in a basal vertebrate lineage and
validate the now-widespread use of body-size corrected testes mass as a
proxy for sperm competition risk and intensity.