Data from: Macroevolutionary patterns of sexual size dimorphism in copepods

Major theories compete to explain the macroevolutionary trends observed in
sexual size dimorphism (SSD) in animals. Quantitative genetic theory
suggests that the sex under historically stronger directional selection
will exhibit greater interspecific variance in size, with covariation
between allometric slopes (male to female size) and the strength of SSD
across clades. Rensch's rule (RR) also suggests a correlation, but
one in which males are always the more size variant sex. Examining
free-living pelagic and parasitic Copepoda, we test these competing
predictions. Females are commonly the larger sex in copepod species.
Comparing clades that vary by four orders of magnitude in their degree of
dimorphism, we show that isometry is widespread. As such we find no
support for either RR or for covariation between allometry and SSD. Our
results suggest that selection on both sexes has been equally important.
We next test the prediction that variation in the degree of SSD is related
to the adult sex ratio. As males become relatively less abundant, it has
been hypothesized that this will lead to a reduction in both inter-male
competition and male size. However, the lack of such a correlation across
diverse free-living pelagic families of copepods provides no support for
this hypothesis. By comparison, in sea lice of the family Caligidae, there
is some qualitative support of the hypothesis, males may suffer elevated
mortality when they leave the host and rove for sedentary females, and
their female-biased SSD is greater than in many free-living families.
However, other parasitic copepods which do not appear to have obvious
differences in sex-based mate searching risks also show similar or even
more extreme SSD, therefore suggesting other factors can drive the
observed extremes.