Data for: Static allometries do not reflect evolutionary allometry in exaggerated weaponry of male New Zealand sheetweb spiders (Cambridgea spp.)

Across the animal kingdom, exaggerated weaponry is frequently used by one
sex to contest access for potential mates. Within species, if
disproportionate investment in weaponry confers an advantage to larger
individuals, this may result in positive static allometry. It is predicted
that the same selective pressures may also lead to positive evolutionary
allometry, where larger species bear disproportionately large weapons on
average, compared with smaller species. However it is unclear whether the
slopes of species-specific static allometries are steeper among larger
species, or remain consistent. All adult males across the New Zealand
sheet-web spider genus Cambridgea bear exaggerated chelicerae which are
used to compete for control of females’ webs. Here, we characterise the
distribution of chelicera lengths within each sex of 12 Cambridgea species
to show that chelicerae almost always exhibit positive static allometry in
males while female chelicera lengths are consistently isometric. We use
comparative phylogenetic methods to demonstrate that the slopes of static
allometries steepen in males of larger species but that the ratio of
average chelicera length to cephalothorax width is tightly conserved
across taxa, leading to an isometric evolutionary allometry. While sexual
selection may drive weapon exaggeration within species, resulting in
steeper or shallower static allometries, this conservation of relative
trait size suggests that chelicera length is subject to other stabilising
selective pressures. Changes to species body plans might be constrained,
while allowing for disproportionate investment in weapon traits at the
extremes of body sizes within species.