Predicting sex bias in mobility from functional traits in flying insects

Understanding the movement patterns of organisms is crucial for effective
biodiversity conservation in the increasingly dynamic and fragmented
landscapes. Since the colonization of habitat patches relies largely on
females, sex differences in movement capability must also be considered.
However, obtaining direct measurements of mobility and dispersal, and sex
biases in these traits, is often challenging. This underscores the
importance of predicting sex-specific estimates of mobility based on
species’ functional traits. Our phylogenetic comparative study aims at
identifying species traits that could serve as proxies for sex bias in
mobility in flying insects. We rely on a comprehensive dataset on the sex
ratios of 454 moth species (Lepidoptera: Macroheterocera), captured by
light traps of the Finnish national moth monitoring scheme. We first
demonstrate that, in the vast majority of species, males outnumber females
among the captured individuals. Our phylogenetic regression models reveal
that species-specific sex ratios correlate with traits typically predicted
to be associated with sex differences in mobility. Female proportions
decrease as sexual dimorphism in wingspan becomes more male-biased and
female wing loading relative to males increases. Proportions of females
are also lower in larger species. Females are particularly scarce in trap
samples of species in which the reproductive output of females is
primarily determined by larval-derived resources (i.e., capital breeders).
These associations suggest that the observed variations in sex ratios do
indeed mirror the variation in sex bias in mobility across species. Our
findings highlight the potential of trait-based approaches to identify
meaningful indicators of insect mobility, including sex biases in
mobility. The availability of such proxies facilitates predictions about
how different species might respond to contemporary challenges, such as
light pollution and habitat loss and fragmentation. The detected
associations also advance ordination schemes of insect life histories by
integrating mobility measures into relevant analyses.