Selection on weapon allometry in the wild

Allometry is the scaling relationship between a trait and body size. This
relationship can often explain considerable morphological variation within
and among species. Nevertheless, much remains unknown about the factors
that underlie allometric patterns. For example, when different allometric
relationships are observed amongst closely related species, these
differences are regularly considered to be products of selection. However,
directional selection on allometry (particularly the slope) has rarely
been tested and observed in natural populations. Here, we investigate
selection on the scaling relationship between weapon size and body size
(i.e., weapon allometry) in a wild population of giant mesquite bugs,
Pachylis neocalifornicus (previously Thasus neocalifornicus). Males in
this species use their weapons (enlarged femurs) to compete with one
another over access to resources and females. We found that large males
with relatively large weapons successfully secured access to mates.
However, we also found that small males with relatively small weapons
could access mates as well. These two patterns together can increase the
allometric slope of the sexually selected weapon, suggesting a
straightforward process by which the allometric slope can evolve.