Extreme range in adult body size reveals hidden trade-offs among sexually selected traits

Sexually selected weapons used to monopolize mating opportunities are predicted to trade-off with traits used in competition for fertilization. Yet, the limited size-range typically found among adults of a species often precludes clear comparisons between population-level and individual-level relative trait investment. The jousting weevil, Brentus anchorago (Coleoptera: Brentidae), varies more than 26-fold in body mass, which is among the most extreme adult body size ranges of any solitary terrestrial species. We reveal a trade-off at a population-level: hypermetric scaling in male weapons (slope =1.59) and a closely-mirrored reversal in allocation to post-copulatory traits (slope =0.54). Yet, at the individual-level we find the opposite pattern; males that invest relatively more in weapons for their size class also invest more in post-copulatory traits. In support of our findings from jousting weevils, across 36 dung beetle species we find the relative effect of population-level scaling increases in species with a larger range in adult body size. Our findings reveal that population-level allometries and individual level trade-offs can both be important in shaping relative trait allocation; we highlight that the adult body size range is rarely examined, but may be integral to gain a deeper understanding of trade-offs in reproductive allocation.

Methods
Datasets were collected:

1) from published literature

2) morphological measurements of external morphology from natural populations

3) morphological measurements of internal morphology from natural populations