Size‐dependent investment in tusk length, testis size and sperm length in a biparental geotrupid beetle

Males of extreme small or large sizes often experience different levels of pre‐ and postcopulatory sexual selection that result in alternative ways to gain fertilization success. Here, we investigated such alternative mating tactics in Lethrus apterus, a biparental geotrupid beetle species with noticeable intermale size variation by comparing tusk (mandibular process) length, testis size and sperm length between large and small males. Firstly, we demonstrated evidence for male polymorphism as extremely large males had longer tusks relative to their body size than extremely small individuals. Secondly, large males had bigger testes than small males throughout the breeding season, however, a significant decrease was observed over time in large but not in small individuals. Thirdly, sperm length did not differ between the two size categories. Although these results do not explain how the observed large body size variation can be maintained in L. apterus, the non allometric relationship found between tusk length and body size and the lower decrease in testis size over the breeding season in case of small individuals might refer to the presence of alternative mating tactics in this species. In species with significant intermale size variation, small individuals are often expected to adopt alternative mating tactics to ensure reproduction, for example by investing less in their pre‐ and more in their postcopulatory reproductive traits. We investigated this pattern in a biparental beetle, Lethrus apterus by comparing tusk (mandibular process) length, testis size and sperm length of extremely large and small males. We showed that males in this species are polymorphic in respect to tusk length as large individuals possess longer tusks relative to their body size. However, contrary to our predictions, large individuals also had relatively bigger testes than small ones and sperm length did not differ between the two size categories. These results do not explain the observed body size variation in this species, but indicate that tusk length and testis size depend mostly on individual condition, while sperm may have an optimal length regardless of body size.