Reproductive behaviors and structures in animals are extremely diverse, but the functional and evolutionary causes of this diversity remain unclear. Has diversification been driven by conflict between males and females or by female selection for valuable male traits? Recent work by Shultz and colleagues on a promising model system, the eastern North American harvestmen (daddy-long-legs), has identified combinations of features that are consistent with long-term evolutionary effects of mating conflict: a reproductive arms race. The researchers hypothesize that the diversification of reproductive structures in harvestmen is a result of conflict rather than the more widely accepted mechanism of female selection. This hypothesis will be tested using a biomechanical approach to measure key physical variables in males and females of several species. Rigorous statistical and quantitative comparative methods will be used to determine whether significant differences exist between species and whether the mechanical parameters of male and female structures co-vary. If diversification is due to mating conflicts, reproductive structures should be correlated in their abilities to deliver and resist mechanical forces, but this result would not be expected if male traits evolve due to female selection. This study is a novel application of biomechanics for the explanation of an evolutionary process, as well as the use of unusually accurate quantitative data for comparative methods. Opportunities for the continued involvement of undergraduate trainees will be provided. As a biracial minority scientist, the continued research and mentoring of Co-PI Burns serves to increase diversity in science and ensures that previous investments in Co-PI Burns by the NSF will prove to be resources well-allocated.
We see incredible diversity in the reproductive structures of animals, even between closely-related species. This is particularly true for males, which, based on the system of mating within a species, may attempt to mate coercively with females, or provide benefits to a female to be able to mate with her. Although we expect to see evolutionary change in male reproductive structures as a cause or consequence of change in the mating system, few research studies investigate the mechanical properties of reproductive traits. We measured male reproductive mechanical force traits in 10 species of harvestmen ("daddy-longlegs"), an understudied group of species. In five of these species, males typically provide benefits to females before mating, but in the latter five, males appear to have adaptations to improve their ability to overcome female defenses. We found that force traits such as mechanical strength were significantly correlated to the phylogeny, or inferred relationships between species, such that closely related species tend to have very similar force measurements. However, there were a big differences between some traits of males within the group of species that may be adapted to overcoming female defenses. This seems to be due to differences in behavior between these species, as they belong to different evolutionary lineages, but experimental considerations such as the small number of specimens sampled may also play a role in increasing the variance within the group. This study combined the fields of evolutionary biology and biomechanics in an innovative way. We showed that we could use displacement and force transducers typical to mechanical engineering to measure fine forces in the reproductive structures of an arthropod, and these measures were consistent enough as to be similar between closely related species. The graduate co-Principal Investigator was trained in the use of of new equipment at the University of Maryland, as well as analysis methods at the Bodega Bay Applied Phylogenetics workshop. Support from this grant was acknowledged in the co-PI's successful doctoral dissertation, several oral presentations delivered at scientific meetings, and a published peer-reviewed research paper.
