Dr. Jeffrey Shultz, our resident arachnologist, gave us insight into the sex life and morphological diversity of harvestmen or Opiliones, also known as “daddy longlegs”. These arachnids are often ignored and thus provide an open territory for new research. A trek through the woods surrounding the Washington DC metropolitan area will often yield a few harvestmen. According to Dr. Shultz, some of these may be new species that have been overlooked even in the most populated areas of the Eastern United States. Your own back yard could be host to a “new” species of harvestman, undescribed by the scientific community.
The Shultz lab now focuses most of their research on the genera Leiobunum and Hadrobunus. Until recently, Hadrobunus was thought to contain only two species, both first described in the 19th century until the recent additions of 11 new species by the Shultz lab. According to Dr. Shultz most species within the genus occur in the southeastern US, with a few lineages reinvading the north following the retreat of the glaciers The pattern suggests that the South served as a glacial refugium. However, what really makes the story interesting is the resulting diversity in the sex lives of both Leiobunum andHadrobunus, which suggests that there has been a hidden “sexual arms race” were males either coerce or entice females to mate with them.
Graduate student Mercedes Burns in the Shultz lab explored a facet of this hypothesis via a phylogenetic analysis of the relationships between harvestmen, focused on the states of the presence or absence of nuptial sacs. In essence two methods were used. One assumed that the “ancestral” state of species is the presence of nuptial sacs, but these could be lost evolutionarily. The second method attempts to determine the rate of change from having sacs to not having sacs and overlays these over the relationships between species of harvestmen on a tree. The results provided evidence that, over time, female barriers were erected in response to males “cheating the system” and no longer using sacs with nuptial gifts. However there was insufficient evidence to suggest that lack of a sac also resulted in males morphologically forcing their way into the female operculum.
Dr. Shultz is now collecting data on the strength of male penile protractor muscles and comparing that to female operculum muscles. His laboratory is also working on determining if there is a difference in the flexibility of the harvestmen penis and if this is correlated with an ability to force their way into the operculum. In the end, Dr. Shultz would aims for a “big picture” where he can test whether environmental conditions drive this sexual arms race and create shifts between enticement vs. antagonistic strategies in harvestmen. This may provide insights into sexual selection in other organisms and how changes in the environment can shift reproductive strategy.