Measuring the strength of the horned passalus beetle, Odontotaenius disjunctus: revisiting an old topic with modern technology.

Over a century ago, a pioneering researcher cleverly devised a means to measure how much weight the horned passalus beetle, Odontotaenius disjunctus (Illiger) (Coleoptera: Passalidae), could pull using a series of springs, pulleys, and careful observation. The technology available in modern times now allows for more rigorous data collection on this topic, which could have a number of uses in scientific investigations. In this study, an apparatus was constructed using a dynamometer and a data logger in an effort to ascertain the pulling strength of this species. By allowing beetles to pull for 10 min, each beetle's mean and maximum pulling force (in Newtons) were obtained for analyses, and whether these measures are related was determined. Then, whether factors such as body length, thorax size, horn size, or gender affect either measure of strength was investigated. Basic body measurements, including horn size, of males versus females were compared. The measurements of 38 beetles (20 females, 18 males) showed there was no difference in overall body length between sexes, but females had greater girth (thorax width) than males, which could translate into larger muscle mass. A total of 21 beetles (10 females, 11 males) were tested for pulling strength. The grand mean pulling force was 0.14 N, and the grand mean maximum was 0.78 N. Despite the fact that beetles tended to pull at 20% of their maximum capacity most of the time, and that maximum force was over 5 times larger than the mean force, the 2 measures were highly correlated, suggesting they may be interchangeable for research purposes. Females had twice the pulling strength (both maximum and mean force) as males in this species overall, but when the larger thorax size of females was considered, the effect of gender was not significant. Beetle length was not a significant predictor of pulling force, but horn size was associated with maximum force. The best predictor of both measures of strength appeared to be thorax size. There are a multitude of interesting scientific questions that could be addressed using data on beetle pulling strength, and this project serves as a starting point for such work.