Skilful attacking trades off with defensive ability in hermit crabs

The ability to win contests is crucial for securing the valuable resources individuals need to survive and reproduce. Fighting ability can thus be a vital determinant of fitness, and much research has been carried out to characterize the traits that influence fighting ability, or resource-holding potential (RHP). RHP is known to be influenced by a wide range of traits including morphology, physiology, vigour and, more recently, skill. These traits are typically analysed in isolation to identify their individual contributions to winning fights. However, relatively little is known about how different RHP traits may interact and potentially trade off against one another to determine overall RHP. Using the European hermit crab, Pagurus bernhardus , we investigated the relationship between two potential RHP traits: strength and skill. We identified a potential trade-off for attackers, indicating that stronger individuals fight less skilfully. This result suggests that performing well in one aspect of RHP may come at the cost of the capacity to excel in another. Furthermore, we found a trade-off across roles, with a negative relationship between investment in offensive and defensive RHP traits. Our study highlights the benefits of researching multiple components of RHP together and illustrates how the influence of a single RHP trait can fluctuate based on an individual's role in the contest.