The evolution of defensive shields used in male-male combat

Many animals compete with other members of their species over access to resources, such as food, shelter, and mates. As a result, some species have evolved weapons to help them outcompete their rivals. Examples include the branching antlers of deer and the elongated horns of rhinoceros beetles. However, weaponry alone is only half of the picture, as many species have also evolved ways to defend themselves against their rivals. For example, goats have evolved thicker skin to limit horn damage during male-male combat. Such defensive shields can have an important role in determining who wins a fight. However, despite its importance, defensive capacity has largely been overlooked. This project will investigate the costs, benefits, and evolutionary consequences of defensive structures used in male-male combat. Synergistic broader impacts include a public museum exhibit about armor in animals, and training scientists at the undergraduate, graduate, and postdoctoral level. Previous work investigating male-male combat has almost exclusively taken an offensive perspective, while the role of defense has almost been completely ignored. How well an individual is defended during a fight can determine whether that individual will win or lose. Thus, an individual’s defensive capacity is critical to their fighting success. However, almost nothing is known about why some individuals are better defended than others. Moreover, evolutionary arms-races between weapons and defensive structures are postulated to have a role in weapon diversification. Thus, our lack of knowledge about these defensive traits is also a critical obstacle to understanding how such arms-races influence the evolution and diversification of weapons. To address these gaps in knowledge, this project will take an experimental manipulation approach to determine why some individuals are better defended than others, and use phylogenetic comparative methods to evaluate the causes and consequences of evolving these defensive structures. Overall, this project will make new conceptual insights into an understudied dimension of male-male combat by integrating data across biological scales. This work will expand the frontiers of knowledge and train students to enter the STEM workforce. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.