The Behavior and Energetics of Ritualized Weapon Use in Mantis Shrimp (Stomatopoda)

Contests are essential parts of an animal’s life history, as they dictate access to critical resources like mates, food, or territory. Studying how animals efficiently assess competitive ability to resolve contests is a central goal of research in animal behavior. Additionally, studies of how animals use traits like signals and weapons in contests lends insight to the evolution of those traits. In this thesis, I study assessment and resolution of territorial contests – as well as the function of signals and weapons in contests – in the mantis shrimp Neogonodactylus bredini (Stomatopoda: Crustacea).

Behavioral theory predicts that animals may use visual or other displays to communicate reliable information on ability, resolving contests without the use of potentially costlier combat, such as biting, grasping, or striking with weapons. In Chapter 2, I show that N. bredini do not match these predictions – the size of structures presented during visual weapon displays did not correlate with strike performance, and almost all contests involved weapon use via high-force striking. Because most strikes were exchanged on the armored telson (tailplate), I hypothesized that the ritualized “telson sparring” behavior helps competitors avoid contest costs and functions as a signal, instead of dangerous combat.

Studies of assessment help show what information competitors use to make decisions during contests and can reveal the role specific behaviors play. In Chapter 3, I show that N. bredini use mutual assessment during both size-matched and non size-matched contests; that is, competitors gather information about both themselves and their opponent. I also show the role telson sparring and other behaviors play during this assessment.

Finally, in Chapter 4, I test how the energetic cost of delivering sparring strikes scales with body size. I find that larger competitors used proportionally more energy when striking, that this positive scaling of energy resulted in constant scaling of velocity across size, and that these results matched predictions from a mathematical model of the strike mechanism. Furthermore, I show that these scaling dynamics are different from those of strikes delivered in another behavioral context: feeding on hard-shelled prey.

Overall, this thesis shows that the use of deadly weapons in contests should not be assumed as dangerous combat; instead, I show how ritualized behaviors allow for weapon use to function in assessment. The approaches used and conclusions made from this thesis can inform work in contest behavior, functional morphology, and biomechanics.