Symmetric fights as a measure of escalation potential in a symbiotic, territorial snapping shrimp

Summary

The shrimp Alpheus armatus territorially defends the sea anemone it occupies, using as a weapon its large, specially modified ‘snapping’ claw. This behavior was studied in experimental contests which were symmetric (matched individuals) with respect to sex, size, and residency. Characteristics of these contests were compared for two size-classes of male and female shrimp.

There were no significant differences between these groups of shrimp in the number of bouts required to establish dominance or in the number of snaps exchanged. Large females had shorter contests than either small or large males, and losers of contests between large females were injured more frequently and more severely.

This distinctiveness of large female contests could be interpreted as evidence that (i) controlling anemones is more important for large females, (ii) injuries are less important for large females, or (iii) large females lose the ability to assess one another because their contests are less frequent.

If injuries are an accurate measure of the most important costs associated with fighting, then these results indicate that short contests are not necessarily the least costly, and that females can be more aggressive than males, as measured by escalation potential, in sexually selected species.