Thursday, August 6, 2009

PS 62-26: Stability of crayfish dominance hierarchies in an unstable environment

Nathan M. Klar and P.H. Crowley. University of Kentucky


Social hierarchies in animal societies are thought to be stable once established. This has been attributed to a variety of mechanisms such as winner-loser effects and spatial segregation of ranks. When these dominance structures are formed in response to competition for shelters it is often assumed that the distribution of shelters is generally static. What happens when hierarchies are formed in unstable environments? To address this question we used groups of the crayfish Orconectes rusticus. These animals are sometimes subjected to disturbances in streams that may relocate them or otherwise modify their shelter options. Groups of five size-matched crayfish were placed in small arenas (abundant shelter or scarce shelter) and were video-recorded for a period of four hours. Individuals were then isolated for three days and the same groups were re-formed and placed in the alternate shelter arrangement. From video recordings we measured shelter use, number of contests, and the linearity of the dominance hierarchies.

Dominance hierarchies were linear within all trials despite all individuals of a group being the same size. This suggests the existence of a strong social mechanism driving hierarchies toward linearity besides differences in resource holding potential. Individuals moving from scarce-shelter to abundant-shelter environments tended to maintain their original ranks while individuals that started with abundant shelters often changed ranks when shelters became scarce. Interestingly, preliminary analysis indicates that the number of contests did not decrease when groups were re-formed in new environments, but there were more contests in groups that began with abundant shelters. These results suggest that groups in limited-shelter environments establish dominance structures more quickly than those in environments with abundant shelter, perhaps due to increased competition for shelter. Future analyses should include duration and intensity of contests to better understand what promotes the rapid formation of dominance hierarchies.