image: When no pseudo-copulation occurred between the dominant and subordinate males, the subordinate male was killed by the sustained aggression of the dominant in half of the animal pairs observed. Here the dominant male crayfish is shown at right with elevated claws and posture, and the body of the recently killed subordinate male is at left, ventral side up. view more
Credit: Donald H. Edwards
Pseudo-copulation--an interaction that mimics sexual copulation--is a behavior known in mammalian communities that reduces aggression and signifies social dominance, particularly among males. However, this complex, so-called ritualized behavior is not widely known to occur among invertebrate species. Researchers have now reported that male crayfish--invertebrates--also use pseudo-copulation to signify formation of a dominance hierarchy and reduce aggression and lethal battles between male rivals.
The findings, reported by Fadi Issa and Donald Edwards of Georgia State University, appear in the November 21st issue of the journal Current Biology, published by Cell Press.
The researchers found that pseudo-copulation among male crayfish was nearly identical to normal sexual copulation, in that the eventual dominant male displays male copulatory behavior and the subordinate male displays female copulatory behavior. Bouts of pseudo-copulation were intermingled with aggressive interactions as the eventual dominant male attempts to mount the eventual subordinate male, who either accepts or refuses. In pairs that pseudo-copulated, initial aggressive interactions became less intense and less frequent over the first hour of interaction, and all the animals survived over the first 24 hours.
However, in male pairs that did not pseudo-copulate, similar initial levels of aggression were maintained over the first hour, and half of the subordinate males were killed during the first 24 hours. Therefore, to a large extent, subordinate males depend on pseudo-copulation for survival--an illustration of how this mutual behavior directly affects the fitness of crayfish and how it may have proven beneficial over the course of evolution.
The researchers include Fadi A. Issa and Donald H. Edwards of Georgia State University in Atlanta, GA. This work was supported by National Science Foundation research grant IBN 0135162 to D.H.E. and the Brains and Behavior Program of Georgia State University.
Issa and Edwards: "Ritualized Submission and the Reduction of Aggression in an Invertebrate." Publishing in Current Biology 16, 2217–2221, November 21, 2006 DOI 10.1016/j.cub.2006.08.065. www.current-biology.com
Journal
Current Biology