Learning of submissive behavior in mice: A new model.

The experience of winning or loosing fights plays an important role in subsequent aggressive or submissive behaviors. In this study agonistic behavior of male mice was chosen to investigate learning mechanisms in the context of a biologically meaningful situation. An ICR mouse introduced into a group of five C57BL/6 mice was attacked by mice of high social status (Fighter, F), but not by lower ranking animals (Non-Fighter, NF). On this basis the following model was developed to study learning of submissive behavior. Day 1 (baseline trial): An ICR mouse was introduced to a single NF-C57 mouse. Few submissive behaviors (crouch) were observed in naive ICR mice upon contact with NF-C57 mice. Day 2 (learning trial): The same ICR mouse was defeated by an F-C57 mouse until it showed defensive upright posture upon approach. This criterion was reached after a mean latency of 3.5 min and after being exposed to a mean number of 14 bites. Day 3 (retest trial): The same pairs as on day 1 confronted each other. Without being attacked, the ICR mouse showed a significant increase of submissive behavior (crouch, defensive sideways and upright) upon mere contact with the NF-C57 mouse when compared to day 1 and to control mice on day 3. Controls, confronted on all three days with NF-C57 mice, showed no increase in submissive behaviors. The results are discussed in terms of acquisition, memory, retrieval and extinction of learned submissive behavior. It is suggested that the mechanisms underlying learning of submissive behavior include generalization of conditioning and specific extinction processes. The further use of the learning scheme to assess drug effects is illustrated.