Motivational Systems in Muroid Rodents

The responses of muroid rodents to the attack of familiar conspecifics is similar in many respects, but different in several important respects from the response to predators. Against predators, defense often involves a damaging attack (lunge-and-bite) while against conspecifics it usually does not. In response to the attack of conspecifics or consociates (familiar animals regardless of species) the behavior often involves submissive postures or vocalizations, whereas against predators these or similar behaviors are only shown under special circumstances such as the immobility response after being captured and held by a predator. Under most circumstances one would not expect submissive postures and vocalizations to inhibit predator attack whereas they are effective in inhibiting the offensive attack of a conspecific [Lehman and Adams, 1977]. In other respects, there is a great deal of similarity in the motivating stimuli and motor patterning mechanisms of defense and submission. Stimuli such as pain and dorsal tactile stimulation are motivating stimuli for both defense and submission, and many motor patterns such as flight, squealing, and various warning signals are activated by both motivational mechanisms.

Wild animals tend to exhibit defense while laboratory animals tend to exhibit submission. This is probably due to the fact that laboratory animals have become habituated to the odors of the other animals in the laboratory and to their human handlers; for them, all opponents are relatively familiar. A similar phenomenon has been noted above regarding differences in offense between wild and laboratory muroid rodents. It should be noted that this is a different analysis than the one I previously proposed in Lehman and Adams [1977]. At that time I suggested that submission and defense might be seen simultaneously in laboratory conspecific encounters, whereas the present analysis suggests that all the behaviors seen in that context were motor patterns of submission.

Unity of Submission Motivational System

On the basis of brain research and behavioral data, I have suggested that submission may be organized by a motivational mechanism that evolved phylogenetically as a variant of the set of neurons constituting the defense motivational mechanism [Adams, 1979a]. Most of the inputs to the two mechanisms, and most of the outputs to motor patterning mechanisms, except those for lunge-and-bite and specifically submissive acts and postures, remain the same. There is one critical input, however, that separates the two systems: A hypothetical "consociate modulator" is activated by familiar consociate stimuli, inhibits the defense motivational mechanism, and facilitates the submission motivational mechanism.

Since the submission motivational system is apparently a variant of defense motivational system, the evidence for its organization and unitary nature is similar to that described earlier for defense.