Motivational Systems in Muroid Rodents

Although it is proposed here that the motivational systems of offense, defense, submission, and patrol/marking are separable in terms of their neural organization, they are often difficult to distinguish in the course of behavioral observations. This is due, in part, to the complex interactions and overlaps among the systems as enumerated below.

The same motor pattern may be activated in a given species by more than one motivational mechanism. This gives rise to the so-called "ambivalent acts or postures" which get their name from the fact that their presence does not tell the observer which of two possible motivational mechanisms are active at that moment [Grant and Mackintosh, 1963; Lehman and Adams, 1977]. A majority of the motor patterning mechanisms illustrated in Figure 1 fall into this category, especially those activated by both defense and submission. Other especially notable examples are approach locomotion which is activated by either offense or patrol/marking, and the sideways and upright postures which can be activated by offense, defense, or submission.

The same sensory analyzer and synthesizer may be tuned to motivating stimuli that affect more than one motivational system. Such analyzers and synthesizers give rise to the simultaneous activation of more than one motivational system in some cases, eg, both offense and patrol/marking in the case of unfamiliar conspecific odors. For this reason one usually sees both the behaviors of offense and those of patrol/marking at more or less the same time in muroid rodents.

Motivational systems may also respond similarly to the internal circulating hormones of an animal, leading to simultaneous activation of several systems. Thus, for example, not only patrol/marking but also male and female sex motivational systems may be activated by androgens and estrogens and, therefore, one is likely to see the behaviors of each system correlated across animals and at the same time. In fact, the behaviors of patrol/marking have been included as the component of sexual behavior called "proceptivity" [Beach, 1976]. Another example is the parallel increase in offense and defense associated with the hormonal state of lactation, which may be due to parallel hormonal effects upon the consociate modulator and the offense motivational system [Adams, in press].

There may be direct inhibitory interactions among motivational mechanisms. A direct inhibition from the motivational mechanisms of submission and defense to the motivational mechanisms of offense may account for the fact that animals are less likely to show offense in a strange cage than in the home cage [Barfield et al, 1972; Jones and Nowell, 1973b]. The specific mechanism of this effect is indicated by the fact that handling and neophobic stimuli, both motivating stimuli of defense and submission, serve to inhibit both isolation-induced fighting and competitive fighting, which depend upon different motivating stimuli [Mink and Adams, in preparation]. It is also possible that similar inhibitory connections go to the motivational mechanism of patrol/marking, as indicated by the data in Adams [1976].

Motor patterning mechanisms may interact directly. As pointed out with regard to defense, the motor patterns of freezing, flight, and fighting back are organized in such a way as to suggest that their motor patterning mechanisms are interconnected by a network of mutual and reciprocal inhibition. I have suggested elsewhere [Adams, 1979a] that a "master switch" responsible for these interactions might be located in the older portion of the cerebellum. Other motor patterning mechanisms might be mutually facilitative: For example, lunge-and-bite attack with squeal or "chit" vocalization, and crouch or full submissive posture with ultrasonic "piping" vocalization, which would explain why they occur in combination.