Motivational Systems in Muroid Rodents

In muroid rodents as in other small mammals, defense against predators and, in some cases, against conspecifics consists principally of freezing, flight, or fighting back. The clearest evidence of the evolutionary utility of these behaviors may be obtained from studies of the hunting behaviors of the natural predators of muroid rodents. Freezing probably evolved because most predators of muroid rodents are more likely to attack moving prey than stationary prey; this has been shown for snakes [Smith and Watson, 1972], owls [Payne, 1971; Kaufman, 1974], hawks [Snyder, 1975], weasels [Erlinge, 1975], cats of various species [Leyhausen, 1973], and even other predatory rodents such as Onychomys [Clark, 1962]. Not only the initial approach for an attack, but also the killing bite may be triggered by movement of the prey for the owl [Payne, 1971], snake [Diefenbach and Emslie, 1971], and cat [Leyhausen, 1973]. The escape leap and flight are also a particularly effective defense against predators including the owl [Lay, 1974], weasel [Erlinge, 1975], and cats [Leyhausen, 1973], although it is necessary that the potential victim have a known flight route to safety [Metzgar, 1967]. Fighting back against a predator with the upright posture and lunge-and-bite attack has only been studied in the case of muroid rodents defending against cats [Leyhausen, 1973], but judging from our experience with the attack of wild rats upon human handlers in the laboratory, it would seem to be a very effective response as well.

Unity of the Defense Motivational System

Although the principal motor patterns of defense - those comprising freezing, flight, and fighting back - cannot occur simultaneously for obvious reasons, they may occur in extremely rapid succession without any intervening pause or other behavior. This suggests that they are all simultaneously activated by a single neural mechanism (called the defense motivational mechanism), but that they are further interconnected at the level of the motor patterning mechanisms by a network of mutual and reciprocal inhibition that ensures only one of them is dominant at anyone moment. Evidence that they are all simultaneously activated by a single defense motivational mechanism comes from two sources: 1) As enumerated below, all of the motor patterns of defense can be activated by all of the motivating stimuli of defense. 2) As documented previously [Adams, 1979a], there is evidence that a single neural center in the midbrain central gray integrates all of these motivating stimuli with all of these motor patterns.