Motivational Systems in Muroid Rodents

Although under most circumstances muroid rodents show only one motivation at a time, ie, they are either completely offensive or completely defensive, there is one important exception: The lactating female. In our experience with R norvegicus, lactating females show a combination of the attack behaviors of both offense and defense. They may alternate rapidly back and forth between bite-and kick attack and lunge-and-bite attack. Similar descriptions of lactation aggression in R norvegicus as consisting of both offense and defense motor patterns have recently been published [Price and Belanger, 1977; Erskine et al, 1978a]. Although increased aggressiveness has been reported in practically every well-studied muroid rodent species as a function of lactation, the precise postures have rarely been described in sufficient detail to say if they were offense, defense, or both. In three species, both types of attack have been noted: In M musculus bite-and-kick attack has been noted by Rosenson and Asheroff [1975] and lunge-and-bite attack has been noted by Gandelman [1972]; in Peromyscus leucopus both bite-and-kick attack and lunge-and-bite attack have been reported [Rowley and Christian, 1976]; and in Me auratus both types of attack have been described [Wise, 1974]. The hormonal basis for the simultaneous offense and defense of the lactating female is being discussed in another publication [Adams, in press].

Unity of the Offense Motivational System

A motivational mechanism of offense was initially proposed in order to explain the temporal coincidence and sequencing of certain motor patterns of resident rats during isolation-induced fighting [Adams, 1976; Lehman and Adams, 1977]. In a typical sequence, following approach to the opponent, the resident engaged in a series of offensive sideways postures and offensive upright postures accompanied by piloerection, followed by full aggressive posture and a bite-and- kick attack. Similar temporal sequences have been reported for other muroid rodents including Me auratus [Lerwill and Makings, 1971] and D groenlandicus [Allin and Banks, 1968]. We have proposed that the temporal association of these motor patterns is due to common inputs to their motor patterning mechanisms from a single set of neurons, the offense motivational mechanism. The sequencing patterns are presumed to be caused by the dependence of the motor patterns upon specific releasing and directing stimuli which are encountered at different stages in the course of the changing spatial relationships between the animals.

The same motor patterns as those of isolation-induced fighting also occur in competitive fighting, which also may be taken as evidence that a single neural mechanism underlies both types of fighting [Zook and Adams, 1975].