Motivational Systems in Muroid Rodents

The motor patterns of threat and alarm also vary among muroid rodent species, but the variability does not appear to be related to reproductive isolating mechanisms. Instead, it may be related to the process of ritualization as described by Lorenz [1966, pp 110-114] and Eibl-Eibesfeldt [1975, pp 104-125]. According to their analysis, movements or postures that originally preceded attack become prolonged and exaggerated during the course of evolution, to the point that they may be differentiated as separated motor patterns of threat. Alarm signals may be analyzed in similar fashion, ie, noises associated with flight might become ritualized so that the actions that create the noises develop into independent motor patterns made prior to the flight itself. Although Lorenz and Eibl-Eibesfeldt imply that the processes are primarily genetic, there is no proof of this in muroid rodents and it is possible that there are important developmental aspects of ritualization.

The fitness value of threat motor patterns may be complex. Their effectiveness would appear to depend not only upon the extent to which they imitate the motivating stimuli of predators or of conspecifics which have previously defeated the opponent, but also to the extent to which the opponent fails to discriminate between these various stimuli. Those motor patterns of threat that produce sudden noise or visual movement may frighten the opponent because such stimuli can be effective motivating stimuli for defense. On the other hand, such stimuli are particularly susceptible to conditioning and habituation; as noted earlier they are not usually seen in laboratory animals. It would seem to be to the advantage of an animal to discriminate the noises and movements of conspecifics from those of potential predators and to respond with defense only in the case of the latter. Motor patterns of threat may also be effective because they provide conditional motivating stimuli similar to those of conspecifics which previously defeated the opponent. In addition to noise and visual movement which accompanied previous attacks, these motor patterns may present an exaggerated body size. Since during the course of ontogeny larger animals usually defeat smaller ones, such motor patterns as the offensive sideways posture, piloerection, tail-raising, and cheek-pouch inflation may be effective as conditional motivating stimuli of defense. On the other hand, one may assume that it is to the advantage of the opponent to learn to respond only to threats which represent true superior strength, and to learn not to respond defensively to simple "bluffs."

Given the complexity of the fitness value of threat motor patterns, one might expect it to change frequently during the course of evolution and development. At one point, a particular threat pattern might be effective in eliciting defense in conspecific opponents or potential predators, but later on, the conspecific opponents might develop a method of "seeing through" the bluff, or the type of predator might change and the new predator might not be affected by the threat.

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