Motivational Systems in Rats and Monkeys:
Are They Homologous
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Motor patterning mechanisms of social behavior have been transformed from predominant use of the mouth in the rat to predominant use of the hands in the monkey. This occurs in practically every motivational system. Offense in the monkey includes hitting and rough restraint with the hands, the latter replacing the full aggressive posture of the rat. Sexual behavior includes firm restraint with the hands prior to mounting in the monkey, and use of the hands in masturbation replacing use of the mouth in cleaning of the anogenital region after mounting by the rat. Both self-grooming and allogrooming have shifted from predominant use of the mouth in the rat to use of the hands in the monkey. It should be noted, however, that occasionally the monkeys employed the mouth as well as the hands during grooming in the present study.

Several types of motor patterns were used by rats that have no apparent equivalents in monkeys: ultrasound vocalization, upright postures, and sideways postures. Ultrasound vocalization is being found now in many types of small mammals; apparently it is suited for communication among these animals because of the rapid decrease in sound intensity within a radius of a meter of the animal. In larger animals such as the stumptail macaque, this property of ultrasound would not be so useful. Upright postures in the rat and other muroid rodents reflect the powerful influence of vibrissal releasing stimuli [Kanki and Adams, 1978] ; thus the upright posture involves mutual stimulation with vibrissae which would not be functional for higher primates. Sideways postures, unlike other postures of agonistic behavior in muroid rodents, are quite variable from one species to another [Adams, in press] ; therefore, it is not surprising that they also vary from one order of mammals to another.

The stumptail macaques, like other primates, have a richer repertoire of vocalizations and a repertoire of facial expressions that are not present in muroid rodents. With the rat, and practically all mammals, monkeys share the motor pattern of screaming during submission. But their other vocalizations, that occur in various contexts including submission, sexual behavior, and display, have no equivalents in the rat. Similarly, their facial expressions find no equivalents in rodents. The development of vocalization and facial expression presumably reflects the same evolutionary process that transformed motivating stimuli from olfactory to complex auditory and visual stimuli.

If one takes into account the overall trends noted above, i.e., from the use of mouth to hand, increased use of vocalization and facial expression, and loss of ultrasound, upright and sideways postures, then the motor patterns of offense, submission, sexual behavior, self-grooming, and allogrooming are remarkably similar in rat and monkey. These trends are not confined to anyone motivational system, but are more general and apply to most or all of the motivational systems in question.

Given the remarkable underlying similarity of the motor patterns of offense, submission, sexual, and grooming behaviors in rat and monkey, it seems likely that the motivational mechanisms for these behaviors have been retained with little change since their evolution from a common mammalian ancestor. Similarities of data for the brain mechanisms of offense and submission between rat and monkey also support this hypothesis [Adams, 1979a].

There is no reason to believe that patrol/marking and display involve homologous motivational systems. Although these behaviors perform similar functions for the rat and monkey, advertising the presence and reproductive and dominance status of the individual, they involve completely different motivating stimuli and motor patterns and have little in common other than their function. In fact, in the prosimians both systems apparently exist independently. Many prosimians engage in elaborate olfactory investigation and scent-marking behaviors, yet also have systems for visual and auditory display. Whether or not the display behaviors of the stumptail macaque reflect the operation of a single motivational mechanism is a question that cannot be answered with the present data.

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