A comparison is presented between the behaviors of male rats and male stumptail macaques that occur when two unfamiliar males are paired for 10 min. Results which have been presented in detail in separate papers elsewhere are compared and discussed here in terms of the hypothetical organization of motivational systems. The following motivational mechanisms appear to be homologous in the rat and monkey: offense, defense, submission, male sex, self grooming, and allogrooming. Two motivational mechanisms are analogous but not homologous: patrol/ marking in the rat and display in the monkey. There is a shift in sensory modality of motivating stimuli from olfaction in the rat to complex visual and auditory stimuli in the monkey. There is also a shift in the modalities of motor patterns from predominant use of the mouth by the rat to use of the hands by the monkey. Both shifts affect most or all motivational systems of social behavior. It is hypothesized that the outer parts of motivational systems, their sensory and motor interfaces with the environment, change more rapidly and incrementally during the course of mammalian evolution than does the inner part, the integrational portion that consists of the motivational mechanisms.
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