Motivational Systems in Muroid Rodents

The primary motivating stimuli of offense are unfamiliar conspecific odors. This is supported by five types of evidence derived principally from experimental studies on the laboratory rat and mouse. 1) Offense is usually preceded by olfactory investigation of both the opponent and the test area [Adams, 1976; Lehman and Adams, 1977]. In addition to our observations on R norvegicus, similar observations have been made on L lemmus [Arvola et al, 1962], M musculus [Banks, 1962] , and Me auratus [Swanson, 1974]. In some cases, preliminary investigation is not observed; however [Adams, 1976; Crowcroft and Rowe, 1963], 2) offense is usually abolished by complete olfactory bulb destruction in some species such as M musculus [Ropartz, 1968] and R norvegicus [Bandler and Chi, 1972] or greatly reduced in other species such as Mer unguiculatus [Lumia et al, 1975] and Me auratus [Murphy, 1976]. 3) Offense is more likely to occur against unfamiliar opponents in the rat and mouse [Scott, 1966], as well as in other muroid rodent species, including A sylvaticus [Gurnell, 1977], P maniculatus [Healey, 1967], Rh opimus [Naumov, 1975], Mi agrestis [Clarke, 1956], Mer unguiculatus [Daly, 1977], and species of Sigmodon [Wolfe and Summerlin, 1968]. This may be due to olfactory stimuli, as shown by an experiment in M musculus by Mackintosh and Grant [1966]. They could increase attack on a familiar opponent by painting its back with urine from a stranger, or decrease attack on a strange opponent by painting its back with urine from a familiar mouse. 4) Offense is more likely to occur if the opponent has been removed from the attacker's cage or colony for two or more weeks. These data, obtained in M musculus [Rowe and Redfern, 1969] and Mer unguiculatus [Norris and Adams, 1972] are interpreted to indicate that the opponent remains familiar for up to two weeks following separation, but then it becomes increasingly unfamiliar after that time. This change could be due to a fading of olfactory memory in the attacker, a change of odors in the victim, or to both. 5) Offense is more likely to occur if the attacker has been isolated from social contact for two or more weeks. This effect has been reported by many investigators, including DaVanzo et al [1966] and Poshivalov [1974] in M musculus, Rieder and Reynierse [1971] in Mer unguiculatus, and Brain [1972] in Me auratus. We have found similar results in laboratory R norvegicus. The effects of social isolation of the attacker may be interpreted similarly to the effects of social isolation of the opponent. The fact that social isolation produces measurable changes in brain chemistry and in other behaviors [Valzelli, 1973] is not inconsistent with this analysis; presumably, all behavioral changes are associated with physiological changes of one sort or another.

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