Motivational Systems in Muroid Rodents

Unlike the other motivational systems discussed in this paper, it is likely that the motivational system of patrol/marking can be activated without any motivating stimuli, but simply in response to high levels of androgen or estrogen. Thus, the enhanced locomotion of the estrous female may apparently occur in clean, air-tight chambers where there are apparently no motivating stimuli. Also, it has been shown that large doses of androgen can reinstate ventral-rub in Mer unguiculatus after the motivating stimuli have been blocked by removal of the olfactory bulbs [Thiessen et al, 1970].

The primary motivating stimuli of patrol/marking are olfactory. This is shown by the fact that all of the various motor patterns of patrol/marking may be decreased or abolished by suppression of olfactory functioning. Undirected locomotion is reduced by olfactory bulb removal in Me auratus [Borer et al, 1974]. Approach locomotion is decreased by olfactory bulb removal in males of Me auratus although not in females [Hilger and Rowe, 1975], and is decreased by olfactory blockade made by the application of zinc sulfate to the olfactory mucosa in R norvegicus [Thor and Flannelly, 1977]. The ventral rub of Mer unguiculatus is abolished by removal of the olfactory bulbs [Baran and Glickman, 1970; Thiessen et al, 1970], and reduced by damage which is confined to the vomeronasal division of the olfactory system [L'Hommedieu and Hull, 1977]. The urine-marking of M musculus is greatly reduced by anosmia caused by application of zinc sulfate [Maruniak et al, 1975b].

The other motor patterns of patrol/marking are probably elicited by the same olfactory motivating stimuli, but confirming data have not yet been obtained by olfactory blockade. Secretion by Harderian glands in Mer unguiculatus has been reported to occur in response to conspecific odors [Thiessen and Yahr, 1977]. Sand-digging by M musculus occurs more if the sand has been scent-marked by a conspecific [Wilsoncroft, 1975], and sand-bathing occurs at the sites where other conspecifics have previously sand-bathed [Eisenberg, 1967], both effects presumably due to olfactory-motivating stimuli. The vaginal marking of female Me auratus is done in response to conspecific male odors [Johnston, 1977a]. In species of Neotoma all of the following motor patterns of patrol/marking are stimulated by conspecific odors: Sniffing, perineal drag, sand-bathing, and ventral-rub [Howe, 1977].

The principle olfactory stimuli which serve as motivating stimuli for patrol/marking are probably the odors of unfamiliar conspecifics. Early studies emphasized that novelty or unfamiliarity with the enviroriment was an important component of the motivating stimuli of locomotion [Shillito, 1963; Barnett, 1958b]. During intermale encounters in R norvegicus, all of the motor patterns of patrol/marking are increased, including locomotion, approach, stop-and-sniff, urine-marking, and facial gland secretion [Lehman and Adams, 1977]. The home animal directs its investigation toward the unfamiliar conspecific intruder, while the intruder directs its investigation partly towards the cage which has been scent-marked by the home animal; we interpreted these data to indicate that the motivating and directing stimuli depended upon the unfamiliarity of the conspecific odors. Studies of urine-marking by M musculus [Maruniak et al, 1974] and ventral-rub by Mer unguiculatus [Baran and Glickman, 1970] came to the conclusion that the primary motivating stimuli for those behaviors was unfamiliar conspecific odors. Confirming data have also been obtained for the urine-marking of male R norvegicus; they are more likely to mark objects that have been marked by other animals than objects that they have themselves marked previously [Brown, 1975]. Similarly, facial grooming that presumably reflects the secretion from facial glands is also stimulated by unfamiliar odors in R norvegicus [Jolles et al, 1979], M musculus [Fentress, 1972], and Mer unguiculatus [Thiessen and Yahr, 1977].

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