Motivational Systems in Muroid Rodents

Although gonadal hormones can also affect defense (as noted below) offense can occur in males, females, and castrates in response to unfamiliar conspecific odors, which is why I have suggested that these are the primary motivating stimuli for offense. Although in laboratory species males may show more offense, castrates can also exhibit offense (Heilman et al, 1977] and, in many species, females are as likely as males to engage in offense against unfamiliar intruders. These species include Me auratus (Vandenbergh, 1971], Mer unguiculatus (Swanson, 1974], S fulviventer and hispidus (Peterson and Helland, 1978], N alexis (Stanley, 1971], R rattus (Ewer, 1971], O irroratus (Davis, 1972], and Miochrogaster and pennsylvanicus (Getz, 1962]. In laboratory rats and mice, females are usually reported to show no offense except during lactation, although a few positive reports are available in both rats (Erskine et al, 1978b] and mice (Haug, 1978]. In the wild, females of these species are more aggressive; females of both wild M musculus (Anderson, 1961; Ebert and Hyde, 1976] and wild R norvegicus (Calhoun, 1962, p 184; Telle, 1966] have often been reported to show offense. The decreased offense of laboratory females may be due to a tendency for colony odors of laboratory animals to be more homogeneous while the odors of wild rats, which are exposed to more varied dietary and bacterial influences, might be expected to be more diverse, unfamiliar and, therefore, provide more effective motivating stimuli for offense.

The fact that males and females are more likely to attack same-sexed intruders must be due to the presence of additional sensory analyzers tuned to sex-specific pheromones. The data may be explained in terms of three hypothetical sensory analyzers: 1) A sensory analyzer tuned to odors of androgen-dependent pheromones, activated by androgens, and facilitating the offense motivational mechanism; 2) a sensory analyzer tuned to estrogen-dependent pheromones, activated by androgens, and inhibiting the offense motivational mechanism; and 3) a sensory analyzer tuned to odors of androgen-dependent pheromones, activated by estrogen, and inhibiting the offense motivational mechanism. Data in support of the first two have been obtained in the laboratory mouse by Mugford and Nowell (1970; 1971a, b], Dixon and Mackintosh (1971], and Haag et al (1974]. They have reported that substances contained in the urine of intact (but not castrate) males will facilitate offense when painted onto the back of a "neutral" test animal, while substances contained in the urine of intact females will inhibit attack. Some data are available from other muroid rodents as well: Offense in the laboratory rat, R norvegicus, is more likely to occur against intact than castrate male opponents (Barfield et al, 1972; Thor and Flannelly, 1976b] and against mature rather than immature male opponents (Thor and Flannelly, 1976a ]; and offense in Me auratus males is facilitated by urine from conspecific males (Payne, 1974b]. Attack is inhibited against females of R norvegicus (Thor and Flannel1y, 1976b ]. Although urine from females of Me auratus does not inhibit offense when painted onto the victim (Payne, 1974b], there is an inhibitory pheromone in the vaginal secretions of this species [Murphy, 1973]. Discussion of the third hypothesized sensory analyzer is presented in a forthcoming publication [Talavera and Adams, in preparation] concerning the estrous inhibition of offense in female muroid rodents.

There is another type of motivating input for offense which is not dependent upon olfaction, but is associated with competition for food or water. This type of offense, which has been called competitive fighting in the rat and mouse, is as likely in females as in males [Zook and Adams, 1975; Fredericson and Birnbaum, 1954] and is not abolished by olfactory bulbectomy [Rowe and Edwards, 1971]. It includes exactly the same motor patterns as other types of offense and may occur with the same time course and intensity [Mink and Adams, in preparation]. Competitive fighting has also been reported in Me auratus and Mer unguiculatus [Boice et al, 1969].

Ultrasound vocalization by the opponent has been shown to inhibit offense in the laboratory rat [Sales, 1972; Lore et al, 1976; Lehman and Adams, 1977]. Although ultrasound is a common motor pattern of the defense motivational system of other muroid rodents, its effect on offense has not been systematically studied except in the rat. Presumably, the ultrasound acts as an inhibitory motivating stimulus on the offense motivational mechanism.