Factors in the waning of muricide in the rat: I. Analysis of intra- and intersession decrement

If each mouse killed by a rat is removed from the rat's home cage and replaced immediately by another live mouse, the rate of killing declines within 1- and 3-hr sessions. Muricide could not then be dishabituated by either a “nonspecific” stimulus (a loud noise) or a specific change in target characteristics (a frog substituted for the mouse). By systematically varying intersession intervals, we found that 1 hr of ad lib killing produced a monotonically decreasing suppression of muricide over the succeeding 96 hr. Subjects performed an interesting kind of forward didng which was influenced by both dishabituating stimuli and killing; this may be a form of die placement behavior.     

5, 7-dihydroxytryptamine lesions: Effects on body weight,irritability, and muricide

Forty-four male rats were subjected to injections of 5,7dhydroxytryptamine (40 μg/4 μ1) in the ascending 5-hydroxytryptamine projection or control operations. The injections resulted in depletion of forebrain 5-hydroxytryptamine, weight loss, irritability, and muricide. No significant correlations were found between any of these effects. It is suggested that the neural systems mediating regulation of body weight and muricide are substantially distinct.     

Factors in the waning of muricide in the rat. II. Digging behavior

If each mouse killed by a rat is removed from the rat's home cage and replaced immediately by another, the decline in the rate of killing within the one-hour sessions is accompanied by an increase in digging in the woodchip bedding material. Deprivation of the opportunity to dig by removal of the bedding material results in a statistically significant increase in kill rate. Since no other behaviors monitored showed a similar increase with this manipulation, it appears that digging may be a mechanism important in the waning of muricide. Furthermore, digging may be, in some sense, a functional equivalent of killing.     

Ethanol effects on shock-induced fighting and muricide by rats

Ethanol (0.25-1 gm/kg body weight; IP) did not significantly alter shock-induced fighting, regardless of whether it was administered to both rats or to only one rat of the pair. Higher doses tended to decrease shock-induced fighting. Ethanol (0.25-2 gm/kg body weight; IP) also did not induce “nonkiller” rats to kill mice and only high doses (1.5 and 2 gm/kg body weight) decreased the incidence of muricide in “killer” rats. The depressant effects of ethanol on both shock-induced fighting and muricide appeared to result from drug-induced ataxia rather than from a direct effect of ethanol on aggressive behavior.