Motivational Systems in Muroid Rodents

The presence of defensive upright posture and defensive sideways posture in muroid rodent species parallels the presence of the related patterns of offensive upright posture and offensive sideways posture. The defensive upright posture, like its offensive counterpart discussed earlier, is present in every muroid rodent which has been thoroughly studied [See Figure 2]. When both offensive and defensive individuals show the posture during conspecific fighting, the result is usually the mutual behavior called "boxing" or "sparring" in which the two animals stand facing each other and alternate forelimb movements as if in locomotion. Defensive sideways posture has not been reported for those muroid rodents which do not exhibit offensive sideways posture (see list under Offense). Defensive upright posture may be distinguished from offensive upright posture by its more vertical or backwards lean [Davis, 1972; Grant and Mackintosh, 1963; Allin and Banks, 1968], and by the presence of more locomotor movements in the latter [Lehman and Adams, 1977].

Although the defensive upright posture is most commonly described from conspecific encounters, it is also used by muroid rodents when cornered by a predator. As such, it has been described in Me auratus, R norvegicus, and M musculus [Leyhausen, 1973], and L lemmus [Myllymaki et al, 1962] .

Another low level of defensive posture, which one might call a "defensive quadrupedal posture," has been reported as a low intensity variation of the defensive upright posture. The animal remains on all four legs, but with the forelegs extended and head thrown back, often with the teeth bared. The posture, often called "threat" has been described for some species which do not have a defensive sideways posture, L lemmus [Arvola et al, 1962] and N alexis [Stanley, 1971], and for species in which the defensive sideways posture is rare, Mi agrestis [Clarke, 1956] , Mi pennsylvanicus and ochrogaster [Krebs, 1970] , and P californicus and maniculatus [Eisenberg, 1962]. Apparently, in these species it takes the place of the defensive sideways posture as a low intensity, defensive posture.

The lunge-and-bite attack is probably present in some form in all muroid rodents, although it is not often used against familiar consociate opponents, especially in laboratory animals (see section on submission). It is usually used against predators and in the defense of the lactating female against conspecifics. In its typical form, the attack is launched from a quadrupedal or upright defensive posture. The head and/or body is thrust forward rapidly with the forepaws extended in front of the face, and a damaging bite is administered to the nearest protruding part of the opponent, often its face. The attacker then returns rapidly to the initial position or else initiates escape, being aided in its withdrawal by its forepaws which push against the opponent at the same time as the bite is given. This type of attack has been described in detail in L lemmus by Arvola et al [1962] and in Peromyscus species by Eisenberg [1962] who notes that in the latter species it may be accompanied by a "chit" vocalization. We have analyzed the sequence by slow motion television analysis in lactating R norvegicus during nest defense and between males during shock-elicited fighting under certain circumstances. As noted earlier, it has been described in lactating females in a number of muroid rodents. Presumably, it is the attack involved in the face-biting described in Mi agrestis [Clarke, 1956], R norvegicus [Blanchard and Blanchard, 1977], L trimucronatus [Banks and Popham, 1975], Cricetomys gambianus [Ewer, 1967], P maniculatus and californicus [Eisenberg, 1962], Ne fuscipes and lepida [Macmillen, 1964], Ne floridana [Rainey, 1956], and Psammomys obesus [Daly and Daly, 1975].

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