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Noises with sudden onset, such as the snapping of a twig, are particularly effective motivating stimuli of defense, especially in wild muroid rodents. Thus, Calhoun [1962, p 87] noted in R norvegicus that "fights, chases, rustling of dry leaves, in fact anything that produced a sudden or unusual noise made rats scatter even when they were some distance away." Similar observations have been made on M musculus [Crowcroft, 1966, p 13] and R rattus [Ewer, 1971]. Other motor patterns in addition to flight may also be elicited. Tail-rattling may be elicited by sudden noises perceived by Ne fuscipes [Linsdale and Tevis, 1951, p 236]; freezing may be elicited by sudden noise in Peromyscus sp [Vestal, 1975]; and upright posture and boxing may be elicited by noise in R norvegicus [Pond et al, 1977].
The noise that motivates defense may also be one made by a conspecific, in which case it may be considered as a warning signal. These noises, which usually have a design function of sudden onset and wide-band sonic frequencies (ie, those which optimally activate defense) may be made by tail-rattling, teeth-chattering, squealing, forefoot-pattering, and hind-foot-thumping. Their effectiveness as warning stimuli have been documented in R rattus [Ewer, 1971], R norvegicus [Calhoun, 1962, p 87], Peromyscus [Eisenberg, 1962], and Ne fuscipes [Linsdale and Tevis, 1951, p 237].
Visual movement may activate defense in wild muroid rodents and in laboratory animals that have been raised with an opportunity to escape. The effectiveness of such stimuli to elicit freezing or flight has been documented in Mi agrestis and C britannicus [Fentress, 1968], P maniculatus [Rosenmann and Morrison, 1975], R norvegicus [Bronstein and Hirsch, 1976], and Mer unguiculatus [Clark and Galef, 1977]. Presumably, other motor patterns of defense could also be elicited if the relevant releasing and directing stimuli were available.
Defense may be elicited in wild muroid rodents by confronting them with an unfamiliar object or unfamiliar place, a phenomenon that has been called "neophobia" [Barnett, 1958c]. This has been studied in wild R norvegicus [Barnett, 1958c; Cowan, 1977], R fuscipes and villosissimus [Cowan, 1977], M musculus [Wolfe, 1969], and species of Peromyscus [Price, 1972]. Among the motor patterns evoked have been freezing [Barnett, 1958c], flight [Blanchard et al, 1974], lunge-and-bite attack [Galef, 1970a], and corticosteroid secretion [Pfister, 1979; Hennessy et al, 1979]. The mechanism of neophobia has not been well studied, although it presumably involves a complex synthesis involving remarkable memory and learning abilities [ Adams, 1979b]. Neophobia is not as well developed in laboratory as in wild rats [Barnett, 1958c].
It is possible that defense pheromones or the odor of a predator may function as motivating stimuli of defense. Thus, it has been found that M musculus [Miiller-Velten, 1966] and R norvegicus [Stevens and Koster, 1972] avoid areas with urine from conspecifics which have been stressed, and R norvegicus avoids areas with odors from muscle or blood of conspecifics [Stevens and Saplikoski, 1973]. One study has reported that there is an innate fear response in Cri cricetus in response to odor from predators [Dieterlen, 1959], but another study has failed to find any evidence of defense in laboratory R norvegicus due to odors from a cat [Blanchard et al, 1975b]. The general area deserves considerable further research.