as a function of age, gender, physiological state, and genetic constitution. Third, rearing practices and housing environments may affect expected typical behavior.
Age: Many young mammals engage in high levels of social play whereas adults rarely do. Thus, play may be normal for young animals but not necessarily so for adults (Ruppenthal et al. 1974; Vanderschuren et al. 1997). All animals display physiological, behavioral, and cognitive changes as they age. Some of these changes, for example changes in coat/ hair appearance and locomotor ability, are overt and easily recognizable. Many laboratory animals display an age-related decline in exploratory activity, which is sometimes correlated to weight gain (as observed, for example, in mice of various strains; Ingram 2000; Ingram et al. 1981). In addition, a number of neurosensory, cardiovascular, endocrine, gastrointestinal, musculoskeletal, and reproductive changes occur with aging, some of which cannot be directly observed in the living animal. Such age-related changes (e.g., hearing and vision deficits) have been documented for a number of murine strains (Hawkins et al. 1985; for more references see Additional References), while cognitive deficits have been reported in aging mice and rats. It is postulated that some of these changes may be gender- and strain-related (Decker et al. 1988; Fischer et al. 1992; Frick et al. 2000). Changes in pain sensitivity and in emotional behavior that may have direct implications for stress and distress have also been reported in aging animals (Berry et al. 2007; Lamberty and Gower 1992).
Gender: Female mammals generally care for infants, whereas the extent of male involvement varies across species. Thus, species-typical behavior may be gender-biased. Moreover, gender-related differences in stress markers can be profound and occur in all vertebrate species. For example, female rats and mice exhibit marked elevations in basal and stress-induced corticosterone release relative to males, although these are buffered by high levels of corticosteroid-binding globulin, thus making free corticosterone levels similar in both sexes (McCormick et al. 1995). Absolute corticosteroid levels in females fluctuate in relation to the stage of estrus, presumably affected by circulating levels of estrogens (Figueiredo et al. 2002). Thus, assessment of HPA activity as a measure of stress (see below) needs to account for the gender of the animal and the type of steroid measurement (i.e., total [plasma] or free [saliva]). Males and females also appear to differ in other aspects of their stress response(s); for example, while females have greater anhedonic and HPA axis responses to chronic mild stress than males, they score lower on tests of behavioral depression caused by chronic stress exposure (Dalla et al. 2005).