Reproductively suppressed subordinates do not have higher CORT levels than breeders and may have lower levels (Clarke and Faulkes, 1997 and Clarke and Faulkes, 2001). While it is not yet clear how stress relates to status in this species, social subordination must be considered in the context of how it affects the individuals involved. Notably, social defeat may be more

universally stressful than low status. Housing density affects rodent behavior, and both crowded and isolated social environments have been used as stressors in rodents. Crowding is a naturalistic stressor especially for social or gregarious species that relates to high population density and resource competition in the field. In house mice, several studies have shown that crowding can impair

reproductive function and may be part of population size regulation (Christian and Lemunyan, 1958 and Christian, PS-341 order 1971). In the highly social, group-living rodent species the degu (Octadon degus), increased group size is associated with greater dispersal consistent with a “social competition” hypothesis ( Quirici et al., 2011). In the laboratory, crowding typically consists of large numbers of mice or rats (e.g. >6 rats/cage (Brown and Grunberg, 1995 and Reiss et al., 2007)) with ad libitum access to resources such as food and water. Crowding must be somewhat extreme to induce stressful outcomes, as group-housing (e.g. 4–6 rats or 12 mice in a sufficiently large EPZ-6438 in vivo area) tuclazepam is often used as a key component of environmental enrichment ( Sztainberg and Chen, 2010 and Simpson and Kelly, 2011). Social crowding has been shown to impact many different

physiological outcomes in male mice, rats, and prairie voles. These include changes in organ weights, hormone secretion, HPA reactivity, pain sensitivity, telomere length, and cardiac outcomes (Gamallo et al., 1986, Gadek-Michalska and Bugajski, 2003, Kotrschal et al., 2007, Grippo et al., 2010, Tramullas et al., 2012 and Puzserova et al., 2013). Crowding of pregnant dams also produces changes in the offspring birth weight, pubertal timing, and reproductive behavior (e.g. Harvey and Chevins, 1987 and Ward et al., 1994) and may lead to lasting changes through a subsequent generation (Christian and Lemunyan, 1958). There appear to be important sex differences in the consequences of crowding, with one study in rats finding that crowding is a stressor for males but has the capacity to calm females (Brown and Grunberg, 1995). At the opposite extreme, solitary housing can be a potent stressor for social species. Social isolation is employed as a stressor in previously group-housed mice and rats (Heinrichs and Koob, 2006); in both species, extended (2–13 week) solitary housing produces an “isolation syndrome” particularly in females, consisting of hyperadrenocorticism, reduced body weight, altered blood composition, and enhanced pain responsiveness among other outcomes (Hatch et al., 1965 and Valzelli, 1973).