Social stress in female Columbian ground squirrels: density-independent effects of kin contribute to variation in fecal glucocorticoid metabolites

Abstract

Social interactions among conspecifics can have marked effects on individual physiology, especially through its modulation of the stress axis by affecting the production of adrenal glucocorticoids (GCs). Previous research has focused on how individual GC levels may be influenced by social status, but few studies have considered how the balance between positive (e.g. cooperation) and negative (e.g. competition) social interactions shape individual GC levels. A lack of association between individual GC levels and social factors may be confounded by opposite effects of social competition on the one hand, and social cooperation on the other. We tested for these effects in the Columbian ground squirrel (Urocitellus columbianus), a colonial rodent. During the breeding season, females are exposed to territorial unrelated neighbors and to territorial, but more tolerant, close kin. On the one hand, territoriality and competition for resources led us to predict a positive association between local colony density and female GC levels. On the other hand, higher tolerance of philopatric kin females and known fitness benefits led us to predict a negative association between kin numbers and female GC levels. We compared levels of fecal cortisol metabolites (FCMs) in females at two different spatial scales during lactation: local (a female's core territory during lactation, 30m-radius about her nest burrow) and colony-wide. At the local scale, female FCM levels were neither related to colony density nor to the number of co-breeding female kin, but FCM levels increased with age. At the colony scale, female FCM levels varied in a quadratic fashion with female kin numbers. FCM levels decreased from 0 to 1 co-breeding kin present and increased with >1 kin present. Among females that had only one co-breeding kin present, daughters (and littermate sisters and mothers, but not significantly) led to a 14% reduction in FCM levels compared with females that had no kin. Our results reject the idea that local colony density is associated with increased GC levels this species, but indicate subtle (positive and negative) effects of kin on individual GC secretion. They further call into question the importance of the nature of social relationships in modulating the stress experienced by individuals.

Territorial scent-marking effects on vigilance behavior, space use, and stress in female Columbian ground squirrels

“…Because mothers favor the establishment of yearling (but not older) daughters (Neuhaus et al 2004), it is possible that the scent of a mother is perceived as a potential threat to an older daughter because of the possibility of a mother relinquishing a former territory to a yearling daughter. The glucocorticoid differences found here are consistent with our previous findings (Sosa et al 2020): when compared to females that had no co-breeding kin present in the population, lactating female having only a daughter or a sister present in the population showed decreased FCM levels, whereas females having only their mother presented similarly elevated FCM levels as females having no close kin around.…”

Phenotypic plasticity in reproductive and somatic efforts of female Columbian ground squirrels

“…At the same time, most mothers also allocated resources to themselves by putting on body mass, an aspect of somatic effort. We specifically wanted to know if and to what extent reproductive and somatic efforts during breeding were phenotypically plastic traits, given that ground squirrels are highly iteroparous, maturing at 1-3 years of age (Rubach et al 2020) and some living to ages of 8-10 years (rarely to 14 years of age, Viblanc et al 2016b;Sosa et al 2020).…”

Weathering the storm: Decreased activity and glucocorticoid levels in response to inclement weather in breeding Columbian ground squirrels

“…We thus expected to observe less individuals foraging during periods of inclement weather, but expected foraging activity to re-increase to higher levels afterwards, perhaps compensating for the inclement periods of lowered resource acquisition. We further expected decreases in metabolic activity during inclement weather to be reflected in overall decreased levels of glucocorticoid levels (Haase et al, 2016) shortly after the period of inclement weather conditions (as measured through fecal glucocorticoid metabolites FCM; Bosson et al, 2009;Sosa et al, 2020), reflecting an overall metabolic state geared towards minimized use of stored energy (for associations between metabolic activity and GCs, see Bonier et al, 2009;Haase et al, 2016). In turn, we predicted such adaptations to result in minimal loss of body mass during periods of detrimental weather.…”