UC Berkeley

The prevalence of female-biased affiliations in group living mammalian species suggests that same-sex relationships are of particular importance for females. However, little is known about the influence of environmental and physiological factors on same-sex social bonds. Female meadow voles present an interesting opportunity for the investigation of these questions because free-living females display seasonal variations in same-sex affiliation. From summer to winter, females transition from an aggressive, territorial phenotype to an affiliative, group living phenotype. The thermoregulatory advantages of huddling have been offered as an explanation for winter sociality in meadow voles; thus, I designed a study to assess the effects of ambient temperature, day length, food availability, and frequency of handling on same-sex affiliative behavior and several potential physiological correlates. Adult female pairs were housed in varying combinations of day length (short (SDs) or long days (LDs)), temperature (21¢ªC or 10¢ªC), and food availability (ad libitum or restricted) and regularly assessed for food intake and body mass. After seven weeks of treatment, females were evaluated for same-sex huddling behavior using 3 h partner preference tests. Uterine mass and serum concentrations of corticosterone and estradiol were assessed for all focal voles. In another study, group size and social preferences were evaluated in male and female meadow voles. Voles were housed in same-sex trios at weaning and behavior tested at 70-100 days of age. During behavior tests, focal voles were presented with the options of huddling with one familiar individual (a member of their trio) or a trio of strangers. My findings suggest that:

1) Day length, food availability, and ambient temperature interact to regulate same-sex affiliative behavior in female meadow voles.

2) Low temperature exposure can modify social preferences without increasing huddling behavior.

3) In SDs, lower ambient temperature augments the propensity to interact with strangers without interfering with existing social bonds, whereas lower temperature in LDs disrupts the retention of bonds.

4) Food restriction enhances affiliative behavior in SDs without causing significant body mass decline.

5) Differences in handling modulate uterine mass, plasma corticosterone, and plasma estradiol without modifying same-sex affiliation.

6) Under certain environmental conditions, variations in same-sex affiliative behavior are correlated with plasma corticosterone and estradiol.

7) The propensity to join a group consisting of novel individuals varies by day length and sex.

Collectively, the results described in this dissertation suggest that conditions associated with winter (food scarcity, low temperatures, and short day lengths) increase social tolerance and promote aggregation of females into groups. Corticosterone secretion and the reproductive axis are also responsive to the environmental factors examined; however, current concentrations of blood plasma corticosterone and estradiol do not fully account for the behavioral responses observed.