Animals can benefit from living in social groups for many reasons. For example, individuals living in a group can share the burden of finding food resources, watching for predators and caring for offspring. However, we know very little about the physiological mechanisms that contribute to social advantages. This project will address this deficiency by evaluating the relationships among stress hormones, immune function, behavior, survival and kinship in Belding?s ground squirrels. Specifically, this project will test how living near kin or non-kin affects stress hormones and immune function as well as how increased stress hormones change predation risk. Captive experiments will be conducted with wild-caught squirrels and will measure genetic relatedness, hormone levels and immune function with laboratory assays. It is predicted that individuals living with kin will exhibit reduced stress hormones, increased immune function and will perceive reduced predation risk. It is also predicted that individuals with experimentally increased stress hormone levels will perceive greater predation risk. This research will have broader impacts because it will improve our understanding of how physiology shapes social benefits. This is relevant to all social vertebrates, including humans, because they all share the hypothalamic pituitary adrenal axis that regulates stress responses. In addition, this research program has a strong history of outreach. Two undergraduates and two field assistants will be trained in behavioral, ecological and laboratory techniques. In addition, the results of this research will be used to teach urban Chicago youth about behavioral research and the scientific method.
Living in a social group can be beneficial for individuals. For example, group living may improve health measures and reproductive success by reducing stress, improving immune function or modifying behavior. Belding’s ground squirrels are social rodents that live in large colonies in the western United States. Squirrels are preyed upon by both terrestrial and aerial predators and are therefore under constant predation pressure. Living in a group, however, may reduce predation pressure and improve general health. We can measure the amount of predation pressure an individual perceives by measuring its vigilance behavior, or the time it spends looking for predators. Group living may impact vigilance behavior because individuals feel safer living in a group. Stress hormone levels may also impact vigilance behavior because if an individual perceives a high risk or predation, it may experience an increase in both stress hormone levels and vigilance behavior. We sought to test both how group size and stress hormones impacted vigilance behavior. To do so, we conducted two experiments on temporarily captive squirrels that we trapped in the wild. In the first experiment, we measured how individuals modify their vigilance behavior when group size changes with the expectation that individuals would feel safer in larger groups. Our results supported this idea because individuals were less vigilant as group size increased. We also found that collective vigilance, or the proportion of time that at least one individual was vigilant, increased as group size increased. Squirrels may be monitoring the vigilance behavior of other squirrels because individuals were only vigilant simultaneously 30% of the time. These results suggest that Belding’s ground squirrels alter their behavior depending on group size, likely because they perceive lower predation risk in larger groups. In addition, even though individuals lower their vigilance behavior in larger groups, the group as a whole is more protected against predators because the proportion of time when at least one individual is watching for predators increases at larger group sizes. In the second experiment, we tested whether raising stress hormone levels would increase vigilance behavior with the expectation that higher stress would correspond to an increase in perceived predation risk. The treatment succeeded in raising the stress hormone levels of the treatment animals but did not impact vigilance behavior. This may be because stress hormones do not mediate vigilance behavior in this species or because we did not raise stress hormones enough to see a difference in behavior. Together, the results from our two studies suggest that perceived predation risk, or vigilance behavior, is mediated by social life and not by stress hormone levels. In related studies on wild squirrels, we have found that individuals who are less vigilant are able to spend more time foraging and thus gain more weight over the short summer season. These individuals then have a better chance of surviving the winter hibernation. Thus, a reduction in vigilance behavior can have far-reaching and important effects. Over the course of the project, we trained three undergraduates and two graduate students in laboratory techniques as well as four field assistants in field techniques. Both of the PI’s interacted with Chicago school children on multiple occasions through career days, science fairs, and local museum outreach events. In addition, Co-PI Brooks was a founding member and the secretary of the Chicago chapter of Graduate Women in Science, an organization dedicated both to supporting women scientists and increasing the general public’s appreciation of science.
