Telemetered Heart Rates of Free-Living Uinta Ground Squirrels in Response to Social Interactions

• Main Author: Ruff, Robert L.
• Format: Others
• Published: DigitalCommons@USU 1971
• Online Access: http://digitalcommons.usu.edu/etd/1611; http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2610&context=etd

Free-living Uinta ground squirrels (Spermophilus armatus) were instrumented with miniaturized ECG radio-transmitters to measure their short-term and long-term heart rate responses to social interactions. A continuous trapping and direct observation program prior to and during the study provided complete behavioral and life history information for virtually all ground squirrels on the 22-acre study area. Three broad categories of heart rate information were obtained: responses by squirrels (1) in their burrows, (2) above ground during social interactions, and (3) above ground when animals were not fighting and when conspecifics were both absent and present. Marked changes in the heart rate of ground squirrels occurred even as animals behaved in their burrows and conspecifics were absent. Some changes were caused by motor behaviors and appeared commensurate with different degrees of physical exertion. Others were evident among inactive animals and took the form of diurnal cardiac rhythms which were independent of immediate motor behavior hut nonetheless attuned to aboveground activity cycles. The heart rate of all ground squirrels increased well above burrow baselines when animals behaved aboveground. The greatest elevations occurred during social interactions which ranged from mere threat to physical combat. Combat encounters elicited responses which frequently exceeded 400 beats/minute as compared to burrow and aboveground baselines for inactive animals of 269 and 284 beats/minute , respectively. Threat interactions produced less dramatic increases, but because threat usually lasted longer, it was perhaps as stressful as combat. There were no significant differences between peak responses of dominants and subordinates, initiates and recipients of aggression, or sex and age classes. However, the duration of cardiac response was less for dominants and hence, they apparently experienced less stress than subordinates. Increases in heart rate, independent of motor behavior and immediate social interactions, were also observed as female ground squirrels roamed within and/or made occasional excurs ions outside their home ranges. These cardiac changes occurred regardless of the presence or absence of conspecifics in the vicinity. It was concluded that such cardiac acceleration represented chronic response to social interactions and was elicited by spatial factors, specifically spatial familiarity, which acted as conditioned stimuli (previously paired with combat). Such chronic responses were greatest during the initial establishment of territories by females, decreased during territoriality, and increased again following the breakdown of territoriality. So territoriality acted to reduce the magnitude of heart rate response and hence, stress among females. The mere presence of conspecifics within the immediate vicinity of an instrumented ground squirrel tended to elevate its heart rate. Therefore, even in the absence of overt social interactions, conditions of crowding also contributed towards chronic stress in the population. The effects upon heart rate with respect to an individual's sex, age, reproductive condilion, and its spatial and social relationship with neighboring ground squirrels are also discussed. Finally, the roles which social and nonsocial factors play in regulating animal numbers on the study are described.