Cardioacceleration in alloparents in response to stimuli from prairie vole pups: the significance of thermoregulation.

Autonomic responses, including changes in heart rate and respiratory sinus arrhythmia (RSA) can provide indications of emotional reactivity to social stimuli in mammals. We have previously reported that male prairie voles (Microtus ochrogaster) spontaneously care for unfamiliar infants, showing a robust and sustained increase in heart rate in the presence of a pup, thus providing an opportunity to examine the physiology of care-giving in reproductively naïve animals. However, the purpose of such heart rate increases has not been explained by previous efforts. In the present study, we first compared male and female prairie vole cardiac responses in the presence of a pup and found no evidence of sex differences in heart rate or RSA. Using male prairie voles, we then examined the characteristics of pups that were capable of eliciting physiological responses, including age of the pup and pup odors. As prairie vole pups increased in age they vocalized less and there was an associated decline in alloparental cardioacceleration. Exposure to pup-related odors induced cardioacceleration in adult males, and this effect also diminished with increasing pup age. Finally, we were able to block the cardioacceleratory effect when the testing environment was warmed to a temperature of 36°C [vs ambient room temperature (approximately 22°C)]. These findings suggest that pup-induced cardioacceleration is a robust phenomenon across alloparental prairie voles of both sexes, and depends on multi-modal processing of different stimuli from the pups. Young pups require care-giving behavior, which appears to drive cardioacceleration in the alloparents. This study also supports the usefulness of autonomic measures in the evaluation of social experiences.