AbstractAs part of a comparative study of affiliative behavior, pups of two different vole species, Microtus ochrogaster (prairie voles) and M. Montanus (montane voles), were compared for their responses to social isolation during the first 2 weeks of postnatal life. Previous studies have demonstrated that under both laboratory and field conditions, adult prairie voles show higher levels of affiliation than adult montane voles, although the species closely resemble each other morphologically. In the current study, prairie vole and montane vole pups showed profound differences in the behavioral and physiologic responses to social isolation. During 5 minutes of isolation, prairie vole pups emitted 314 ± 60 (days 4–6) and 601 ± 55 (days 8–10) ultrasonic vocalizations. At these same ages, plasma corticosterone increased 4–6 fold within 30 minutes of separation. The increase in corticosterone was highly correlated with the number of calls (r = 66). In contrast, most montane vole pups emitted no isolation calls under identical conditions. Moreover, montane vole pups had equivalent baseline corticosterone concentrations, but showed only a minor increse in corticosterone following separation at 4–6 days with no increase apparent at 8–10 days. As montane vole pups were capable of producing high levels of ultrasonic vocalizations and increased concentrations of plasma corticosterone in responce to known stressors (tail suspension and halothane vapors), these result suggest that social isolation evokes less distress for montane vole pups than for prairie vole pups. The developmental difference in the amount of parent‐young contact was not a sufficient explanation for the differences in the response to separation as cross‐fostered montane voles failed to show an increase in ultrasonic vocalizations (although a slight increase in corticosterone was observed). Taken together, these studies indicate that species‐typical adult patterns of affiliation may be apparent early in development in the response of infants to social separation.