I compared number of ultrasonic calls infant prairie voles (Microtus ochrogaster) and montane voles (M. montanus) emit when isolated. Typically, murid infants produce ultrasonic calls when they cannot thermoregulate adequately. Infant prairie voles called much more than did infant montane voles (average of 954 versus 17 calls/20 min at ages 0–10 days, P < 0.001) and more than other murids. Prairie voles also differed from montane voles in calling at high rates from day of birth, which is not typical of murids. The 2 species developed thermoregulatory competency at the same ages, so differences in calling could not be explained by differences in ontogeny. I suggest that rates of calling are related to social organization for, unlike montane voles and most murid species, prairie voles display monogamous mating behavior and a high level of biparental care. High calling rates could be adaptive for infant prairie voles, as parents are near almost all the time and can retrieve them; in contrast, montane vole mothers are away from the nest much of the time, increasing the risk of calling infants being overheard by a predator. The high rate of calling from day of birth in prairie voles may relate to prairie voles being nipple-clingers and so being at higher risk than montane voles of being dropped outside the nest when very young.