The effect of the presence or absence of the male, and of decreased ambient temperature (21 degrees C vs. 4 degrees C) on litter survival, pup survival and pup growth was measured from birth through day 18 after birth in Siberian (P. sungorus) and Djungarian (P. campbelli) hamsters. Siberian hamsters were not significantly affected by the experimental manipulations. In contrast, whereas 100% of litters and 95% of pups were successfully raised to weaning at 21 degrees C by paired Djungarian hamsters, survival fell to 47% when the mate was absent and even further, to 32%, when the ambient temperature was lowered. No significant differences in litter size or pup weight at birth were detected between species at the warmer temperature. However, P. sungorus pups gained weight significantly faster through day 12 after birth (while dependence upon the mother for food was absolute) than P. campbelli pups under all experimental conditions. Although the species are closely related, these data show that male Djungarian hamsters are essential to offspring survival under conditions where Siberian hamsters do not require conspecific help. Species differences in metabolism and thermoregulation may account for the differential pup survival.