Total parental expenditure of mammals and birds has been predicted to increase with litter or brood size, but data from rodents provide little support for this prediction. We examined the effects of natural variation in litter size on parental nest attendance in pine (= woodland), prairie, and meadow voles housed in seminatural environments. In all 3 species, mothers spent less time in the nest with large litters than with small; time spent in the nest by fathers was unaffected by litter size. We suggest that failure to meet the prediction of increased parental expenditure with increased litter size reflects basic differences in the manner in which mammals and birds feed altricial young.