The amount of time infant rats 3-13 days old spent emitting ultrasounds was greater when pups were placed in an empty dish than when placed in a dish containing clean bedding or soiled bedding from the nest. Pups from 5-13 days of age vocalized more when on the clean bedding than when on the soiled bedding. When placed on cloth covered dishes, pups vocalized most to the empty dish and least to the dish containing soiled bedding. The odor of clean bedding elicited less ultrasonic vocalization than the odor of the empty dish only when less than 7 days old. Pups vocalized more to room temperature than to heated conditions. The results indicate that olfactory, tactual, and thermal nest cues influence infantile ultrasonic vocalization in the rat.

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Previous investigations reported that providing non lactating maternally behaving adult females to infant rats weaned at 13 days of age substantially reduces the mortality rate of such weanlings. In experiments undertaken to isolate the maternal variable(s) promoting survival of early weaned rats we found that periodically inducing micturition or stimulation by handling did not alter viability. Thirteen-day-old weanlings did survive if provided with a source of heat, but did not survive at normal room temperature, It appears that the role of the nonlactating mother in the early weaning studies, at least in promoting survival, may simply be that of maintaining adequate body temperatures.Normal or spontaneous weaning of young rats is a gradual process that occurs between 14 and 28 days of age (Krecek, 1971). Growing evidence indicates that experimentally induced early and suden separation of rat pups from their mother results in a number of lasting physiological and behavioral effects, either from social and/or nutritional factors. Thus, Novakova (1966a, b, 1970) found that early weaning impaired subsequent acquisition of classical conditioning but that either a high-fat diet or returning the mother to the litter after cauterizing her nipples to prevent lactation reduced the learning deficit. When pups were provided with both a high-fat diet and nonlactating mother, the effects of early weaning on learning were completely blocked. Plaut and Davis (1972) found that the presence of a nonlactating rat "aunt" or a cauterized mother attenuated cerebral threonine deficits in early weaned animals but did not prevent decreases in body weight, brain weight, and/or cerebral aspartic acid in rats weaned at 15 days. When weaning was forced at an earlier age, 13 days, rat aunts were able to increase dramatically the proportion of animals surviving until 21 days (80% vs. 10%). Plaut and Davis suggested that these latter findings might be attributed to the influence that an adult rat can have on directing the eating behavior of infants (Galef, 1971). However, Krecek (1971) in reporting a difference in the viability of I4-day-old vs. l o-day-old early weaned rats, offered an alternative explanation: deaths due to weaning at 14 days resulted from "disruption of bladder function" due to the immaturity of the micturation response.In attempting to isolate the aspect(s) or maternal deprivation responsible for the high mortality of rat pups weaned at 13 days, we conducted several initial experiments which will be briefly mentioned before describing the last and most critical study. The first two experiments were conducted to determine whether periodic elicitation of micturition by stimulation of the genitalia or periodic tactual stimulation by handling would enhance survival of early weaned rats. Surprisingly, virtually all of the I3-day-old weanlings in these studies survived regardless of experimental assignment.Because methodological details differed between our first two studies and those of previous investigators, a third study...