Nesting Behavior of Lactating Norway Rats: Influence of Dams' Choice of Temperature and Pup-Associated Stimuli
Norway rat dams show an intermittent pattern of contact with their young during the first two weeks postpartum. Previous research has shown that one important factor influencing the length of the intermittent contacts (or nest bouts) is the body temperature of the dam. Dams tend to terminate nest bouts when their body temperature rises, and over the first two weeks postpartum nest bout durations decrease, as the dam experiences an elevation in body temperature during a nest bout both more frequently and more rapidly. In the present series of studies, the analysis of the pattern of mother-young contact is extended to include an investigation of the thermal environment that the dam herself chooses to nest in and the consequences of this choice for the pattern of nesting behaviour. Further, not only are nest bout durations considered in these studies, but also nest bout frequencies and interbout intervals.
It was shown that when dams were allowed to choose from a wide range of surface temperatures they placed their pups on a warm surface (approximately 32°C) and seemed to prefer to spend time between nest bouts on a fairly warm surface (approximately 24 - 26°C). In comparison to dams that were not given a range of temperatures to choose from, dams that established a warm nest site and spent time between nest bouts on a warm surface showed much shorter nest bout durations, a higher frequency of nest bouts (although the frequency of nest bouts that were long enough for milk delivery to occur was similar between groups), and longer durations between nest bouts. While the short nest bout durations were most probably due to the fact that the dams were more likely to experience a fairly rapid rise in temperature during nest bouts, it was necessary to test separately whether the temperature that the dams preferred between nest bouts or the warm temperature at which the pups were maintained contributed to the higher frequency of nest bouts and the prolonged durations between nest bouts. It was found that if dams were forced to spend all interbout intervals on a fairly warm surface (approximately 26°), they showed much longer intervals between nest bouts which were typically accompanied by a slow decline in the dam's body temperature. Maintaining the pups at a warm termperature during the dam's absence produced prolonged intervals between nest bouts in which milk delivery could occur and also a much higher frequency of nest bouts.
While previous investigations had assumed that the pups play a passive role in the determination of neat bout termination, it was found in the present investigation that the pups played both passive and active roles in the overall pattern of nesting. It was shown that dams maintain contact with pups if the pups are cold. Pups might also play a more active role in maintaining contact with the dam by attaching to the dam's nipples soon after contact is established, because failure to attach to the nipples soon after contact was established frequently led to rapid nest bout termination. Finally, it was found that pups could actively curtail interbout intervals by emitting ultrasonic vocalizations. While ultrasonic vocalizations are typically emitted only when the pups are subjected to some stress, and thus may not be important in a controlled laboratory situation, this finding is of particular significance, as it indicates that the pups are not merely passive recipients of maternal care, but rather are capable of actively altering the pattern of nesting behaviour.