Transport of Young in the Norway Rat

Abstract

<p>During the first two to three weeks of their life, the altricial young of the Norway rat remain in a nest constructed by the mother. If this nest is distributed, the mother will abandon it and transport her young individually to a new location.</p> <p>When the nests of lactating rats were destroyed daily from the day of birth (day 0) through day 30 of lactation, it was found that the probability of transport of young was high until the beginning of the third week postpartum. After day 14, the probability of transport began to decrease, and 90% of mothers had ceased transporting entirely by day 20.</p> <p>When the nest box of a day 4 or day 12 mother was not disturbed, or when a concealing paper nest remained after the cover of the box was removed, the probability of transport was very low. However, exposure of the mother and young, destruction of the nest, and flooding of the nest were found to be sufficient to elicit transport.</p> <p>The elimination of pup distress signals by anesthetization of the litter did not decrease the probability, nor increase the latency of transport. Therefore, mothers appear to be responding directly to the characteristics of the disturbance itself, rather than to the distress of the pups.</p> <p>Although distress signals from the young are not necessary for the elicitation of transport behavior, they are sufficient to increase the probability of transport when the mother and young remain concealed by a paper nest after the nest box cover is removed. Distress cries emitted by a single pup induced the mother to transport the entire litter. No preference was shown for the pup emitting the cries.</p> <p>The young also facilitate their transport to a safe nest by exhibiting a characteristic transport response in which all of their extremities are tucked in close to the body. This pup transport response is normally elicited by the tactile stimulation of the skin received from the mother's teeth. Grasping the skin of almost any area on the dorsal and lateral surfaces of the body was sufficient to elicit the transport response, and mothers were observed to carry pups by grasping the skin anywhere in these areas.</p> <p>The pup transport response develops gradually over the first 10 days of life, and then can be reliably elicited until the end of the third week postpartum. After this time, the response becomes more difficult to elicit.</p> <p>The ontogeny of the pup transport response suggests its function; that is, the response is seen during the time that mothers will still carry larger pups which may be unwieldy. The elimination of the pup transport response by light anesthesia does not impede the transport behavior of the mother on day 5, when pups are relatively small. On day 12, however, when pups have become relatively large, elimination of the transport response causes the pups limbs to drag along the ground, interfering with the mother's locomotion and increasing the time necessary to transport young, either through a tunnel or in an open runway.</p> <p>The rat mother and young thus interact on several levels to ensure the safety of the litter after nest site disturbance.</p>