Parturition in the rat ( Rattus norvegicus): Normative aspects and the temporal patterning of behaviours

Abstract

The purpose of this paper was to present a quantitative analysis of parturition in the laboratory rat. In addition, the effects of parity were also investigated. Gestational length was 22–23 days (day sperm found was Day 1) in 95% of the pregnancies. The average length of parturition was 97.3 min. The mean litter size was 10.84 pups and the average weight of each pup at birth was 6.49g. The effects of parity were limited to changes in two parturitional behaviours: Mount and Lordosis Contraction. These changes may reflect the conditioning of the uterus in response to a prior pregnancy. Pub births and placental expulsions did not occur at equidistant points throughout parturition. Following the birth of the first pup there was a characteristic lull in births. Indeed, almost two-thirds of the births occurred during the second half of parturition. The expulsion of placentas followed a similar time course though displaced to the right, reflecting the fact that placentas are expelled at some point following the birth of a pup. The main thrust of this paper was the temporal sequencing of parturitional behaviours. The 21 behaviours clustered into five phases which were defined in terms of their peak occurrence during parturition. The first phase — the Initiation Phase — was consonant with the birth of the first pup. During parturition, the female spent much of her time licking the first born, grooming her head, and sniffing in response to the novelty of the birth fluids. The second phase — the Contraction Phase — was marked by a lul in the pup births. During this phase the predominant contraction pattern shifted from Lordosis, the type of contraction seen prior to the birth of the first pup, to the transitional type (Intermediate) and finally to the Vertical Contraction. It was the Vertical Contraction which was closely associated with the birth of pups. The third phase — the Birth-Oriented Phase — included the second half of parturition, when the majority of the litter was born. The behaviours exhibited during this phase were intricately involved with pup births per se. The female spent much time grooming and pulling at the anogenital region, eating placentas, and grooming herself to clean the copious quantities of birth fluids. The fourth phase — the Termination Phase — was marked by the birth of the last pup in the litter. Sniff Pup reached its highest levels during this phase. The fifth phase — the Nursing Phase — began after all the pups had been born, cleaned, stimulated, and clustered. The female then became quiescent over the young and nursing was initiated. A sixth cluster of behaviours exhibited low levels of occurrence and the absence of temporal patterning. Thus, parturition in the rat is characterized by an orderly progression of behaviours through the parturitional period. In addition, several naturally occurring behaviours such as Groom Dorsal and Eat-Drink are maintained at very low levels throughout the delivery.