Contribution of Suckling Stimulation to the Daily Periodic Display of Nursing Behavior in Non-Lactating Virgin Rabbits

Abstract

A female rabbit (does) nurse once a day for three min with circadian periodicity. This timing is controlled by the amount of suckling stimulation received as small litters (2-4 kits) disrupt nursing periodicity and increase the duration of nursing bouts. Yet, as suckling induces: a) the secretion of oxytocin and prolactin and b) milk ejection, the relative contribution of those two types of factors to the daily timing of nursing needs to be independently explored. Therefore, the objective of the present work was to determine the relevance of milk output for the circadian display of rabbit nursing behavior. Methods: We used the model of anosmic virgins, as they can be induced to behave maternally (by lesioning the olfactory epithelium), they allow suckling from (foster) kits, but they are unable to produce milk. Does were given five foster kits, aged 1-5 days, placed inside a transparent nest box within the maternal cage, and replaced daily. The does' behavior was recorded continuously for 15 days through a video system allowing us full view of the animals. Results: By day 6 post-lesion 70% of virgins (i.e., 14/20) were entering the nest box, adopting a crouching posture over the litter, allowing suckling, and leaving ca. 3 min. later. 55% (i.e., 11/20) of lesioned virgins displayed nursing behavior with a ca. 24 hr periodicity, as determined by a Rayleigh analysis (3 does) or a linear regression (8 does). Conclusions: a) daily periodicity of nursing behavior can occur regardless of milk output or a preceding pregnancy; b) such factors, however, contribute to the display of a single daily episode of circadian nursing behavior in most does.