Mammary pheromone-induced odour learning influences sucking behaviour and milk intake in the newborn rabbit

Newborn rabbits, Oryctolagus cuniculus, locate their mother's nipples through typical orocephalic movements elicited by odour stimuli, in particular by the mammary pheromone (MP). The MP also promotes neonatal odour learning: after single pairing with the MP, an initially neutral odorant becomes able to elicit sucking-related head-searching/oral-grasping movements. However, the behavioural significance of the MP-induced odour learning remains poorly understood. We carried out three experiments to explore its influence on milk intake and compare its consequences with those resulting from nursing-induced conditioning. First, pups conditioned to an odorant by pairing with the MP on postnatal days 2-3 were shown to gain more milk on day 4 during nursing by a female carrying the conditioned odorant along the nipple lines. Second, surprisingly, nursing-induced odour learning failed to induce this effect. We therefore determined whether the location of the conditioned odorant on or around the nipples modified the pups' milk intake: it appeared that after nursing-induced conditioning, the pups gained more milk when the conditioned odorant was applied directly on the nipples. Moreover, several results showed that pups could learn different odorants during successive days of conditioning, and that the more recently acquired cue was the most influential on milk intake. This study suggests that the MP plays a critical role to ensure sucking performance in newborn rabbits, not only through its releasing effect, but also through its ability to promote the acquisition of novel odours carried by the mother.