A pheromone that rapidly promotes learning in the newborn.

Mammalian neonates depend on their mother's food supply and use a defined sequence of actions to find her mammary area. Their behavior is initially uncertain and demanding but rapidly becomes optimal. Efficient learning is thus operating in newborns. For instance, European rabbit (Oryctolagus cuniculus) pups localize the nipples through typical orocephalic movements. These movements are released by the mammary pheromone secreted in milk or by prenatally learned odor cues. During daily nursing, they also learn odors associated with the mother, supposedly with sucking as the main reinforcer. We here investigate the role of the mammary pheromone as an enforcer of early olfactory learning in newborn rabbits. In testing more than 950 pups, we show that the mammary pheromone promotes learning of neutral odorants paired with the pheromone in single and short trials. The pheromone-induced learning is efficient from birth and supports successive acquisition of distinct odorants. This reveals that a mammalian pheromone can function as a "cognitive organizer" that promotes early learning of relevant environmental cues.

Chemical and behavioural characterization of the rabbit mammary pheromone.

Mammals owe part of their evolutionary success to the harmonious exchanges of information, energy and immunity between females and their offspring. This functional reciprocity is vital for the survival and normal development of infants, and for the inclusive fitness of parents. It is best seen in the intense exchanges taking place around the mother's offering of, and the infant's quest for, milk. All mammalian females have evolved behavioural and sensory methods of stimulating and guiding their inexperienced newborns to their mammae, whereas newborns have coevolved means to respond to them efficiently. Among these cues, maternal odours have repeatedly been shown to be involved, but the chemical identity and pheromonal nature of these cues have not been definitively characterized until now. Here we focus on the nature of an odour signal emitted by the female rabbit to which newborn pups respond by attraction and oral grasping, and provide a complete chemical and behavioural description of a pheromone of mammary origin in a mammalian species.

Orientation response of newborn rabbits to odours of lactating females: relative effectiveness of surface and milk cues.

To assess temporal and spatial variations in the emission of the odour guiding them to the nipples, we exposed newborn rabbits, Oryctolagus cuniculus, to lactating females in a series of simultaneous olfactory choice tests. They showed a clear preference for females in early, rather than late, lactation, and in the prenursing, rather than postnursing, phase. Furthermore, the female's abdominal, and specifically nipple, regions were more attractive to pups than the back and non-nipple abdominal areas. Finally, the surface odour cues from the abdomen of lactating females were as attractive as the odour of rabbit milk itself. These data suggest that the source of the active compound eliciting preferential orientation is located on the nipple, a site allowing simultaneous evaporation in air and dissolution in milk. Copyright 2001 The Association for the Study of Animal Behaviour.