Researchers show for the first time that a mammal begins to suckle its mother's milk through a learned response built on learning her unique combination of smells. When it is born, the newborn is exposed to the smell of its mother's amniotic fluid and the baby then responds to those smells to feed.
Prevailing thought has been that pheromones -chemicals that trigger an innate behaviour - drove the suckling response as an automatic behaviour. The new work determines that, in mice, the smells must be learned before the behaviour can occur.
Suckling is a critical step for survival in mammals, which are defined by giving birth to offspring that need to feed from their mother's milk. The newborn must begin to feed soon after birth or it will die. It is a crucial, defining behaviour in mammals and offers researchers an opportunity to investigate the biology of instinct.
Previous research into suckling has shown that European rabbit mothers use a pheromone to initiate suckling in their newborn babies. This led most scientists to think that all mammals were likely to use the same mechanism. Keen to discover the pheromone involved in other mammals, the team chose the mouse because they have a parenting style similar to that of humans, nurturing and caring for their young.
"We were expecting to find a pheromone controlling suckling in mice, but we found a completely different mechanism at work," says Dr Darren Logan, lead author of the study from the Wellcome Trust Sanger Institute. "We have shown for the first time that it is not a pheromone response in mice, but a learned response, founded on a mix of odours: the unique signature smell of the mother."
To discover the smells involved in initiating suckling, researchers introduced newborn mice delivered by Caesarean section to breasts that had been washed clean and then soaked in one of the fluids that a baby would first inhale at birth. These included amniotic fluid, the mother's saliva (from being licked clean), breast milk and urine. Only the breasts that smelt of the mother's amniotic fluid initiated suckling.
The team then tested for the presence of a pheromone in the amniotic fluid. They fed pregnant mice strong smelling foods, such as garlic, to change the signature odour of the mother. If a pheromone was involved, the garlic would have no effect on suckling. In fact only those mice that had previous exposure to the amniotic fluid with the strong smell from their mother were able to feed successfully, proving the signature odour must be learned.