Suckling.

Suckling is the only behavior that is common among mammals. In newborn albino rats it is originally elicited by amniotic fluid deposited by the mother during parturition. Subsequent suckling is stimulated by saliva deposited on the nipples by the infant rats. Internal controls over the volume of milk suckled do not appear until infant rats are about 2 weeks of age at which time gastric distension, milk, systemic dehydration, and intestinal hormone cholecystokinin suppress milk intake derived through suckling. The development of controls over suckling appetite appears to parallel that of consummatory control. Until about 2 weeks of age infant rats choose to suckle a nonlactating nipple with the same frequency as a lactating nipple. Thereafter, the lactating nipple is unanimously chosen. These studies suggest differences and commonalities in the suckling behavior of laboratory rats and other mammals.