The emergence of spontaneous licking of salt and water in the rat

Sodium hunger can be evoked in the rat suckling 72-h postnatally by stimulating the brain renin-angiotensin system and at 12 days-of-age by sodium depletion. Yet it is not known whether the suckling utilizes these capacities in the nest. Our observations were designed to reveal whether and when these abilities emerge spontaneously in the suckling rat. Litters with their dams were continuously videotaped from 8 to 21 days of age to study the development of spontaneous salt intake. Pups in a litter spontaneously licked 3% NaCl solution when they encountered it during exploration as early as 13 days of age (range 13-16 days). By 14 days of age pups moved toward the salt repeatedly to lick it. During the 24 h after a pup first tastes salt, it licks it for about 10s each time it approaches it, and makes between one and four approaches. The developmental pattern of salt and water licking is similar in frequency of licking and in timetable, and most licking occurs at night. Note that the behaviors appear before the pups' eyes open at 15 days. We conclude that this behavior may be functionally important for the suckling, guiding it to sources of salts in the nest such as lacrimal secretions and urine. (Support: Israel-US Binational Science Foundation.).