Maternal licking by virgin and lactating rats: water transfer from pups.

Rat dams provide water to their young via milk. Dams reclaim much of this water by licking the pups' anogenital areas, stimulating reflexive urination and consuming the pups' urine. Sensitized virgin rats, induced to act maternally do not provide water to pups, but they nevertheless lick them. To determine whether bidirectional transfer of water between the rat mother and her litter mediates maternal licking, water transfer from pups to sensitized virgins was compared with that to lactating dams. We used time-lapse video recordings to measure anogenital licking of pups. Sensitized virgins and lactating dams spent equivalent amounts of time licking the anogenital regions of test litters. We quantified the amount of water transferred from offspring to both virgins and dams by injecting pups with tritiated water and measuring the radioactive label in maternal plasma after interaction with a litter of 5-day-olds. Dams obtained more than twice as much urine from the litter in 4 hr than did the maternal virgins. Differences in the amount of water obtained from pups were due to differences in urine availability caused by the receipt of milk from the dams. When the dams' nipples were ligated, so that their pups received no milk, ligated dams and virgins consumed equivalent amounts of pup urine. Maternal licking and urine consumption are not dependent solely upon the bidirectional exchange of water between the dam and her offspring.