Maternal licking of young: resource exchange and proximate controls.

Rat mothers lick the anogenital area of their young, stimulate reflexive urination, and ingest the urine. To quantify the transfer of body water from offspring to mother, we injected 5- to 30-day-old rat pups with tritiated water and the next day measured the radioactive label in maternal plasma. Rat dams reclaim about 16 ml of water/day from 5-day-old litters, 20 ml/day from 10-day-olds, and 40 ml/day from 15-day-olds. Urine consumption decreases to negligible amounts by Day 30. Time-lapse video recordings were used to measure maternal anogenital licking from Days 1-31 postpartum. Water transfer correlated with the developmental diminution of maternal anogenital licking. Maternal anogenital licking is controlled, in part, by the transfer of water from litter to dam and by the mother's salt appetite. Anogenital licking of pups was decreased after dams ingested a 0.15 M solution of NaCl but not after consumption of KCl, implicating a sodium specific mechanism. Licking of other regions of the pup's body was not affected. We discuss mother-infant interactions in terms of bidirectional exchanges of resources that constitute a "symbiotic" association.