Maximum sodium chloride intake and thirst in domesticated and wild Norway rats.

T HE POLLOWING STUDIES are concerned with the questions of a) how much salt can be put in a rat’s food before it stops eating; b) how much salt can a rat ingest before toxic symptoms set in; and c) the extent to which salt toxicity depends on the amounts of water that a rat drinks in relation to the salt content of its diet. These studies were originally undertaken because of observations made during an investigation of the effects of domestication on the Norway rat (I). In the course of that investigation comparisons between recently trapped wild Norway rats and domesticated Norway rats from our colony showed: a) that the adrenals are definitely smaller and less active in the domesticated rat; b) that 2 or 4% salt added to the diet resulted in a greater increase of water intake in wild than in domesticated rats; and c) that about 30% of wild rats trapped from the streets have an abnormally high water intake in relation to body weight. These observations indicated that wild and domesticated rats may differ in their salt and water metabolism, perhaps as a direct or indirect consequence of differences in the size and activity of their adrenals. Further experiments have now been undertaken to determine just what differences in salt and water metabolism actually exist between these two strains. The results, which are presented in this report, show that marked differences do exist between the two, but more importantly, they indicate principles that may be involved in the ingestion and metabolism of very large amounts of salt and water by animals in general.