In the 1930s, it was discovered that seals can maintain their water balance through pre-formed water in their diet and water produced during metabolism. This finding has been supported by later studies. Although there is no empirical evidence of true seals drinking saltwater voluntarily, observations suggest that incidental ingestion during feeding may account for a small percentage of their total water intake. In contrast, eared pinnipeds have been observed drinking seawater voluntarily, primarily in warm environments to assist with nitrogen excretion and thermal regulation. Galápagos fur seals drink seawater, but Antarctic and northern fur seals do not. Harp seals have been observed consuming fresh water in the form of ice, and harbor seals have been seen drinking fresh water when available. However, it is unclear if fresh water intake is significant for maintaining fluid balance in seals. Overall, drinking either salt or fresh water does not seem to be a common behavior in seals. In captive seals, dietary salt supplements may be necessary to prevent hyponatremia.