Sodium Availability and Fecal Aldosterone Concentration of Wild and Captive Japanese Macaques and Sika Deer

Sodium acquisition is important for terrestrial plant-feeding animals, since their main food (plants) does not contain a sufficient amount of sodium. To reveal the physiological response to sodium deficiency among terrestrial plant-feeding animals, we investigated the fecal aldosterone concentration of Japanese macaques and sika deer living under various conditions of sodium availability. Aldosterone is a mineralocorticoid hormone that stimulates resorption of sodium in the kidneys. Among Japanese macaques, fecal aldosterone concentration was low in captivity and in one wild population whose members habitually ingest sea water. The captive monkeys fed exclusively on monkey chow, which ensures sodium sufficiently. In other wild populations, even those living near the coast, fecal aldosterone concentration was high, and thus they seem to depend on resorption of sodium in the kidneys as a strategy of sodium acquisition. Among sika deer, fecal aldosterone concentration was lower in captive individuals than in wild ones. Deer pellets fed to captive animals are sodium rich, but no such foods are available to wild deer. This study suggests that the resorption of sodium in the kidneys is a prevalent strategy among terrestrial plant-feeding animals by showing higher aldosterone concentrations, except in rare situations where sodium intake can easily occur.