A method for evaluating hunger and thirst in monkeys by measuring blood ghrelin and osmolality levels

Abstract

Hunger and thirst drive animals consumption behavior and regulate their decision-making regarding rewards. We previously assessed the thirst states of monkeys by measuring blood osmolality under controlled water access and related the thirst states to risky decisions for fluid rewards. However, hunger assessments in monkeys have been poorly performed. Moreover, the lack of precise measures for hunger states leads to another issue regarding how hunger and thirst states interact with each other in each individual. Thus, when controlling food access to motivate subject performances, how these two physiological needs are satisfied in captive monkeys remains unclear. Here, we measured blood ghrelin levels and osmolality for hunger and thirst, respectively, in four captive macaques. Using enzyme-linked immunosorbent assay, we found that the levels of blood ghrelin, a widely measured hunger-related peptide hormone in humans, were high after 20 h of no food access (with ad libitum water), which is a typical controlled food access condition. One hour after consuming the regular dry meal, the value decreased in three out of four monkeys to within the range of individual blood ghrelin levels. Additionally, blood osmolality measured from the same blood sample, the standard hematological index of hydration status, increased after consuming regular dry meals with no water access. Thus, ghrelin and osmolality may provide a precise reflection of the physiological states of individual monkeys for hunger and thirst, suggesting that these indices can be used as a tool for monitoring hunger and thirst levels that mediate the animals decision-making for consuming rewards.