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 concerning rewards. We previously assessed the thirst states of monkeys by measuring blood osmolality under controlled water access and examined how these thirst states influenced their risk-taking behaviour in decisions involving fluid rewards. However, hunger assessment in monkeys remains 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 performance, it remains unclear how these two physiological needs are satisfied in captive monkeys. Here, we measured blood ghrelin and osmolality levels to respectively assess hunger and thirst in four captive macaques. Using an enzyme-linked immunosorbent assay, we identified 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). This reflects a typical controlled food access condition. One hour after consuming a regular dry meal, the blood ghrelin levels in three out of four monkeys decreased to within their baseline range. Additionally, blood osmolality measured from the same blood sample, the standard hematological index of hydration status, increased after consuming the regular dry meal with no water access. Thus, ghrelin and osmolality may reflect the physiological states of individual monkeys regarding hunger and thirst, suggesting that these indices can be used as tools for monitoring hunger and thirst levels that mediate an animal’s decision to consume rewards.

Significance statement Standard methods for behavioral and neurophysiological experiments in non-human primates rely on controlled access to food or fluid rewards to motivate their performance. We previously assessed the thirst state of monkeys by measuring blood osmolality, the most widely used hematological index of hydration status. Here, we assessed the hunger state of monkeys by measuring blood ghrelin levels, a widely measured hunger-related peptide hormone in humans, using an enzyme-linked immunosorbent assay. We measured these indices and found that they reflected the hunger and thirst states of the monkeys before and after consuming dry meals, with no relation to each other. Thus, these two physical indices can be utilized to monitor hunger and thirst in primates.