All animal procedures complied with the NIH Guide for Care and Use of Laboratory Animals, and were approved by the institutional animal care and use committee of the Royal Netherlands Academy of Arts and Sciences. Two male macaque monkeys participated in the experiment. They were 5 (M1) and 8 (M2) y old at the start of the experiments and weighted between 7 and 8 (M1) and 8 and 9 (M2) kg over the course of the recordings. The monkeys were socially housed in pairs in a specialized primate facility with natural daylight, controlled humidity and temperature. The home cage was a large floor-to-ceiling cage that allowed natural climbing and swinging behavior. The cage had a solid floor, covered with sawdust, and was enriched with toys and foraging items. The diet consisted of monkey chow supplemented with fresh fruit. The access to fluid was controlled, according to a carefully designed regime for fluid uptake. During weekdays the animals received diluted fruit juice in the experimental set up upon correctly performed trials. We ensured that the animals drank sufficient fluid in the set up and supplemented extra fluid after the recording session if the monkeys did not drink enough. In the weekend the animals received at least 700 mL of water in the home cage supplied in a drinking bottle. The animals were regularly checked by veterinary staff and animal caretakers and their weight and general appearance were recorded in an electronic logbook daily during fluid control periods.