After publication of this article [1], questions were raised about the use of fluid control in the study protocol. The “Subjects and housing” subsection of the Material and methods [1] includes a description of fluid control and assessment of animals’ hydration status: “Access to water was regulated prior to experimental sessions conducted by the lab in order to maintain task motivation for experiments that use fruit juice as reward. … Hydration status was assessed by general appearance (bright, alert, responsive), body weight, skin turgor, and fecal output or consistency by members of the laboratory and veterinary staff.” However, no fluid or food control (restriction) was used to motivate the animals to perform the task in the PLOS ONE study [1]. The fluid control statements were included in the paper as background information on protocols approved for other studies in the lab, some of which involved animals that were also used in the PLOS ONE study. Only one monkey used in the PLOS ONE study [1] also participated in a concurrent study involving fluid control. In that study, the 20 mL/kg/day level stated in the Materials and methods section was a minimum allowable value, not the level actually used, as recommended in [2]. When fluid control was used with this animal, a gradual taper procedure was employed to find the level of daily fluids that would incentivize the animal to perform tasks within the relevant study protocol for which food treats were used as rewards, as described in [3]. Working with a veterinarian to monitor the monkey’s health and welfare, the individual monkey’s daily fluid access was gradually titrated each week until the necessary performance on the task was reached, within the 20mL/kg/day guidelines. While on fluid control, any signs of dehydration would immediately trigger free access to water; however, no signs of dehydration were ever observed. The authors’ prior work showed that this type of fluid control protocol motivates performance while leaving physiological parameters such as body weight and weight gain unaffected [4].
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