Abstract

Simple SummaryMacaques are highly social animals and commonly used in biomedical research. These studies often require group-living animals to be pair-housed in a controlled environment. This controlled environment likely affects stress and body fat levels. This study investigates the effect of a change from group- to pair-housing on cortisol, as measure for stress, and body fat levels of 32 female rhesus macaques. Besides individual differences, cortisol levels were higher in pair-housing compared to group-housing. Body fat levels did not differ between housing conditions. Accordingly, there was no clear association between cortisol and body fat levels.Macaques are among the most commonly used non-human primates in biomedical research. They are highly social animals, yet biomedical studies often require group-living animals to be pair-housed in a controlled environment. A change in environment causes only short-term stress in adapting individuals, while non-adapting animals may experience long-term stress that can adversely affect study results. Individuals likely differ in their ability to adapt depending on individual characteristics. Changes in cortisol and body fat levels may reflect these different individual responses. Here, we investigate the long-term effect of a change from group- to pair-housing on cortisol and body fat levels in 32 female rhesus macaques, exploring whether age, dominance rank, original cortisol, and body fat levels are related to long-term stress in pair-housing. Hair samples were analyzed for cortisol levels, while anthropometric measurements and computed tomography were performed to quantify body fat. Monkeys served as their own control with a 7.5-month period between the measurements. Cortisol levels increased, while average body fat levels did not differ when individuals were moved from group- to pair-housing. Cortisol and body fat levels were not significantly correlated. Changes in cortisol were independent of age and dominance rank, whereas individual variation in body fat alterations was related to the group-housed body fat level and dominance rank. Although this study did not identify individual characteristics related to long-term stress in pair-housing, the individual variation confirms that some individuals are more resilient to change than others and provides possibilities for future refinement studies.

Highlights

  • Macaques are among the most commonly used non-human primates (NHPs) in biomedical research due to their high level of similarity to humans [1,2]

  • Age had no significant effect on weight-for-height index (WHI) (F (1,24) = 2.030, p = 0.167), but older individuals did have a higher Total body fat percentage (TBF) compared to younger macaques (F (1,24) = 6.145, p = 0.021)

  • This study investigated the effect of moving female rhesus macaques from groupto pair-housing on hair cortisol concentrations (HCCs) and body fat levels and aimed to identify individual characteristics associated with long-term stress in pair-housing

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Summary

Introduction

Macaques are among the most commonly used non-human primates (NHPs) in biomedical research due to their high level of similarity to humans [1,2]. All studies need to comply with the principles of the 3Rs: replacement, reduction, and refinement [3], and welfare of the animals must be ensured as well as possible. Since the expression of natural behavior is a commonly used indicator of animal welfare, housing conditions should provide possibilities to perform species-specific natural behavior [4,5]. Optimal housing conditions for macaques consist of naturalistic group-housing in large enclosures that mimic their natural habitat. These conditions are generally not feasible in biomedical research, since this requires a more controlled environment [13,14,15]. NHPs in biomedical studies are usually pair-housed in smaller cages in an indoor facility [13,16]

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