Performance of Marmoset Monkeys as Embryo Donors Is Reflected by Different Stress-Related Parameters

SIMPLE SUMMARY: Does chronic distress from repeated anesthesia and frequent handling impair fertility and well-being in marmosets, or do the animals become used to study procedures over time? In a retrospective study, we calculated the average number of embryos isolated from early-pregnancy-stage marmoset monkeys/monthly cycle (embryo retrieval rate; ERR) in an experimental setting, including monthly anesthesia for uterus flushing and frequent (~twice/week) blood sampling in an awake state. Based on this, the monkeys were subdivided into two groups: one with high ERR and one with low ERR. ERRs were then related to markers of chronic stress (e.g., blood cortisol and weight fluctuations) in the animals. During the first year of study, the group with a higher ERR already seemed to be more stress-tolerant, as indicated by (1) low blood cortisol levels, (2) only minor body weight fluctuations, and (3) lower stress ratings in a subjective evaluation. By contrast, animals from the group with a low ERR showed signs of increased stress. Importantly, the animals from the low ERR group did not improve over time, whereas animals from the high ERR group exhibited constantly high ERRs over time. We conclude that some marmoset monkeys can handle frequent blood sampling and monthly anesthesia for embryo collection without being stressed, while others appear to be stressed from the beginning and do not improve over time. The determination of stress characteristics of animals before inclusion in future studies could reduce animal numbers and improve animal welfare. ABSTRACT: Non-human primates (NHPs) serve as embryo donors for embryo collection in order to mimic genetic diseases in humans by genetic modification. Reproductive health of the embryo donors is crucial, and chronic distress needs to be avoided. Embryo retrieval rates (ERR), anti-Müllerian hormone (AMH) concentrations, cortisol levels, and body weight fluctuations were assessed as markers for fertility and distress. With regard to successful embryo retrievals (total n = 667), the animals were either used for extended periods (long-term group; LTG) or only for short periods (short-term group; STG). Retrospective evaluation expectedly showed that animals in the LTG had a higher ERR than animals in the STG (p < 0.0001). Importantly, ERR in the LTG remained stable throughout the experimental period, and high embryo rates were already encountered during the first year of experimental use (p = 0.0002). High ERR were associated with high AMH and low cortisol levels, and minimal body weight fluctuations following anesthesia, indicating a superior ability of the LTG animals to handle distress. We conclude that the long-term experimental use of marmosets does not impair their fertility or health status per se, supporting the view that animal reuse can be in accordance with the 3R-principle, implying reduction, replacement, and refinement in animal experimentation.