Effects of social housing of sexually mature male cynomolgus monkeys during general and reproductive toxicity evaluation

We investigated the effects of social housing on reproductive parameters and body weight in mature cynomolgus monkeys, the predominant nonhuman primate model in (reproductive) toxicology. Group housing (n = 7) delayed body weight gain compared to single housing. Testicular volumes decreased to approx. 45% of baseline within 13 weeks in low ranking animals followed by return to baseline during weeks 21–26. Interestingly, ejaculate and endocrine parameters did not exhibit corresponding changes. Following separation of this group into a high rank group (n = 4) and low rank group (n = 3), testicular volumes varied within background variation (±15%). Re-allocation of new animals with prior contact/group housing experience into groups of 3 animals also prevented body weight and testicular effects. In conclusion, group formation markedly, albeit transiently, altered body weights and testicular size, and these effects could largely be avoided by previous social interaction for at least 26 weeks. These findings should be considered during social housing of mature animals in toxicity studies.