Enhancing Breeding Success Through Simple Intrauterine Insemination in Large-Scale Cynomolgus Macaque Colony

Ultrasound-guided intrauterine insemination (IUI) has been successfully employed in cynomolgus macaques, yet its effectiveness in large-scale, multi-age cohorts has not been fully explored. Here, we implemented IUI within large breeding colonies to boost pregnancy rates in cynomolgus macaques. Ovulation timing was estimated by monitoring menstrual bleeding, after which IUI was performed. We then compared single menstrual cycle (SMC) pregnancy rates between IUI groups and harem-style mating groups. Our findings revealed that the overall pregnancy rate in the IUI group was significantly higher than in the harem-style breeding group across all ages. Notably, this advantage was mainly attributable to monkeys over 10 years old; in contrast, no significant difference was observed among 4–5-year-old and 6–10-year-old monkeys. Although pregnant females in the IUI group exhibited a higher birth rate compared to those in harem-style breeding, this difference was not statistically significant. Collectively, these results establish a basis for refining reproductive technologies in older cynomolgus macaque breeding programs and highlight potential improvements in managing large-scale colonies.