Longitudinal Changes in Testicular Volume and the Additional Value of Body Weight for Estimating Histology-Linked Testicular Development in Male Cynomolgus Macaques (Macaca fascicularis)

Individual variation in pubertal development limits the usefulness of chronological age alone for assessing testicular development in male cynomolgus macaques. This retrospective study characterized longitudinal changes in externally measured testicular volume and evaluated the additional information provided by body weight in Chinese- and Cambodian-origin animals. Routine records collected between 2006 and 2026 were analyzed. After data cleaning, 31,310 observations from 22,305 animals were available, including 4331 animals with repeated measurements. A prespecified mean bilateral testicular volume threshold of 8.68 cm3, previously associated with bilateral histological Grade ≥ 4, was applied. Testicular volume increased markedly between approximately 4 and 6 years of age, with substantial inter-individual variation. At 5 years, the estimated probability of attaining the threshold was 54.3% in Cambodian-origin and 51.6% in Chinese-origin animals, with no clear evidence of different age-dependent threshold trajectories between origins. In animals aged 2–8 years, body weight remained strongly associated with threshold attainment after adjustment for age and origin (OR 4.18 per kg, 95% CI 3.93–4.44; p < 0.001). These findings demonstrate substantial variation in testicular development among similarly aged animals and support direct testicular measurement as the primary practical screening measure, with age and body weight providing complementary information.