Functional assessment of sexual maturity in male macaques (Macaca fascicularis)

► Sperm in a semen sample correlates with sexual maturity. ► A reliable functional marker for sexual maturity is now available. ► Body weight, age and testis do not reliably predict male maturity. ► Cynomolgus monkey origin may determine age at male maturity. Selection of suitable criteria for assessing sexual maturity in the male long-tailed macaque (Macaca fascicularis) has yielded conflicting results. The present retrospective work investigates whether the sole presence of sperm in the baseline semen sample unequivocally (i.e. for every animal) hallmarks complete testicular maturation. For 956 animals providing the baseline semen sample, neither age, body weight nor testes volume unequivocally predicted the presence of sperm in that sample, and for 322 animals these parameters failed to predict testicular histology. In contrast, the presence of sperm in the baseline semen sample correlated with mature testis histology at study termination in every single animal (n=197/322). Surprisingly, for the 125/322 animals without sperm in the baseline semen sample, spermatogenesis was also mature in 95 animals. Thus, the mere provision of a semen sample without sperm – implying peripheral reproductive tract maturation – was associated with mature spermatogenesis in approx. 75% of animals. Interestingly, testicular maturation occurred approx. 2years earlier in Mauritian compared to Asian mainland animals. In conclusion, a single semen sample that contains sperm provides unequivocal evidence for mature spermatogenesis and, thus, is suggested as a functional parameter for sexual maturity assessment in this species.