Multidimensional ejaculate-level variability in sperm quality in cynomolgus monkeys

Semen analysis in non-human primates is widely used for colony management, reproductive studies, and experimental design. However, variability in sperm quality across repeated ejaculates and its implications for semen interpretation remain insufficiently characterized. In this study, repeated semen samples were collected from individual cynomolgus monkeys and evaluated using conventional semen parameters, computer-assisted sperm analysis (CASA)-derived kinematics, and functional sperm assessments, including viability, morphology, acrosome integrity, and acrosome reaction. Substantial variability was observed across repeated ejaculates. Progressive motility exhibited marked variability and was not consistently associated with sperm concentration at the ejaculate level. Kinematic parameters, such as amplitude of lateral head displacement, varied independently and did not uniformly correspond with motility percentages. Functional assessments further revealed heterogeneous patterns, with relatively stable viability and morphology but greater variability in acrosome-related parameters. Collectively, these findings indicate that sperm quality parameters do not vary in a concordant manner and reflect distinct biological properties. Sperm quality in cynomolgus monkeys therefore represents a multidimensional and ejaculate-dependent biological state rather than a fixed individual characteristic. Variability across repeated ejaculates may influence semen interpretation, indicating that single-ejaculate assessment may incompletely represent male reproductive status.