Sperm Morphometric Subpopulations as the Seasonal Biomarkers of Sperm Quality in Squirrel Monkeys (Saimiri collinsi)

Abstract Sperm subpopulations are distinct cell clusters that define sperm function and serve as biomarkers for optimizing donor selection and reproductive success in seasonally breeding species. Male squirrel monkeys ( Saimiri collinsi , an Amazonian primate), exhibit marked seasonal reproduction linked to the fattening phenomenon, an androgen-dependent weight gain coinciding with the mating season. We investigated sperm subpopulations and semen quality in this primate across fattening and non-fattening periods. We collected semen samples monthly via electroejaculation from eight males over 14 months and assessed the samples for sperm motility, vigor, membrane functionality and integrity, morphology, and morphometry. Statistical analyses, including principal component and generalized linear mixed models, revealed three subpopulations. Subpopulation 1 (39% of the variation) was stable across periods and featured sperm with rounded heads adapted for motility in viscoelastic media, such as seminal coagulum. Subpopulation 2 (27%) predominated during the non-fattening period, showing smaller heads and an increased in major defects, likely due to epididymal stasis. Conversely, subpopulation 3 (33%) significantly increased during the fattening period, exhibiting large, elongated heads. The maintenance of high proportions of normal morphology across periods, despite seasonal shifts in subpopulation frequencies, suggests that postcopulatory sexual selection maintains high-quality standards, with the fattening period representing the physiological peak for adapted phenotypes. Our findings show that ejaculate composition is seasonally modulated, with different subpopulations predominating in each period. Subpopulation analysis is a useful predictive tool for evaluating seasonal semen quality in S. collinsi , with implications for sample selection and the advancement of assisted reproductive biotechnologies for primate conservation.