Individual Variation in Sperm Quality and Resilience to Cryopreservation with Different Dimethyl Sulfoxide Concentrations in Captive Greater Rheas ( Rhea americana )

Introduction: Rhea americana are ecologically important, as they disperse seeds and participate in the food chain of various predators in South America. However, populations are declining in the wild. Objectives: The objective was to characterize the resilience of greater rhea spermatozoa to cryopreservation by assessing a commercial semen extender supplemented with different concentrations of dimethyl sulfoxide (DMSO). Methods: Spermatozoa from the vas deferens of six mature males were evaluated for kinetic parameters, viability, membrane functionality, and morphology. Samples were first diluted in Ovodyl TM plus 6%, 10%, 14%, or 18% (v/v) DMSO, loaded in 0.25-mL straws frozen, and plunged in liquid nitrogen. After one week, samples were thawed and reevaluated. Sperm resilience to freezing temperatures was analyzed through preservation efficiencies (percentage of postfreezing values relative to the fresh ones). Results: Marked interindividual variability was observed, particularly in motility-related parameters. Fresh sperm motility ranged from 15.5% to 61.0%, while postthaw values did not exceed 11%. Similarly, viability ranged from 59% to 71% in fresh samples and from 3% to 26% postthaw. Membrane functionality varied from 64% to 95% in fresh samples compared with 7%–43% postthaw. Sperm morphology ranged from 66% to 86% in fresh samples. Coefficient of variation analysis confirmed high variability for motility parameters, whereas other variables showed moderate to low dispersion, indicating a heterogeneous response among individuals to the cryopreservation procedure. There was no difference ( p < 0.05) between the different DMSO concentrations evaluated. The average efficiency of preserving total motility, progressive motility, viability, and membrane functionality in 14% DMSO was 18.3 ± 11.6%, 47.0 ± 28.5%, 21.0 ± 5.9%, and 27.1 ± 11.6%, respectively. Conclusion: DMSO concentrations did not influence postthaw sperm quality. However, marked interindividual variation, particularly in motility-related parameters, played a key role in sperm resilience to cryopreservation.