Regional changes in sperm morphology and function during epididymal transit reveal early functional maturation in the red-nosed rat (Wiedomys pyrrhorhinos)

The red-nosed rat (Wiedomys pyrrhorhinos) is a Neotropical rodent threatened by accelerated habitat degradation resulting from unsustainable anthropogenic activities. Therefore, this study aimed to characterize the morphofunctional parameters of epididymal spermatozoa during epididymal transit in W. pyrrhorhinos. Eight adult males, weighing 41.7 ± 4.2 g and aged 6.3 ± 0.9 months, were used. Following euthanasia by cervical dislocation, testis-epididymis complexes were collected, and spermatozoa were recovered from the caput, corpus, and cauda epididymis using the slicing technique. Samples were evaluated for sperm concentration, chromatin integrity, structural and functional plasma membrane integrity, sperm kinetics (CASA), morphology, and ultrastructure. Data were analyzed using a linear mixed-effects model followed by Tukey's post hoc test, or Friedman followed by Dunn's test (P < 0.05). Sperm concentration, chromatin integrity, membrane functionality, and the proportion of morphologically normal spermatozoa increased significantly during epididymal transit, reaching their highest values in the cauda epididymis (138.8 ± 31.8 × 10⁶ sperm/mL; 92.1 ± 2.2%; 80.3 ± 2.8%; and 93.8 ± 0.9%, respectively). In contrast, sperm viability and kinetic parameters showed no significant changes throughout epididymal transit, indicating an earlier acquisition of functional competence than those typically described for most mammals. Scanning electron microscopy revealed spermatozoa with an elongated smooth oval head, hook-shaped acrosome, asymmetric head-midpiece implantation, and filamentous tail. Overall, these findings demonstrate a distinctive pattern of epididymal sperm maturation in W. pyrrhorhinos, in which functional attributes are established early during epididymal transit, providing new insights into the reproductive biology of this threatened Neotropical rodent and supporting future conservation strategies.