A Descriptive Study of Brown Bear (Ursus arctos) Sperm Quality and Proteomic Profiles Considering Sperm Origin

The conservation of small and genetically vulnerable brown bear populations, such as the Cantabrian subpopulation in Spain, depends on developing species-specific assisted reproductive technologies and genetic resource banks. However, the lack of standardized sperm collection and cryopreservation protocols hinders their application. This study provides the first comparative analysis of sperm quality and proteomic profiles from three different origins: epididymal, pre-ejaculated, and ejaculated. Sperm quality parameters -motility and kinetic, viability, apoptosis, and oxidative stress- and protein expression were assessed. Although yields were similar, marked differences were observed in sperm quality and protein profiles. Sixty-three proteins involved in metabolism, stress response, and oxidative balance were differentially expressed depending on sperm origin. Epididymal sperm showed the highest viability and motility, lowest apoptosis, and a proteomic profile indicative of active spermatogenesis and enhanced oxidative stress defense. In contrast, ejaculated sperm had increased oxidative stress and reduced expression of metabolic proteins, while pre-ejaculated sperm exhibited lower motility, likely due to urine contamination and mitochondrial protein alterations, despite comparable viability and apoptosis. These findings offer novel insights into brown bear sperm biology and highlight the importance of sperm origin in developing optimized assisted reproduction strategies, ultimately supporting ex situ conservation efforts for this species.