Sperm functional and morphometric differences between Iberian and European ecotypes of capercaillie (Tetrao urogallus...

The capercaillie (Tetrao urogallus L.) is a threatened forest bird with distinct Iberian and European ecotypes. This study examines the functional and morphometric differences between the sperm of these ecotypes. In a first experiment, semen volume, sperm concentration, motility, viability, and DNA integrity were assessed in 'Iberian' and 'European' capercaillies, both maintained at breeding centres in Spain. In a second experiment, morphometric traits were measured in Iberian capercaillies maintained at a breeding centre in Spain, and in European capercaillies also maintained at breeding centres in Spain (EmS birds) and in Poland (EmP birds). In the first experiment, the European males produced larger ejaculate volumes (39.80 ± 3.56 µL) than the Iberian males (29.68 ± 4.64 µL). However, the Iberian males returned significantly higher sperm concentrations (501.99 ± 83.90 × 10⁶ spz/mL vs. 77.66 ± 26.09 × 10⁶ spz/mL). In the second experiment, the origin of the birds also affected (P < 0.001) sperm head dimensions. These were smaller in the EmP birds compared to the EmS birds (P < 0.01), and compared to Iberian males (always maintained in Spain) (P < 0.001). Within each of these groups, three sperm subpopulations were identified according to head dimensions, with differences (P < 0.001) between these groups in terms of the proportion of each subpopulation. This is the first comprehensive study of sperm morphometric characteristics in these capercaillie ecotypes. These results may provide critical insights into the reproductive and evolutionary strategies of capercaillies and contribute to improving the success of reproductive technologies across different ecotypes and populations.