Asinara male donkey (Equus africanus asinus var. Albina) and stallion (Equus ferus caballus) reproductive characteristics: Correlations between testicular blood supply and sperm production

abstract: The Asinara donkey is a rare small albino variety of feral donkey listed as ''critical'' by the Food and Agriculture Organization of the United Nations (FAO) and by the Domestic Animal Diversity Information System (DAD-IS) in 2022. The objectives of the present study were to evaluate the reproductive characteristics of healthy male Asinara donkeys through andrological evaluation and testicular echo-color-Doppler examination to assess correlations between testicular blood supply and sperm production and to compare it with horse reproductive characteristics. Eight healthy donkeys and stallions were subjected to semen collection and evaluation, ultrasound and Doppler evaluation of the testicles and serum testosterone determination. The testosterone concentrations in donkeys were 1.42 ± 0.69 ng/ml while in horses were 1.90 ± 0.63 ng/ml within the normal values reported in these species. Horses had greater mean and total testicular volume, semen gel-free volume, and total sperm number than donkeys (P < 0.001). Blood flow analysis of the testicular artery in both locations was a relatively simple procedure. The waveforms recorded in a convoluted location (-con) of the testicular artery were characteristic of the high-resistance vessel, while in the marginal location (-marg) were of low-resistance. It was also recorded a gradual decrease in peak systolic velocity (PSV), resistive index (RI), pulsatility index (PI), mean velocity (MV) and testicular arterial blood flow (TABF) values along the peripheral course of the testicular artery. Therefore, donkeys had greater PSV-con (P ≤ 0.01), PSV-marg (P < 0.01), MV-con (P < 0.001), MV-marg (P < 0.01), testicular arterial blood flow rate (TABFr-con) (P < 0.001) and TABFr-marg (P < 0.01) than horses. Conversely, pulsatility index (PI-con) was higher in horses (P < 0.05). The comparative analysis between the Doppler parameters and the reproductive/seminal characteristics showed a positive correlation of TTBF-con with Testosterone (ρ = 0.976, p < 0.01 and ρ = 0.905, p < 0.01 in donkeys and horses, respectively), with Total Testicular Volume (ρ = 0.952, p < 0.01 and ρ = 1.000, p < 0.01 in donkeys and horses, respectively), and with Total spermatozoa concentration (ρ = 0.905, p < 0.01 and ρ = 0.813, p < 0.05 in donkeys and horses, respectively). Additionally, only in the donkey there was a positive correlation with Semen gel free volume (ρ = 0.881, p < 0.01) and Spermatozoa concentration × ml (ρ = 0.786, p < 0.05). The testosterone concentration was positively correlated in both species with Total testicular volume (ρ = 0.976, p < 0.01 and ρ = 0.905, p < 0.01 in donkeys and horses, respectively) and Total sperm concentration (ρ = 0.881, p < 0.01 and ρ = 0.976, p < 0.01 in donkeys and horses, respectively). It was also correlated positively with Semen gel free volume (ρ = 0.857, p < 0.01) and spermatozoa concentration (ρ = 0.762, p < 0.05) in donkeys. Finally, the PI-con (ρ = − 0.786, p < 0.05) was negatively correlated with Semen gel free volume in the donkeys. The results of this study show that Total Testicular Blood Flow is the parameter, together with testosterone, most positively correlated with testicular volume and total sperm concentration in both species. Furthermore, differences in both reproductive and testicular hemodynamic characteristics between the Asinara donkey and stallion are highlighted, suggesting that caution should be taken in transferring knowledge from one species to another. New data on reproductive aspects, seminal characteristics, testicular blood flow perfusion in healthy Asinara donkeys were reported and these may represent reference value for further research or clinical applications.