Seasonal variation in serum testosterone, testicular volume, and semen characteristics in the coyote (Canis latrans)

The coyote is a seasonally breeding mammal, with most copulations occurring between December and April (depending on location). The objective of this study was to characterize seasonal changes in serum testosterone concentrations, testicular volume, and ejaculate quantity and quality in captive male coyotes. There were seasonal differences in testicular volume, with the greatest volume (20.2 ± 5.4 cm2, mean ± S.E.M.) in February, corresponding with peak breeding season. Circulating serum testosterone concentrations peaked (3.31 ± 0.9 ng/mL) during January and were positively correlated (P ≤ 0.001, r = 0.413) with testicular volume. Ejaculate volume (1.67 ± 0.4 mL) and sperm concentration (549.2 × 106 ± 297.7 spermatozoa/mL) both peaked during January and February, consistent with the height of the breeding season. Ejaculate volume and sperm concentrations were positively correlated with testicular size (r = 0.679, P ≤ 0.001 and r = 0.499, P ≤ 0.001, respectively) and with serum testosterone concentrations (r = 0.368, P ≤ 0.01 and r = 0.208, P ≤ 0.05). Progressively motile, viable, and morphologically normal spermatozoa fluctuated seasonally, peaked (90.4 ± 4.5, 84.8 ± 4.1, and 87.9 ± 2.9%) during the breeding season, and then subsequently declined (period of aspermatogenesis). All three of these end points were positively correlated with testicular size (r = 0.589, P ≤ 0.001; r = 0.586, P ≤ 0.001; and r = 0.469; P ≤ 0.001) and serum testosterone (r = 0.167, P ≤ 0.05; r = 0.190, P ≤ 0.05; and r = 0.221, P ≤ 0.01). In conclusion, there were intricate relationships among testosterone concentrations, testicular volume, and the production of both functionally intact and morphologically normal spermatozoa.