Diurnal rhythms of heart and respiratory rates in donkeys of different age groups during the cold‐dry and hot‐dry seasons in a tropical savannah

The aim of this study was to determine the effect of season on diurnal rhythms of heart (HR) and respiratory rates (RespR) in the adult, yearling, and foal donkeys during the cold‐dry and hot‐dry seasons under natural light/dark cycle. The resting HR and RespR were recorded bihourly for 24 consecutive hours from 06:00 to 06:00 h (GMT +1) in 30, clinically healthy donkeys (10 adults,10 yearlings, and 10 foals). Dry‐bulb temperature (DBT), relative humidity (RH), temperature‐humidity index (THI) and wet‐bulb globe temperature index (WBGT) inside the pen were recorded bihourly from 06:00 to 06:00 h. Values of DBT, THI, and WBGT obtained during the hot‐dry season were significantly (P < 0.05) higher than corresponding values recorded during the cold‐dry season. Application of single‐cosinor procedure showed that HR and RespR exhibited daily rhythmicity in both seasons. The mesors of the HR in adult (41.51 ± 0.34 beats/min [bpm]), yearling (40.80 ± 0.43 bpm), and foal (47.55 ± 0.40 bpm) donkeys during the cold‐dry season were significantly (P < 0.01) lower, compared to the corresponding values of 48.4 ± 0.40 bpm, 50.42 ± 0.52 bpm and 58.10 ± 0.50 bpm, respectively during the hot‐dry season. The mesors of RespR in adult, yearling, and foal donkeys during the hot‐dry season were higher (P < 0.05), when compared to the corresponding values recorded in the cold‐dry season. The HR and RespR of foals were significantly (P < 0.05) higher than those of the adult and yearling donkeys. Amplitudes of HR and RespR were higher during the hot‐dry season than the cold‐dry season. In conclusion, seasonal changes affect diurnal rhythmicity of HR and RespR of adult, yearling, and foal donkeys during the cold‐dry and hot‐dry seasons. The HR and RespR of donkeys vary with age, with higher values in the foals than the adult and yearling donkeys in both seasons.