Thermal, cardiorespiratory and cortisol responses of impala (Aepyceros melampus) to chemical immobilisation with 4 different drug combinations.

Thermometric data loggers were surgically implanted in 15 impala (Aepyceros melampus) to investigate the consequences of chemical capture. Impala were darted and chemically immobilised for 30 min with each of the following drug combinations: etorphine and azaperone; etorphine and medetomidine; thiafentanil and azaperone, and a thiafentanil medetomidine combination. During immobilisation, pulse oximeter readings, respiratory rhythm, the plane of immobilisation and plasma cortisol concentrations were measured and recorded. The impala developed an extremely high rise in body temperature, which peaked 20-30 min after reversal of the immobilisation. The magnitude of the rise in body temperature was similar for all the drug combinations (F = 0.8, P = 0.5), but the duration of the hyperthermia was shorter when the thiafentanil and azaperone combination was used (F = 3.35, P < 0.05). Changes in body temperature were related to the time that it took for an animal to become recumbent after darting (r2 = 0.45, P = 0.006) and not to the effect of the drug combination on time to recumbency (r2 = 0.29, P = 0.46). The relationship between time to recumbency and body temperature change, and also to plasma cortisol concentration (r2 = 0.67, P = 0.008), indicated that physiological consequences of capture were related to the duration of exposure to a stressor, and not to the pharmacology of the capture drugs. Although shorter time to recumbency in individuals resulted in the benefit of smaller stress responses and body temperature changes, those individuals were predisposed to developing hypoxia and possibly induction apnoea. When animals are chemically immobilised, reducing the thermal consequences of capture requires limiting the exposure of the animal to a psychological 'fright stress'.