Thermal and metabolic adaptations to heat and exercise in two East African herbivores, the domestic donkey (equus asinus asinus) and the one-humped camel (camelus dromedarius

Abstract

In this study, thermal and metabolic adaptations to heat and exercise have been investigated in the two species during treadmill running at speeds between 0- 15 km.h-1 and on gradients between 0-70• An open flow system was employed to measure oxygen consumption while the animals ran at the different speeds and gradients both while loaded and unloaded. A heat balance evaluation while animals ran and while at rest in a climatic chamber at ambient temperatures of 15- 400c was also ascertained. The following series of experiments were carried out under controlled laboratory conditions: a) Energy expenditure during exercise; b) Energy expenditure and heart rate during exercise; c) Energetic cost of carrying loads; d) Thermoregulation during dehydration; f) The role of the extremities in thermoregulation. The energy cost of running increased proportionately with running speed. Regression lines describing the relationship between metabolic rate and -xxirunning speed (Vr) for the two species are given below :- Donkeys 0.94 + 0.41 Vf (r 0.89, n 60) Camels = 0.62 + 0.32 Vf (r = 0.91, n = 20) where metabolic rate is in W.kg-1 (assuming 20.1 J.ml -102) ,V . . -1 f 1S speed 1n km.h , n is the number of observations in each experiment and r is the regression coefficient. It was observed that the camel and the_.donkey consumed less energy than would be predicted on the basis of their body weight at anyone given speed between 0-15 km.h-1. The minimum net costs of transport were 1.7 J.kg-1.m-1 and 1.3J.kg-1m-1 for the donkey and camel respectively. Similarly; the 'optimal' (speed at which the mechanical energy changes of the centre of mass are minimised by pendulum-like transfers between the kinetic and potential energy of the centre of mass) walking speeds were about 4 km.h-1 and 6 km.h-1 for the donkey and camel respectively. There was a proportionate increase in the heart rate as the running speed increased in both species. A relationship between the heart rate and the energy expenditure was described by the following equations: Heart rate i) Donkeys 6.56 + 23.80H (r = 0.82, n = 60) ii) Camels 25.61 + 14.99H (r = 0.87, n = 20) where heart rate is in beats. min-1 and H is metabolic rate in W. kg-1,n is the number of observations in each experiment and r is the regression coefficient. The mechanical efficiency of performing external work while carrying loads ranging between 20-80 kg at speeds between 2-6 km.h-1 and at treadmill gradients of 30, 50 and 70 was 22%. Efficiency ranged between 12-35% depending on the walking speed and load being carried. The regulation of body temperature as well as pulmocutaneous evaporation at rest and during exercise was investigated. It was observed that the donkey and camel allowed their body temperatures to rise during treadmill exercise. The degree of body temperature rise was dependent on the speed and duration of exercise. At speeds of up to 6 km.h-1, however, the animals were able to regulate their body temperatures at levels slightly higher than the resting. At speeds higher than 6 km.h-1 body temperature tended to keep on rising as long as the exercise lasted. At speeds of 10 km.h-1 and 15 km.h-1, heat storage accounted for 57% and 56% of the total heat production in the donkey, respectively. In the camel at the same speeds, heat storage was 77% and 87% of the total heat production, respectively. Total evaporative heat loss (TEHL) during exercise increased with increasing - - speed. This was particularly so in the donkey where TEHL accounted for 35% of the total heat production at speeds of 10 km.h -1. In the camel, however, l.t was observed that percentage TEHL tended to decline as speed increased accounting for only 19% of the total heat productlo.n at a speed of 10 km.h -1.. Following exposure to high ambient temperatures in the climatic chamber, cutaneous evaporative heat loss (CEHL) was the major route of heat dissipation in both species. After exposing the animals to an ambient temperature of 400C, sweating rates of 198.6 ±19.3 g.H2o.m-2.h-1 were measured in the donkey. Similarly, in the camel, sweating commenced at an ambient temperature of 350C and at 400C, sweating rate I was 164.5 ± 14.9 g.H -2 -1 . 2.m .h . Dehydratlon not only delayed, but drastically reduced the rates of sweating at all ambient temperatures, especially in the camel. There was, however, an apparent increase in respiratory evaporative heat loss during dehydration in both species though TEHL decreased. Exposure to high ambient temperature was accompanied by an increase in body temperature in both species. Subjection of dehydrated animals to high ambien temperatures led to more rise of their body temperatures than during hydration at anyone temperature. At 40°C dehydrated camels had temperature of 39.2oC while at 15°C their body temperature was 35.6; a rise of 3.6oC. Exposure of camels and donkeys to ambient within the range 5°C to 40°C led to large differences between the skin temperatures extremities and the trunk. Large variations skin temperature of the extremities were r between ambient temperatures of 5°C·:to 20( At ambient temperatures above 25°C, the ext rem: temperatures and the trunk skin temperatUJ similar. At 40°C, however, the skin temperat the extremities were higher than that of the tl It is concluded that both the donkey and 1 are efficient pack animals. They have lower el costs of transport than other domestic species including man. Body temperature increased dur running and heat storage accounted for a major the heat balance in both species. variations skin temperatures of the extremities of both s in hot and cold environments have a thermoreg function.