The African Wild Dog
Water, Salts, & Excretions
Figure.
The rate of evaporative water loss experienced by mammals exposed to heat stress depends strongly on their body size.
The graph shows weight-specific rates of evaporative water loss for exercising animals (red) and resting animals (blue) directly exposed to hot conditions. The resting humans and burros were studied while resting in the desert; the resting mice and rats were studied in a laboratory chamber main- tained at an air temperature of 40°C (104°F).The equation for the red line is E = 6.03W –0.33, where E is rate of evaporative water loss in percentage of body weight per hour and W is body weight in kilograms. The equation for the blue line is E = 1.38W–0.21.The two species of kangaroo rats included in the resting studies are Merriam’s kangaroo rats (Dipodomys merriami) and desert kangaroo rats (abbreviated “Desert k-rats”; D. deserti ), weighing 34 g and 114 g, respectively. (After Schmidt-Nielsen 1954; Soholt et al., 1977)
To further explore the physiological implications of body size, we’ll need to compare large and small mammals under two simplified assumptions:
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animals thermoregulate physiologically by use of evaporative cooling
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animals are actually confronted with hot, stressful conditions (rather than evading them)
In order for an animal to thermoregulate under these assumptions, water must be evaporated to void both exogenous heat (which enters the body from the hot environment) and endogenous heat (which is produced by its metabolism). In both of these circumstances, larger body size is an advantage.
Large mammals, like the wild dogs, are advantaged in the way that they have less body-surface area per unit of weight than smaller mammals do. In dry, hot conditions when the exogenous ambient temperature is higher than internal body temperature, heat will enter an animal’s body across its body surfaces. Large mammals typically experience less heat entry per unit of weight than small mammals and have relatively little body-surface area per unit of weight.
Large mammals have a greater benefit than small ones from endogenous heat because of the allometric relation between an animal’s body size and its metabolic rate. Large mammals typically have lower weight-specific metabolic rates than small mammals, as well as lower weight-specific water costs to relieve their internal heat. Therefore, the African wild dog’s water costs to void the incoming heat are lower per unit of weight.
The African wild dog can survive for longer periods than other mammals without a regular water supply. The length of the dogs intestine helps the animal to absorb as much moisture as possible from the food. Further evolutionary adaptations of their carnassial teeth help the dogs consume their food quicker which promotes quick replenishment and in reabsorption to retain water and maintain homeostasis while thermoregulating.
Large body size is a physiological advantage in terms of water costs.
WATER DEPENDENCY
ELECTROLYTES & SECRETIONS
In vertebrates, a good amount of electrolytes and water secrete from salivary glands, pancreas, biliary system, and the gastrointestinal tract which helps food through the digestive tract. Additionally, these secretions provide necessary pH needed to activate endogenous enzymes and maintain indigenous microbes.
Electrolytes consist mainly of the sodium (Na+), potassium (K+), and hydrogen (H+) cations as well as chloride (Cl-) and bicarbonate (HCO3-) anions. Water passively diffuses into the digestive tract which results in secretion.
Most secretions need to be reabsorbed in order to prevent dehydration and any major body fluid disturbances in the electrolyte-water and acid-base balance. A good amount of energy that gets absorbed by the digestive system gets used for secretion and reabsorption of electrolytes.
Water and food resources in deserts and dry savannas are often difficult to obtain. Some animals that are accustomed to live in dry, hot places (like the African wild dog), are classified as drinking-water-independent since they can survive days or weeks without access to drinking water. Though the wild dogs are water independent and receive plenty of nutrition and water consumption from their food, these animals do need to drink on occasion.
On the other hand, animals that are drinking-water-dependent are required to drink roughly each day to maintain water balance.