The larger surface area and basal metabolic rates per unit of body weight of smaller animals means that they evaporate more water and lose more heat per unit of body weight than larger animals. The practical implication is that smaller animals are more susceptible to changes in temperature (cold or hot), wind speed, and humidity. The core temperatures of smaller animals can decrease in cold environments more quickly than those of larger animals.

Smaller animals also become dehydrated more quickly than larger animals and cannot live without water as long. That is because of the larger evaporative skin area and/or respiration rate of small animals. Some species have special adaptations to conserve water, but even among these species, the general relationships between young (smaller) and older (larger) animals apply.

Smaller animals generally have higher metabolic rates per unit of body weight than larger animals. That means that smaller animals can go without food for less time than larger animals, which, because they are larger, have relatively greater nutrient reserves.


Provision of a proper thermal environment is the most important element of safe and humane animal transportation. Temperature has been implicated as the major factor in leading to animal mortality during transportation in many species (Abbott et al., 1995; e.g., Bayliss and Hinton, 1990; Slanetz et al., 1957). The principles of a safe thermal environment during transportation are not different from those in normal housing. The goal is to identify the range of ambient temperatures over which an animal is able to maintain a physiologically normal core body temperature. In this section, the basic principles of thermoregulation in warm-blooded animals are discussed to provide the scientific basis of the committee’s recommendations and to inform the professional judgment of researchers, staff, and institutional animal care and use committees in meeting performance standards.

Principles of Thermoregulation

Warm-blooded animals are known as homeotherms because they maintain a constant body temperature through a high metabolic rate. That process keeps body temperature constant, independent of the ambient temperature. The average body temperatures of the most common research animal species are listed in Table 3-1.

The thermoneutral zone (TNZ) is the range of ambient temperatures within which an animal’s metabolic rate is at a minimum and body tem-

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