do not appear to exhibit as large an HPA response as do mature cattle (Cole et al., 1988; Mormede et al., 1982). At 8 weeks, the response begins to change. Corticosteroids increase, but glucose is variable, either increasing or remaining unchanged (Crookshank et al., 1979; Kent and Ewbank, 1983, 1986a, 1986b; Simensen et al., 1980). In general, those changes return to baseline immediately after transportation (Knowles et al., 1999; Warriss et al., 1992), although some genotypes may have altered endocrine concentrations for months after transport (Nyberg et al., 1988). Young pigs and calves have also been found to have an unstable metabolic rate after transportation, requiring 6 to 9 days to stabilize (del Barrio et al., 1993; Heetkamp et al., 2002; Schrama et al., 1992).

The limited physiological responses observed in adult animals can be much more pronounced after extended periods of food and water deprivation. Livestock are often transported without access to food and water for safety reasons, and the longer the period of deprivation, the longer the time necessary for normalization. Extended periods (more than a day) without food and water may result in 5 days or more before normalization of physiological measures (Warriss et al., 1995).

ALLOMETRIC SCALING AND IMPLICATION FOR TRANSPORTATION PRACTICES

Transported research animals vary greatly in size, from small rodents to very large sea mammals, and within each species animals can vary in size from neonates to adults. As the size of animals varies, so do the biological processes that affect transportation practices. However, variations in processes such as heat production, metabolic rate, and space requirement are not linear functions of animal size (Lindstedt and Schaeffer, 2002). In other words, an animal that is twice the size of another animal does not have twice its metabolic rate. Rather, the relationship is exponential. The term allometric scaling is used to describe methods of quantifying the dependence of biological processes on body mass (West et al., 1997). Implicit in allometric scaling is the principle that small animals occupy more space per unit of body weight than larger animals. Small animals also produce more heat per unit of body weight than larger animals.

The relationships between surface area, metabolic rate, and space required by mammals are defined by the following allometric equations:

Surface area (m2) = 0.1 × weight2/3

(Curtis, 1983)

Basal metabolic rate (kcal/hr) = 3.0 × weight3/4

(Curtis, 1983)

Floor area (lateral recumbency, m2) = 0.1 × weight1/3

(Baxter, 1984)



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