of 28° to 29°C, began to lose weight when the environmental temperature during feeding rose above 32°C. Numerous other investigators have found equivalent results with rats (Fletcher, 1986; Hamilton and Brobeck, 1964; Jakubczak, 1976; Leon and Woodside, 1983) and with other species, such as goats (Appleman and Delouche, 1959) that eat less and hoard less (Fantino and Cabanac, 1984) and frequently lose weight in hotter ambient temperatures. Kraly and Blass (1976) found that rats will work harder for food and consume more unpalatable food in the cold. Mice eat 43 percent less in an ambient temperature of 33°C than at 17°C (Thurlby, 1979; see also Donhoffer and Vonotzky, 1947), and pigs that were maintained at 32° to 35°C ate only half as much as pigs maintained at 10° to 12°C (Heath, 1980; Macari et al., 1986). Note that Swiergiel and Ingram (1986) found that piglets maintained at 35°C gained more weight than did piglets maintained at 10°C, but the intake levels appeared to have been controlled in this study; the higher BW of the 35°C piglets may have represented their attempt to store energy rather than bum it. Cafeteria-fed rats maintained at 29°C ate less, but if anything gained more weight than did those maintained at 24°C, presumably because in the hotter environment the rats became much more energy efficient, storing their excess calories rather than burning them and risking hyperthermia (Rothwell and Stock, 1986). Presumably in severe heat, thermogenic disposal of calories would pose enough of a threat so that the animal would quickly learn to cut back on its intake.
More proximal heating (that is, in the preoptic and anterior hypothalamic regions) serves to inhibit feeding in much the same way as does distal heating (that is, in the external environment) (Andersson and Larsson, 1961). This simply indicates that the effects of environmental heat must be registered somewhere in the central nervous system if they are to affect feeding; the hypothalamic tracts remain the prime candidates for the coordination of heat-and-feeding regulation. Other loci, such as the liver (DiBella et al., 1981) and even the skin (Booth and Strang, 1936), have also been nominated as crucial in producing regulatory thermal feedback in the control of eating.
In general, "reduced intake in warm environments [has] been shown in several endothermic animals" (Refinetti, 1988). One exception to the rule that animals eat less when it is hot is contained in a study by Bellward and Dauncey (1988); in this study, mice ate more at above-normal temperatures than at below-normal temperatures. The explanation for this contrary effect supports the general approach here: mice had to choose between heat (exposure to a heat lamp) and food. When it was cold, they tended to choose heat, at the expense of food. Presumably if they had been allowed access to food but not the heat lamp, they would have eaten less as the temperature rose.
One mechanism possibly contributing to increased intake in animals