Physiology & Behavior, Vol. 66, No. 1, pp. 137–143, 1999 © 1999 Elsevier Science Inc. Printed in the USA. All rights reserved 0031-9384/99/$–see front matter PII S0031-9384(98)00287-X 137 Effects of Fasting on the Circadian Body Temperature Rhythm of Japanese Quail HERBERT UNDERWOOD, 1 CHRISTOPHER T. STEELE AND BORA ZIVKOVIC Department of Zoology, North Carolina State University, Raleigh, NC 27695 Received 17 July 1998; Accepted 24 September 1998 UNDERWOOD, H., C. T. STEELE AND B. ZIVKOVIC. Effects of fasting on the circadian body temperature rhythm of Japanese quail. PHYSIOL BEHAV 66(1) 137–143, 1999.—The effect of food deprivation on the body temperature and ac- tivity rhythms of quail was assessed in birds exposed to both light–dark (LD) cycles and to continuous darkness (DD). Quail normally exhibit a daily rhythm of body temperature in LD that will persist in DD (that is, the rhythm is circadian). In LD, 3 days’ food deprivation caused the body temperature to drop below its normal nighttime levels, whereas daytime body tem- perature was unaffected. In DD, food deprivation caused the body temperature to drop below normal at all phases of the cir- cadian rhythm of body temperature. Accordingly, the lack of hypothermia during the light phase of the LD cycle following food deprivation must represent a direct exogenous or “masking” effect of light, and is not an endogenous property of the cir- cadian system. Blind birds exposed to LD 12:12 exhibited an entrained body temperature rhythm, and food deprivation caused a drop in body temperature below normal levels during both the light and dark phases of the LD cycle. Accordingly, the masking effects of light observed in normal birds on LD cycles is mediated via retinal photoreceptors and not via extraret- inal photoreceptors. Measurements of activity levels before and during fasting indicate that fasting-induced hypothermia can- not be explained simply as a consequence of decreases in activity levels. Food deprivation was also observed to cause signifi- cant phase shifts in the endogenous rhythm of body temperature. © 1999 Elsevier Science Inc. Circadian Temperature rhythm Fasting Activity rhythm Quail MOST, if not all, homeotherms normally exhibit a daily rhythm in body temperature (11). Because the body tempera- ture rhythm persists under constant conditions, the body tem- perature rhythm is truly “circadian”; that is, it is driven by the animals’ internal circadian clock. The daily pattern of body temperature normally seen under cyclic environmental cycles (most notably, light), therefore, represents “entrainment” of this endogenous body temperature rhythm. In addition to en- training effects, external stimuli such as light may also directly affect the expression of the body temperature rhythm. Direct effects of a stimulus on the expression of an overt rhythm have been termed “masking” (1). Consequently, depending on the species, the daily body temperature rhythm under daily light– dark conditions may reflect both entraining and masking ef- fects of light. In response to food deprivation, some diurnal birds will show nocturnal hypothermia; that is, their body temperature drops below that normally observed (3,5,9). This response, of course, has adaptive significance because it leads to significant energy savings. To our knowledge, all of the studies to date that have examined the hypothermic response to food depri- vation were conducted under 24-h light–dark (LD) conditions or, if examined in constant conditions, no comparisons were made between the body temperature rhythm in constant con- ditions to those expressed under LD cycles (3,5,6). Under LD conditions birds, including Japanese quail, will show a hypo- thermic response during the dark phase of the LD cycle, but not during the light phase (3,5,11). Small diurnal mammals also seem to show a hypothermic response to food depriva- tion only during the dark phase of the LD cycle (11). The mechanism responsible for the day–night difference in the re- sponse to food deprivation has not been explored. The observation that food deprivation seems only to affect the night phase of a LD cycle could be interpreted to mean ei- ther that: 1) the circadian rhythm of body temperature is sen- sitive to the effects of food deprivation only during the ani- mal’s subjective night, or 2) the circadian rhythm of body temperature could potentially be influenced by food depriva- tion during all phase of the circadian rhythm of body tempera- ture, but light overrides (i.e., masks) the effects of food depri- vation. The current study was undertaken to determine which of these two hypotheses is correct by examining the effects of 1 To whom requests for reprints should be addressed. E-mail: Herbert_Underwood@ncsu.edu