We tested three hypotheses regarding the cues that elicit facultative hypothermia in Japanese quail (Coturnix japonica): H(1)) Ambient temperature (T(a)), alone, influences the onset and depth of hypothermia; H(2)) Fasting, alone, influences the onset and depth of hypothermia; H(3)) T(a) acts synergistically with fasting to shape the use of hypothermia. Eight quail were maintained within their thermoneutral zone (TNZ) at 32.6+/-0.2 degrees C, and eight below their lower critical temperature (T(lc)) at 12.7+/-3.0 degrees C. All quail entered hypothermia upon food deprivation, even quail kept within their TNZ. Body temperature (T(b)) decreased more (38.36+/-0.53 degrees C vs. 39.57+/-0.57 degrees C), body mass (m(b)) loss was greater (21.0+/-7.20 g vs.12.8+/-2.62g), and the energy saved by using hypothermia was greater (25.18-45.01% vs. 7.98-28.06%) in low the T(a) treatment than in TNZ treatment. Interestingly, the depth of hypothermia was positively correlated with m(b) loss in the low T(a) treatment, but not in TNZ treatment. Our data support H(3), that both thermoregulatory costs and body energy reserves are proximate cues for entry into hypothermia in quail. This outcome is not surprising below the T(lc). However, the quail kept at their TNZ also responded to food deprivation by entering hypothermia with no apparent dependence on m(b) loss. Therefore inputs, other than thermoregulatory costs and body condition, must serve as cues to enter hypothermia. Consequently, we address the role that tissue sparing may play in the physiological 'decision' to employ hypothermia.