Over a decade ago it was hypothesized that the rapid cold hardening process allows an organism’s overall cold tolerance to track changes in environmental temperature, as would occur in nature during diurnal thermal cycles. Although a number of studies have since focused on characterizing the rapid cold hardening process and on elucidating the physiological mechanisms upon which it is based, the ecological relevance of this phenomenon has received little attention. We present evidence that in Drosophila mel-anogaster rapid cold hardening can be induced during cooling at rates which occur naturally, and that the protection afforded in such a manner benefits the organism at ecologically relevant temperatures. Drosophila melanogaster cooled at natural rates (0.05 and 0.1 ° C min21) exhibited significantly higher survival after one hour of exposure to 27 and 28 ° C than did those directly transferred to these temperatures or those cooled at 0.5, or 1.0 ° C min21. Protection accrued throughout the cooling process (e.g., flies cooled to 0 ° C were more cold tolerant than those cooled to 11 ° C). Whereas D. melanogaster cooled at 1.0 ° C min21 had a critical thermal minimum (i.e., the temperature at which torpor occurred) of 6.5 – 0.6 ° C, those cooled at an ecologically relevan
