The fly that came in from the cold:: geographic variation of recovery time from low-temperature exposure in Drosophila subobscura

1. The time required for an ectotherm to recover from cold exposure is a useful, non-lethal index of cold tolerance. We explore how recovery times are affected by exposure to low temperatures, develop statistical methodologies, and study geographic variation in recovery time in four populations of Drosophila subobscura , a cold-tolerant species. 2. We exposed flies to a low temperature (-7 degreesC to 1 degreesC) for 16 h, returned them to ambient temperature, and recorded the elapsed time ('recovery time') until they stood. Other flies were exposed to even colder temperatures (-11 degreesC to -7 degreesC), but for shorter times. 3. Recovery times were inversely related to exposure temperature, but had a plateau between -6 degreesC and -4 degreesC. 4. Populations had similar recovery times at 'warm' temperatures, but two subtropical populations had relatively long recovery times at colder temperatures. 5. Inter-population differences were also evident in a regression analysis, and recovery times were inversely related to latitude (ordered-factor analysis). Populations differed slightly in the slopes of regressions but differed strongly in their intercepts. 6. The physiological mechanisms underlying the non-linear responses are unknown, but the plateau region suggests that recovery time is governed by the interplay of two temperature-dependent processes. Two models are proposed for the interaction of these processes.