Characterizing the thermal environment of small mammals: what should we be measuring, and how?

Operative temperature and standard operative temperature have been proposed as providing an estimate of the thermal load experienced by endotherms and ectotherms. Common techniques of measuring operative temperature include black-bulb temperatures or simplified unheated metal models, typically made of copper. We quantified the thermal environment perceived by a small, arboreal rodent using a number of methods at three study sites in winter and summer. Our area of interest was how well these methods accurately portrayed the actual temperatures that small mammals are exposed to. We predicted that black-bulb and copper model temperatures would more accurately predict operative temperature during photophase when compared with other direct measurements/devices, and that black-bulb temperature would record the greatest variation in temperatures. Temperature differences between the methods were largest during the midday, when temperatures were highest. All methods recorded a greater range of temperatures during photophase than during scotophase. Black-bulb and model temperatures produced more accurate, rapid measurements when compared with measurements produced by direct temperature recording devices, particularly during photophase, when solar radiation is the major influence of heating. Other methods lagged behind black-bulb measurements. Although mean temperatures of some of the methods were significantly different, there was a high degree of correlation between all methods, even after randomization and generation of 25% and 10% subsamples. In studies requiring accurate time series measurements, it is suggested that black-bulb or copper models be employed rather than direct temperature recording devices. Simpler measurement devices would suffice for studies requiring an estimate of the temperature variation and trends in the microclimate of small mammals, particularly arboreal or cavity dwelling species.