Into thin air: Physiology and evolution of alpine insects

Numerous physical parameters that influence insect physiologyvary substantially with altitude, including temperature, airdensity, and oxygen partial pressure. Here, we review existingliterature and present new empirical data to better characterizethe high-altitude environment, and then consider how this environmentaffects the physiology and evolution of insects. Using weatherballoon data from fifty-three sites across the globe, we estimatea mean altitudinal temperature lapse rate of 6.0 °C/km.We also present empirically determined lapse rates for P_O2 andair density. The temperature decline with elevation may substantiallycompromise insect thermoregulation at high altitude. However,heat-transfer models predict that lower air density at elevationreduces convective heat loss of insects by to a surprisinglylarge degree. This effect combined with behavioral thermoregulationand the availability of buffered microhabitats make the netthermal consequences of high-altitude residence strongly context-specific.The decline in P_O2 with elevation may compromise insect developmentand physiology, but its effects are difficult to predict withoutsimultaneously considering temperature and air density. Flyinginsects compensate for low air densities with both short-termresponses, such as increased stroke amplitude (but not wingbeatfrequency), and with long-term developmental and/or evolutionaryincreases in wing size relative to body size. Finally, in contrastto predictions based on Bergmann's Rule, a literature surveyof thirty-six insect species suggests that those living in colder,higher altitudes do not tend to have larger body sizes.