Meeting Abstract
Understanding the influence of abiotic environments on physiological traits has been a long-standing goal in evolutionary physiology. Janzen’s hypothesis predicts that organisms from relatively seasonal environments (e.g. temperate latitudes) will evolve comparatively broad thermal tolerances to cope with greater environmental variation. In contrast, organisms from less seasonal environments (e.g. tropical latitudes) are expected to have narrower thermal tolerances, which may make less tolerant of environmental variation. Understanding how thermal tolerances vary across latitude is crucial in predicting whether tropical organisms have narrower tolerances and thus, greater susceptibility to climate change than their temperate-zone counterparts. We used the breadth of the thermoneutral zone (TNZ) to assay thermal tolerances in suites of tropical and temperate-zone resident bird species using flow-through respirometry. Temperate-zone species had significantly broader thermal tolerances than their tropical counterparts. Latitudinal patterns in thermal tolerance breadth were driven by variation at the lower end of the TNZ (Lower Critical Temperature: LCT), which was significantly lower in temperate-zone species. The upper end of the TNZ (Upper Critical Temperature: UCT) was not significantly different between tropical and temperate-zone species. Both UCT and LCT values were independent of body size and were consistent among individuals of the same species. Thermal tolerance (i.e. TNZ breadth) thus varies predictably with latitude and appears to be driven by exposure to cold temperatures. Our data confirm Janzen’s hypothesis and suggest that environmental variation (i.e. past selective regimes and/or current environmental conditions) influences avian thermal tolerances.
