Heat is deadlier than hypoxia for an elevationally migrating moth

Abstract Oxygen availability declines with increasing elevation, yet some animals nevertheless migrate to high elevations each summer. For this seasonal migration to be a beneficial strategy, the threats at lower elevations must be greater than those posed by insufficient oxygen at high elevations. One threat at low elevations that could outweigh the risk of hypoxia at high elevations is overheating in the hot summers. We tested if an elevationally migrating moth, Euxoa auxiliaris , is more sensitive to the hot summer conditions in the lowlands than to the low oxygen conditions in the highlands. We conducted a 2 × 2 factorial experiment, where we measured mortality rates under (1) an extreme temperature (40°C) vs. warm temperature (34.5°C) and (2) a hypoxic condition ( P O2 = 95.7 mm Hg) vs. a normoxic condition ( P O2 = 130.7 mm Hg). Hotter conditions dramatically increased the rate of mortality in these elevationally migrating moths, but lower oxygen conditions did not affect mortality. These results indicate that, at least for E. auxiliaris , a major threat that would be faced at low elevations is far more dangerous than is the low oxygen they do encounter at high elevations. Our study provides mechanistic support for a key tenet of evolutionary theory in seasonal environments: species will migrate to a risky habitat if the alternative would be worse.