Structure and thermal biology of subterranean army ant bivouacs in tropical montane forests

Active brood-warming in army ant nests (bivouacs) is well documented for surface-dwelling Eciton burchellii and E. hamatum colonies in lowland tropical forests. However, little is known about thermoregulation by the below-ground bivouacking army ants that comprise all other species in subfamily Dorylinae. Here we report the first observations of subterranean Labidus praedator bivouacs in tropical montane and premontane conditions (Monteverde, Costa Rica), and present the first evidence for active nest warming in underground bivouacs. We measured bivouac temperatures at depth increments of 10 cm through the center of a 1565 m elevation bivouac and compared these to simultaneous measurements at the same soil depths 1 m outside the bivouac. The bivouac was actively heated to over 6 A degrees C higher than the adjacent soil. Another bivouac showed warming of up to 3.7 A degrees C above surface ambient. We measured critical thermal maxima (CTmax) and minima (CTmin) of L. praedator workers of a range of body sizes including callows, as well as thermal tolerances of inquiline millipedes from the bivouac. CTmax varied positively with worker body size. CTmin was lower for mature than for callow workers. Symbiotic millipedes had lower CTmax and higher CTmin than ant workers. Temperatures below the thermal tolerance ranges of symbiotic millipedes and near the bottom thermal tolerance range for callow workers were recorded in the bivouac periphery and in adjacent soil, suggesting active bivouac warming protects some members of L. praedator bivouac communities from cold-limitation at high elevations in the tropics.