Social thermal physiology: How superorganismal homeostasis confronts elevational thermal clines (Formicidae: Ecitoninae: Eciton burchellii parvispinum)

Temperature variation along elevational clines generates locally stable but spatially diverse temperatures in the tropics, potentially selecting for the evolution of thermally specialized animal populations. The Neotropical army ant Eciton burchellii parvispinum is distributed across a wide elevation range spanning at least 7°C differences in mean annual temperature. These ants form some of the largest and most complex animal societies. Specialized group behaviors for dealing with thermal variation include homeostatic bivouacs (temporary living nests) composed of interlocking workers that cradle the brood and queen. Data from lowland (warm) forests suggest bivouacs create warmed and stable thermal conditions. We compared bivouac buffering of temperature and humidity, and performance of putative thermoregulatory behaviors, across elevations in Costa Rica to test whether lowland and high-elevation bivouacs maintain similar internal conditions across varying temperature conditions. We then manipulated temperatures of bivouacs at high and low elevations, recording group behavioral responses, to test whether and how army ant bivouacs accommodate extreme high and low temperatures at each site. Results suggest colonies have the capacity to maintain similar internal temperatures across high and low elevation sites, despite recorded ambient temperature variance of over 15°C. We demonstrate novel thermoregulatory behavioral responses to temperature manipulations, and we compare our findings to patterns observed in other army ant taxa. Tropical ectotherms are predicted to face strong pressures of directional climate change. While previous studies have largely focused on the thermal tolerance of adult individual workers, this study examines the collective capacity of the colony to accommodate thermal variation.