Plastic collective endothermy in a complex animal society (army ant bivouacs: Eciton burchellii parvispinum)

Abstract

Endothermic animals do not always have a single adaptive internal temperature; some species exhibit plastic homeostasis, adaptively allowing body temperature to drop when thermoregulatory costs are high. Like large-bodied endotherms, some animal societies exhibit collective thermal homeostasis. We tested for plasticity of thermoregulation in the self-assembled temporary nests (bivouacs) of army ants. We measured core bivouac temperatures under a range of environmental conditions and at different colony developmental (larval vs pupal brood) stages. Contrary to previous assertions, bivouacs were not perfect thermoregulators in all developmental stages. Instead, bivouacs functioned as superorganismal facultative endotherms, using a combination of site choice and context-dependent metabolic heating to adjust core temperatures across an elevational cline in ambient temperature. When ambient temperature was low, the magnitude of metabolic heating was dependent on colony developmental stage: pupal bivouacs were warmer than larval bivouacs. At cooler high elevations, bivouacs functioned like some endothermic animals that intermittently lower their body temperatures to conserve energy. Bivouacs potentially conserved energy by investing less metabolic heating in larval brood because the high costs of impaired worker development may require more stringent thermoregulation of pupae. Our data also suggest that site choice played an important role in bivouac cooling under high ambient temperatures at low elevations. Climate warming may expand upper elevational range limits of Eciton burchellii parvispinum, while reducing the availability of cool and moist bivouac sites at lower elevations, potentially leading to future low-elevation range contraction.

Behavioral response to heat stress of twig-nesting canopy ants

“…We found that Cephalotes nested in smaller cavities that were slower to heat up, suggesting that the choice of nests might mitigate the effect of heat stress. One-way social insects passively thermoregulate their nests by choosing an adequate nest location (Jones and Oldroyd 2006), and this behavior even occurs in army ants, which are nomadic and endothermic (Soare et al 2011;Baudier et al 2019). Nest site selection in Cephalotes is also governed by the ability to defend their nests (Powell 2016), and Cephalotes are able to survive longer in nests that are easier to defend (Powell et al 2017).…”

Foraging plasticity of Atta cephalotes (Hymenoptera: Formicidae) in response to temperature differences between forest and pasture

“…This is a plausible explanation because mating apparently occurs high in the air, and many colonies conduct their nuptial flights during the same period of the day; therefore, the probability of outbreeding is high (Moser, 1967). Additionally, the ants can change the temperature at which they are exposed by changing the place of the foraging trail; thus, the site choice plays an important role modifying the ant ambient temperatures (Baudier, D'Amelio, Sulger, O'Donnell, & O'Connor, 2018).…”

Multi-level instrumentation of bivouac thermoregulation: current methods and future directions

“…A new bivouac site is chosen collectively by workers during the previous day's raid [7,22]. Site choice varies across elevations and appears to be especially important for cooling in hot climates [2,23]. Eciton burchellii in the humid sites where they predominantly occur are more likely to bivouac in trees at low elevations to enhance convective cooling [23].…”