Does social thermal regulation constrain individual thermal tolerance in an ant species?

Abstract

In ants, social thermal regulation is the collective maintenance of a nest temperature that is optimal for individual colony members. In the thermophilic ant Aphaenogaster iberica, two key behaviours regulate nest temperature: seasonal nest relocation and variable nest depth. Outside the nest, foragers must adapt their activity to avoid temperatures that exceed their thermal limits. It has been suggested that social thermal regulation constrains physiological and morphological thermal adaptations at the individual level. We tested this hypothesis by examining the foraging rhythms of six populations of A. iberica, which were found at different elevations (from 100 to 2,000 m) in the Sierra Nevada mountain range of southern Spain. We tested the thermal resistance of individuals from these populations under controlled conditions. Janzen's climatic variability hypothesis (CVH) states that greater climatic variability should select for organisms with broader temperature tolerances. We found that the A. iberica population at 1,300 m experienced the most extreme temperatures and that ants from this population had the highest heat tolerance (LT50 = 57.55°C). These results support CVH's validity at microclimatic scales, such as the one represented by the elevational gradient in this study. Aphaenogaster iberica maintains colony food intake levels across different elevations and mean daily temperatures by shifting its rhythm of activity. This efficient colony‐level thermal regulation and the significant differences in individual heat tolerance that we observed among the populations suggest that behaviourally controlled thermal regulation does not constrain individual physiological adaptations for coping with extreme temperatures.

Vertical niche and elevation range size in tropical ants: Implications for climate resilience

“…Here, we focus on foraging niche breadth of adult workers rather than the nest sites of colonies. The persistence of a colony at a site will be contingent on both nest success and foraging success but nests can be buffered from external temperature variation, through either nest construction (Jones & Oldroyd, 2006) or social thermal regulation (Villalta et al., 2020), whereas foragers must cope with thermal extremes (Jayatilaka et al., 2011).…”

Ant (Hymenoptera: Formicidae) species diversity in secondary forest and three agricultural land uses of the Colombian Pacific Coast

“…Furthermore, the ants adapt their thermal tolerance to local conditions (Villalta et al 2020). It is well established that E. ruidum is a thermophilous species on the local scale (Schatz & Lachaud 2008;Santamaría et al 2009), which could explain the high densities of this species found in oil palm plantations that consist of open and sunny areas.…”

Colony-level mechanisms of thermal tolerance regulation in the ant Ectatomma ruidum

“…Thermal limits are a major factor in structuring species distributions across broad environmental gradients (Diamond et al, 2012). Ants also adapt their thermal tolerance to local conditions (Bujan & Kaspari, 2017;Diamond, Chick, Perez, Strickler, & Martin, 2017;Villalta et al, 2020); we have known this ever since Mary Talbot invented equipment and protocols to measure critical thermal maxima (Talbot, 1943). In temperate environments, it appears that the ability to withstand cold may be more predictive of distributions than the ability to withstand heat (Bishop, Robertson, Van Rensburg, & Parr, 2017;Bujan, Roeder, de Beurs, Weiser, & Kaspari, 2020).…”

“…Because it is energetically expensive to tolerate high heat, ant colonies must experience selective pressures to decrease investment into thermal tolerance for those experiencing cooler temperatures, such as nest-bound workers and foragers that leave the nest in cooler temperatures (Cerdá & Retana, 2000;Gehring & Wehner, 1995;Ribeiro, Camacho, & Navas, 2012;Talbot, 1934;Willot, Gueydan, & Aron, 2017). In a recent experiment, Villalta et al (2020) demonstrated how colonies of Aphaenogaster iberica ants move their nests and modify the structure of nests to respond to seasonal temperature changes, and showed that colonies manage thermal challenges through a combination of colony-level behaviors, adaptive physiological responses, and individual foraging decisions. We know less about how intracolonial variation in thermal tolerance is actively managed by colonies.…”