Introduction
The rapid anthropogenic warming of the planet has amplified our need to understand how organisms function in challenging thermal regimes. This work is complicated by the broad range of behavioral and physiological mechanisms used for thermoregulation (Kearney, Shine, & Porter, 2009; McGlynn, Dunn, Wayman, & Romero, 2010). The behavioral regulation of thermal tolerance is even more complicated in colonies of social insects (Chick, Perez, & Diamond, 2017). Colonies are comprised of individuals whose thermoregulatory behavior responds to social context (Kaspar, Cook, & Breed, 2018), even though whole colonies can metabolically function as a single unit (Cook, Kaspar, Flaxman, & Breed, 2016; Hou, Kaspari, Vander Zanden, & Gillooly, 2010). It is worthwhile to understand how the sociality of insect colonies may facilitate responsiveness to thermal challenges.
It might be expected that ants are highly capable of flexibly responding to thermal challenges, as they are often regarded as paragons of efficiency (Wilson, 1980). Indeed, ants are capable of responding to thermal challenges at the colony level (Talbot, 1943). In many species, colonies will move to more thermally favorable locations (McGlynn, 2012; Smallwood, 1982). Most of what we know about thermal tolerance in ants comes from interspecific comparisons, among contrasting life histories (Garcia-Robledo, Chuquillanqui, Kuprewicz, & Escobar-Sarria, 2018), habitats (Kaspari, Clay, Lucas, Yanoviak, & Kay, 2015), and evolutionary histories (Diamond & Chick, 2018; Diamond et al., 2012). 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.
Physiological mechanisms of thermal tolerance in insects are well described (Harrison, Woods, & Roberts, 2013). Insects produce heat shock proteins to prevent damage from heat exposure, and ant species that inhabit hotter environments constitutively express more heat shock proteins (Gehring & Wehner, 1995; Willot et al., 2017). Heat shock protein production can also be induced by exposure to high heat environments (Helms Cahan et al., 2017; Moseley, 1997). In tropical environments, daily temperature cycles encompass a greater thermal range than annual temperature cycles, which explains why there are differences in thermal tolerance between diurnally-foraging and nocturnally-foraging ant species in the tropics (Garcia-Robledo et al., 2018; Hodkinson, 2005). Some species forage at all times of day, and earlier work with one such species (Ectatomma ruidum ), has shown that foragers sampled in the heat of the day demonstrated a greater thermal tolerance than those sampled in the relative cool of the evening (Esch, Jimenez, Peretz, Uno, & O’Donnell, 2017). A follow-up study on this disparity found that these differences were not caused by differences between colonies (Nelson et al., 2018). That is, in E. ruidum , differences in thermal tolerance expressed by workers at daily thermal maxima must be accounted for by processes that take place within individual ant colonies. Our present research on E. ruidum is designed to understand the processes that make some workers more thermally tolerant than their nestmates.
Here we hypothesize three mechanisms for colony-level organization of thermal tolerance in E. ruidum . The Thermal Acclimation Hypothesis posits that worker differences in thermal tolerance are the result of ephemeral induced defenses based on prior thermal experiences. The next two hypotheses for the organization of thermal tolerance in ant colonies involve variation in the constitutive expression of thermal tolerance. According to the Division of Labor Hypothesis, variation in thermal tolerance among individuals is explained by their role in the colony (Janowiecki, Clifton, Avalos, & Vargo, 2020). If this hypothesis is true, then we would expect differences in thermal tolerance between foragers and non-foragers, and between foragers depending on the time of day that they forage. Last, under the Circadian Rhythm Hypothesis, variability in thermal tolerance is driven by an endogenous circadian rhythm (Lazzari & Insausti, 2008) that regulates daily cycling of heat shock protein production. According to this hypothesis, we expect that the thermal tolerance of ants inside colonies will differ at thermal minima and thermal maxima, even if colonies are exposed to a constant temperature throughout the day. In this study, we challenged ants with a constant elevated temperature and measured the amount of time before they lost the ability to function, which is a measure that we label “thermal persistence.” The mechanisms for colony-level regulation of thermal persistence are not necessarily mutually exclusive, and we have no a priori reasons to favor any of the hypotheses over the other. Here we present a set of experiments to evaluate these three hypotheses.