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.