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\begin{document}

\title{How does climate change impact social bees and bee sociality?}



\author[1]{Madeleine M. Ostwald}%
\author[1]{Carmen R. B. da Silva}%
\author[1]{Katja Seltmann}%
\affil[1]{Affiliation not available}%


\vspace{-1em}



  
  \date{May 30, 2024}


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\textbf{Abstract}

\textbf{}1.~~~~~Climatic factors are known to mediate the expression of
social behaviors. Likewise, variation in social behavior can dictate
climate responses. Understanding interactions between climate and
sociality is crucial for predicting impacts of climate change on animal
populations.

2.~~~~~These effects are particularly relevant for taxa like bees that
exhibit a broad diversity of social states. An emerging body of
literature aims to quantify bee responses to environmental change with
respect to variation in key functional traits, including sociality.
Additionally, decades of research on environmental drivers of social
evolution may prove fruitful for predicting shifts in the costs and
benefits of social strategies under climate change.~

3.~~~~~In this review, we explore these findings to ask two
interconnected questions: (a) how does sociality impact vulnerability to
climate change, and (b) how might climate change impact social
organization in bees? In doing so, we generate
predictions\textbf{~}about the impacts of climate change on the
expression and distribution of social phenotypes in bees. Furthermore,
we emphasize that social and solitary bee populations could be
differentially impacted by climate change due to important behavioral,
physiological, and life history differences.~

4.~~~~~We highlight important avenues for investigating evolutionary
consequences of climate change for social bees and for bee social
organization. Understanding linkages between social behavior and
environmental conditions is essential for forecasting vulnerability and
resilience to climate change across bee taxa.

\textbf{}

\par\null

\textbf{Introduction}

Climate represents a major selective force in the evolution of
sociality. Climatic conditions can shape developmental rates, nesting
strategies, biotic interactions, and other processes that determine the
relative costs and benefits of group formation (Blumstein et al., 2022;
Fisher et al., 2021; Menzel and Feldmeyer, 2021; Moss and While, 2021;
Wilson, 1971). Social living can give rise to emergent strategies for
coping with climatic stressors (Arnold, 1988; Fahrenholz et al., 1989;
Klok and Chown, 1999). Observed patterns in global biogeography support
hypotheses linking climate to social evolution, with distributions of
social organisms falling along gradients of temperature, precipitation,
and climatic stochasticity (Jetz and Rubenstein, 2011; Lukas and
Clutton-Brock, 2017; Purcell, 2011). These interactions between climate
and sociality raise important questions about the fate of social
organisms and social phenotypes under changing climate.

Climate change is predicted to have major consequences for the survival,
health, and distributions of organisms globally, including ecologically
and economically important animals like pollinators (da Silva et al.,
2023; Forrest, 2016; Potts et al., 2010). Bees (Hymenoptera: Apoidea:
Anthophila) are the primary animal pollinators of terrestrial ecosystems
(Ollerton et al., 2011). Bees possess some of the richest diversity of
social behaviors of any animal taxon, with multiple independent origins
of eusociality and a broad range of non-eusocial forms (Michener, 1974;
Wcislo and Fewell, 2017). Within the past decade, researchers have
increasingly investigated species-level social traits as possible
predictors of bee responses to environmental change (Forrest et al.,
2015; Hall et al., 2019; Harrison et al., 2018). Social organization in
bees correlates with other behavioral and life history traits, like
foraging strategy and phenology, that can influence susceptibility to
climate change stressors. However, a unifying pattern linking sociality
to climate change vulnerability has remained elusive.

Climate change is also likely to shift the expression and geographic
distribution of social behaviors across animal populations. Expected
consequences of climate change include not only increasing average
temperatures, but also increasing climate variability and increasing
frequency and severity of extreme events like drought, flooding, and
wildfire (IPCC, 2022). For bees, many of which can flexibly express
sociality in response to environmental conditions (Wcislo and Fewell,
2017), the social impacts of these shifts could be particularly
profound. Nevertheless, very few studies have directly assessed the
impacts of climate change on bee social organization. Still, these
effects may be extrapolated from known relationships between bee
sociality and local environmental conditions. Drawing from this
literature, we make predictions about the consequences of climate change
for bee sociality and identify promising directions for future research.

Box 1: \emph{Notes on bee social biology and terminology}

The ancestor of bees was solitary, as are the vast majority of extant
bee species (Debevec et al., 2012). Nevertheless, diverse forms of
social living are known to be found in all currently recognized bee
families except the Stenotritidae (Michener, 1974; Wcislo and Fewell,
2017). The extent of and capacity for social living among bees is likely
to be vastly underestimated, given that social strategies may be present
at low levels in otherwise solitary populations (Wcislo, 1997, 1993;
Yagi and Hasegawa, 2012). Social organization in bees takes a diversity
of forms, and includes several independent origins of (and reversals
from) eusociality (Danforth et al., 2003; Kocher and Paxton, 2014).
Among and within taxa, bee societies vary considerably in group size, in
the degree of reproductive skew and morphological specialization, and in
the genetic relatedness of group members (Michener, 1974; Ostwald et
al., 2022b; Wcislo and Fewell, 2017). Particularly instructive for
social evolutionary studies are the many lineages in which social living
is a facultative state, which may be determined by environmental
conditions.

The studies we review here vary considerably in their categorizations of
social states. For the purposes of this review, we will define sociality
as multi-female group living within a nesting context, with or without
reproductive division of labor (\emph{sensu} Wcislo \& Fewell, 2017). We
consider communal living as a social category because it entails
behaviors and characteristics distinct from solitary living that are
relevant to environmental tolerance, for example, by enhancing nest
defense from predators and parasites, by generating advantages in nest
excavation and initiation, and by enabling social thermoregulatory
behaviors. In general, we will broadly compare social groups to solitary
individuals, but emphasize that sociality represents a broad spectrum of
behaviors with corresponding diversity in their responses to climate
conditions.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-1-SCC/Figure-1-SCC}
\caption{{Figure 1. Behavioral, physiological, and life history traits common
among social bees, arranged by their tendency to confer vulnerability
vs. resilience to changing climate. Importantly, not all social bees
possess these traits, and the effect of these traits on climate
responses will vary with environmental context.~
{\label{939092}}%
}}
\end{center}
\end{figure}

\textbf{1. How does sociality impact vulnerability to climate change?}

Sociality shapes bees' life histories, physiological traits, and
behavioral repertoires. These traits can broadly influence how bees
respond to climatic variables, and therefore their vulnerability to
environmental change (Figure 1). Group living has been proposed to
provide buffering effects against environmental variability (Kennedy et
al., 2018; Komdeur and Ma, 2021), which may explain why cooperatively
breeding animals to thrive in regions characterized by strong climatic
variability (Jetz and Rubenstein, 2011; Lukas and Clutton-Brock, 2017;
Sheehan et al., 2015). While a considerable body of literature explores
effects of climate change on bees that are social, many fewer
investigate social effects at the species level, i.e., by assessing
social behavior as a predictor variable across bee species. Where data
do exist, patterns have been mixed. Some studies have found significant
relationships between sociality and climate-relevant functional traits,
like desiccation resistance and thermal tolerance (da Silva unpub.,
Hamblin et al., 2017). Others have shown that sociality was weakly or
not at all associated with climate change responses, including
phenological shifts (Bartomeus et al., 2011; Meiners et al., 2020) and
responses to extreme weather events (Graham et al., 2021). More
commonly, studies have evaluated sociality as a predictor of responses
to anthropogenic landscape change. Social bees may be less susceptible
than solitary bees to urbanization (Banaszak-Cibicka and \selectlanguage{polish}Ż\selectlanguage{english}mihorski,
2012; Harrison et al., 2018) and agricultural intensification (Forrest
et al., 2015; Hall et al., 2019). While these studies are informative
for suggesting general patterns of social resilience to environmental
change, more work investigating climate variables specifically is
necessary to understand social trait-related responses to climate
change. A promising starting-place to generate predictions for these
studies is by examining life history, behavioral, and physiological
trait variation between social and solitary bees.

\emph{Life history factors}

Known differences in social and solitary bee life histories can offer
insights into potential mechanisms of climate resilience. Principal
among these differences is the seasonal life cycle of social bees.
Solitary bees are active as adults for narrow windows of time, averaging
about one month per year, but often for much shorter (Michener, 2007).
This restricted activity period could render solitary bees particularly
vulnerable to phenological mismatch as host plants advance their
flowering times under climate change (Kudo and Ida, 2013; Visser and
Gienapp, 2019). Longer activity periods, however, present other
challenges. Social bees are typically multivoltine (i.e., they produce
multiple broods per year), with an adult activity period spanning
several months (Michener, 1974). The breadth of social bee species'
activity windows may increase their likelihood of being impacted by
resource gaps (Kaluza et al., 2018; Requier et al., 2020). However,
these disadvantages of a long colony lifespan may be counterbalanced by
behavioral adaptations common to social species, including polylecty and
resource storage, that enhance survival when floral resources are
depleted.

Social and solitary bees also differ markedly in their overwintering
life cycles. Most solitary bees spend most of the year in diapause in
the pre-pupal stage. Eusocial bees, by contrast, typically overwinter as
fertilized adult female gynes. Several studies have demonstrated that
bees overwintering as adults are more sensitive to increased winter
temperatures, leading to increased mortality and weight loss relative to
species overwintering in the prepupal stage (Fr\selectlanguage{ngerman}ünd et al., 2013;
Kammerer et al., 2021; Slominski and Burkle, 2019). Importantly, not all
solitary bees conform to the typical, likely ancestral pattern of
prepupal winter diapause, and communal bees exhibit a mix of adult and
pre-pupal overwintering life histories. For the eusocial bees, however,
many of which are adult-wintering, warming winter temperatures may be
disproportionately challenging.

\emph{Behavioral and physiological factors}

Social bees possess a suite of behavioral adaptations related to
resource acquisition that might be advantageous as floral resources
become scarcer, more patchily distributed, and/or unpredictably
available under climate change. In order to support their extended
colony life cycles over the course of the flowering season, the vast
majority of social bees have broad, generalist pollen diets (Michener,
2007), which confers resilience to changing floral communities (Bogusch
et al., 2020). Highly eusocial bees also possess complex communication
strategies (via olfactory, auditory, and dance communication) that
enable them to adaptively coordinate foraging efforts across large
colony workforces (Michener, 1974; Seeley, 1995; von Frisch, 1967). By
accurately communicating presence, location, and/or quality of food
resources, these behaviors enable colonies to more effectively exploit
spatially and temporally unpredictable food landscapes (Dornhaus and
Chittka, 2004; Hrncir et al., 2019; Maia-Silva et al., 2020). Many
eusocial bees also store food in the nest for adult consumption,
buffering against floral dearth periods (Grüter, 2020; Heinrich, 1979;
Seeley, 1985). Food storage enables a perennial lifestyle for the highly
eusocial bees (e.g., honey bees and stingless bees), and even for annual
colonies (e.g., bumble bees) it can provide insurance against short
periods of poor foraging conditions. Social bees can also share
collected food via trophallaxis, even in simpler facultative societies
(Gerling et al., 1983; Kukuk and Crozier, 1990; Sakagami and Laroca,
1971). Finally, social bees have larger foraging ranges (Kendall et al.,
2022) and greater dispersal capabilities (López\selectlanguage{english}-Uribe et al., 2019) than
do solitary bees, potentially allowing them to escape resource-depleted
landscapes. Colonies of the African honey bee (\emph{Apis mellifera
scutellata} Lepeletier, 1836) will seasonally abscond from their
established nest sites, migrating to areas of greater food abundance
(McNally and Schneider, 1992).

These traits can increase social bees' resilience to drought conditions.
Several studies have highlighted eusocial bees as ecological ``winners''
of drought events. Hung et al. found increased representation of
eusocial \emph{Lasioglossum} bees in samples collected in Southern
California following the severe drought year of 2014 (2021). Similarly,
Kammerer et al. examined a long-term bee occurrence dataset in the
mid-Atlantic US and found that solitary bees declined in
low-precipitation years, whereas eusocial bees did not (2021). Other
findings have highlighted polylecty, a trait that co-occurs with
sociality, as a successful strategy under drought conditions. Minckley
et al. surveyed bee abundance in the Chihuahuan Desert and found that
generalist bees were more abundant in drought years (2013).
Alternatively, solitary bee traits may be particularly adaptive in arid
regions with unpredictable rainfall. Minckley et al. suggest that under
severe drought scenarios, the (solitary) specialist species that can
undergo facultative long-term diapause may have competitive advantages
over generalist bees that cannot wait out unfavorable years (2013).
Indeed, the ability of solitary, specialist, univoltine species to time
their active season with short, unpredictable flowering periods
represents one hypothesis for why solitary bees are so species rich in
desert environments (Danforth et al., 2019).

Social bees also possess unique behavioral mechanisms for regulating
their microclimates, buffering against thermal stress under climate
change. Especially in temperate regions, the eusocial corbiculate bees
employ a suite of integrated behaviors to deftly control their nest
temperatures, including direct incubation, metabolic heat production,
fanning, nest evacuation, and evaporative cooling (Heinrich, 1993; Jones
and Oldroyd, 2006; Seeley, 1985). These behaviors enable colonies to
maintain an optimal thermal setpoint despite wide variation in ambient
temperatures. Coordinated thermoregulatory behaviors can promote
recovery from and resilience to extreme heat events. Following intensive
water collection to cool the nest under high ambient temperatures, honey
bee workers can temporarily store water in their combs and their crops
for future distribution, potentially buffering against future
emergencies (Ostwald et al., 2016). While these behaviors are best known
in the corbiculate bees, thermoregulatory behaviors may exist in other
clades. Michener observed fanning at the nest entrance by the
primitively eusocial halictid \emph{Augochlorella aurata} (Smith, 1853);
(1974). In winter hibernaculae, passive clustering of adults in could
minimize heat loss by reducing the group's collective thermal inertia.
For the facultatively social carpenter bee, \emph{Xylocopa
sonorina}Smith, 1874, bees that overwintered in groups maintained body
temperatures nearly 1.5degC warmer than solitary individuals at the
coldest time of day (Ostwald et al., 2022a). Minor differences such as
these could present survival advantages of social nesting when
temperatures approach freezing.

The thermoregulatory behaviors of social bees may have important
implications for their physiological tolerance limits. Eusocial bees are
highly adept at controlling nest temperatures, and they are particularly
sensitive to deviations from their optimal thermal ranges. European
honey bees tightly regulate the temperature of their broodnests within
the range of 33-36degC, even as ambient temperatures drop below freezing
or soar to extreme highs (Fahrenholz et al., 1989; Seeley, 1985). Brood
reared at even a single degree below this range (32degC) experience
significant learning deficits (Jones et al., 2005; Tautz et al., 2003).
Solitary bees, in contrast, may tolerate a much wider range of
temperatures during development and throughout their adult lives (Earls
et al., 2021; Frund et al., 2013; Park et al., 2022), during which they
may be poorly buffered from environmental temperatures. This variation
in the thermal experiences of social and solitary bees might help to
explain corresponding variation in their heat tolerance or ability to
survive in arid environments. For example, the climatic variability
hypothesis proposes that species that experience greater environmental
variability should have greater phenotypic plasticity (ability to shift
underlying physiology with changes in environment) than species that
experience little environmental variability (Janzen, 1967). In contrast,
organisms that evolve in highly variable environments are also expected
to have broad physiological tolerances and limited plastic responses to
changes in climate (Gabriel, 2005). However, there are examples of
species that have plastic physiological responses to changes in
temperature and broad thermal tolerances (da Silva et al., 2019; Healy
and Schulte, 2012). Thus, if solitary bees are evolving in stochastic
and variable environmental conditions, we would expect them to either
have broader thermal tolerances, greater plasticity in their thermal
performance, or both, compared to social bees which are expected to
evolve in more stable environmental conditions. Indeed, determining
whether social or solitary bees are more vulnerable to climate change
will require an understanding of their physiological tolerances and the
microclimates that they inhabit (i.e., social species are less heat
tolerant, but also experience lower extreme thermal environments). For
example, many solitary and communal species live in stem nests that are
exposed to a great deal or climatic variability or, alternatively, live
in underground tunnel nests, which are much more thermally stable (da
Silva et al., 2019; Healy and Schulte, 2012). Eusocial lineages (e.g.,
Apini and Meliponini) often nest in cavities, which we would expect to
experience an intermediate amount of thermal variability compared to
stem nests or underground tunnel nests. Thus, microclimate variability
is likely to be influenced by both sociality and nesting strategy, which
in turn could shape the evolution and plasticity of physiological
tolerances.

\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-2-SCC/Figure-2-SCC}
\caption{{Figure 2. Summary of the impacts of climate on sociality, as mediated by
changes in the selective environment and resulting impacts on social
traits and fitness. Arrows indicate directions of influence.
{\label{162933}}%
}}
\end{center}
\end{figure}



\textbf{2. How might climate change impact social organization in bees?}

Climatic selective pressures have been implicated in social evolutionary
transitions across animal taxa (Guevara and Avil\selectlanguage{ngerman}és, 2015; Jetz and
Rubenstein, 2011; Liu et al., 2020; Lukas and Clutton-Brock, 2017).
These patterns support the hypothesis that sociality can facilitate the
colonization of unpredictable environments, or can expand species'
ranges (Brooks et al., 2017; Cornwallis et al., 2017). In bees, climatic
factors have shaped the diversification and distributions of social
lineages (Brady et al., 2006; Groom and Rehan, 2018; Kocher et al.,
2014). The same selective forces that have historically shaped the
evolution of social behavior in bees could likewise influence social
behavior under climate change.

The bees most likely to experience transitions in social organization in
response to climate change are facultatively social bees with some
degree of social plasticity because they already possess the behavioral
flexibility to express multiple social states. Facultative sociality is
best known among the Halictidae and Xylocopinae (Michener, 1990; Shell
and Rehan, 2017), but may be widespread across bee taxa when one
considers communal nesters and the many typically solitary species that
may have some capacity for communal nesting (Wcislo and Tierney, 2009).
Additionally, climate change may influence colony demography and social
traits (e.g., colony size, reproductive skew, sex ratios) across bee
species more broadly, including the obligately social species.
Importantly, these demographic shifts can impact the social environment,
which can in turn promote changes in social organization and complexity
(Figure 2).

\emph{Phenological and thermal effects}

Some of the best-known impacts of climate on bee social strategy relate
to seasonal constraints. Because eusociality requires adult generation
overlap, eusocial colonies can only form where breeding seasons are
sufficiently long to permit the rearing of a worker brood prior to a
reproductive brood (Davison and Field, 2018a; Hunt and Amdam, 2005). For
this reason, some socially polymorphic species exhibit intraspecific
variation in social behavior along environmental gradients in breeding
season length. This is the case for several temperate halictine species,
for which solitary populations are found at high latitude or
high-altitude portions of their range (where short breeding seasons
preclude the production of a worker generation), and eusocial
populations are found at lower latitude or altitude (Davison and Field,
2016; Eickwort et al., 1996; Packer, 1990; Sakagami and Munakata, 1972).
Similarly, \emph{Ceratina} are more likely to be social in tropical
regions, where long breeding seasons permit bivoltinism, and suggest
that dispersals to temperate regions prompted evolutionary reversions to
solitary living (Groom and Rehan, 2018).

The extent to which social polymorphism is determined by phenotypic
plasticity will dictate how individual species and populations respond
to changing climate. Transplant and common garden experiments represent
promising tools for disentangling genetic and plastic effects on social
strategy. For example, Davison and Field transplanted \emph{Lasioglossum
calceatum} Scopoli, 1763 foundresses from a solitary, high latitude
population to a lower-latitude site where conspecifics are typically
eusocial (2018). Nine of ten transplanted foundresses retained a
solitary lifestyle despite the extended breeding season, suggesting that
the social polymorphism in this species may be largely genetically
determined. In contrast, reciprocal field transplants of \emph{Halictus
rubicundus} Christ, 1791 across a latitudinal gradient induced social
behavior in foundresses from typically solitary populations and solitary
behavior in foundresses from social populations (Field et al., 2010).
Schürch et al. used climate projections to predict that the social
phenotype in \emph{H. rubicundus} will appear at higher latitudes than
previously observed, as increasing average temperatures extend the
breeding season (2016). Importantly, the extent to which sociality is
plastic varies even within species; North American populations
of\emph{H. rubicundus} , for example, show stronger genetic
differentiation between social and solitary populations than do European
populations (Field et al., 2010; Soucy and Danforth, 2002). These
considerations suggest that social responses to climate change will be
strongly heterogenous across and even within socially polymorphic
species.

Phenological effects on sociality are tightly linked to thermal effects.
Temperature not only influences the temporal window in which bees can
rear brood, it also directly impacts development time. These factors
interact to determine the capacity for completing two broods in a single
breeding season. For the allodapine bee \emph{Exoneura
robusta}Cockerell, 1922, faster brood development times at lower
latitude, probably due to warmer temperatures, enabled social nesting
via the production of a second brood (Cronin and Schwarz, 1999). Because
emergence order can determine social dominance for
\emph{Exoneura}(Schwarz and Woods, 1994), accelerated development could
also have consequences for social organization, if warmer temperatures
reduce variation in emergence time. Remarkably, even microclimate
variation within a single site might be sufficient to drive variation in
social phenotype. Hirata and Higashi demonstrated that intra-population
social dimorphism in \emph{Lasioglossum baleicum} Cockerell, 1937
depends on local temperature differences (2008). Brood developed faster
in nests located in sunny areas due to increased soil temperature,
permitting a second brood to be reared before the end of the breeding
season (Hirata and Higashi, 2008). The effects of temperature on
development time could be compounded by increases in foraging rate with
temperature. In temperate climates, foraging activity is limited by the
threshold temperature required for flight initiation (Stone and Willmer,
1989). In some contexts, warming temperatures could increase the thermal
activity window for foraging, enabling foundresses to rear larger
broods. Schürch et al. demonstrated that the number of provisioning
trips completed and the number of offspring provisioned increased with
temperature for\emph{Halictus rubicundus} foundresses (2016). Combined,
these mechanisms may account for the association between warmer climates
or years and increases in colony size (Cronin and Schwarz, 1999; Field
et al., 2010; Packer and Knerer, 1986; Richards and Packer, 1995).
Alternatively, in environments characterized by hot summers that
regularly exceed bees' optimal foraging temperatures, warming could
constrain second brood provisioning by limiting activity windows (Jaboor
et al., 2022).

Thermal effects on colony demography can also impact within-group social
dynamics, for example, by shaping the within-group distribution of
female body sizes. Body size in social bees is strongly associated with
reproductive dominance (Brothers and Michener, 1974; Richards, 2011;
Smith et al., 2008). Specifically, larger females are better able to
physically coerce offspring or other nestmates into worker behaviors
like foraging; and these dominance behaviors seem to be important in
inhibiting worker ovarian development (Brothers and Michener, 1974;
Michener and Brothers, 1974). Environmental impacts on body size thus
represent an avenue through which climate change might impose shifts in
social organization. Richards and Packer found that favorable conditions
(warm, dry years) led to primitively eusocial \emph{Halictus
ligatus}Say, 1837 queens producing larger-bodied workers than they did
in unfavorable conditions (cool, rainy years), likely due to enhanced
foraging opportunities (1996). When the body size differential between
queens and workers is low, queens may be less successful at policing
worker reproduction. Indeed, under favorable conditions, workers were
relatively large and more likely to reproduce. Conversely, under
unfavorable conditions, queens and workers were more dissimilar in size
and worker reproduction was rare, leading to more strongly eusocial
colony organization (Richards and Packer, 1996). Similarly, for the
facultatively social, subtropical small carpenter bee, \emph{Ceratina
australensis} Perkins, 1912, unfavorable years (hot, dry years) produced
smaller-bodied brood (Dew et al., 2018). Because \emph{C.
australensis}females that found social nests tend to be larger-bodied,
climate-mediated body size variation may impact year-to-year variation
in the frequency of social nesting (Dew et al., 2018).

Finally, temperature can shape colony demography through shifts in
offspring sex ratios. Female-biased broods early in the reproductive
season create opportunities for social nesting via worker recruitment
(Boomsma, 1991; Trivers and Hare, 1976). Yanega found that warmer
temperatures were correlated with increasing male bias in the first
brood of \emph{H. rubicundus} , which led to a population-level decrease
in the frequency of eusocial nesting (Yanega, 1993). Future work
tracking first brood sex ratios in flexibly social bees will be
particularly instructive for predicting impacts of warming on colony
demography and the frequency of social nesting.

\emph{Precipitation effects}

Social strategies can also help mitigate environmental stressors
associated with precipitation and water availability. Many of these
effects are intrinsically bound to thermal effects, working in concert
with temperature variation to determine whether conditions are favorable
or unfavorable for foraging and brood rearing. Annual variation in
precipitation can dictate activity periods, either by inhibiting
foraging in times of extended rainfall or by creating floral dearth
periods in times of drought. Indeed, for the facultatively
eusocial\emph{Augochlorella aurata} , drought conditions reduced brood
sizes by two to three offspring (Packer, 1990). Conversely, Schürch et
al. suggest that an increase in spring rainfall under climate change
could reduce the frequency of social nesting in \emph{Halictus
rubicundus} , by delaying provisioning and reducing the time window to
produce a second brood (2016). Likewise for \emph{Halictus ligatus} ,
high rainfall created conditions unfavorable for worker production, with
consequences for social organization (Richards and Packer, 1995).
Finally, precipitation can pose direct survival risks that may affect
the frequency of social nesting. Heavy rainfall and flooding threaten
brood survival, which can reduce worker recruitment, decreasing colony
size and restricting opportunities for social nesting.

Precipitation can also alter features of the physical environment that
determine the costs and benefits of social nesting. For ground nesting
bees, nest excavation may be particularly costly when soils are hard,
especially during drought. Danforth suggests that the high energetic
costs of excavating dry, hard-packed soils favors communal nesting for
the desert-adapted bees \emph{Perdita portalis} Timberlake, 1954 (1991)
and \emph{Macrotera texana} Cresson, 1878 (1996). Drying of soils is a
predicted consequence of climate change in many regions, which may raise
the costs of nest excavation for ground nesting bees, thereby increasing
the benefits of cooperative nest excavation. Indeed, Bohart and Youssef
observed an increase in the incidence of social nesting during drought
conditions for the typically solitary sweat bee \emph{Lasioglossum
lusorium} Cresson, 1872 (1976). In addition to energetic costs,
excavation of dry soils could entail increased cuticular wear,
potentially increasing risk of desiccation. However, the physiological
and behavioral consequences of dry soil excavation remain to be tested
empirically.

\emph{Predation, parasitism, and competition}

For many bee lineages, sociality may have arisen as a strategy for
mitigating the effects of inter- and intraspecific interactions,
especially parasitism, predation, and intraspecific competition (Lin and
Michener, 1972; Wcislo and Fewell, 2017). Climate change will impact the
distribution and abundances of diverse communities of bees' competitors
and natural enemies, with variable consequences for bee social behavior.
In populations under strong selective pressure from parasitism or
predation, sociality can enable bees to forage without leaving their
nests unattended. Active defensive behavior by guard bees, or even
simply the presence of a bee in the nest, can successfully deter natural
enemies (Kukuk et al., 1998; Mikát et al., 2016; Zammit et al., 2008).
Abrams and Eickwort observed cleptoparasitic bees (\emph{Nomada} ,
Apidae) entering solitary nests of the sweat bee \emph{Agapostemon
virescens} Fabricius, 1775 (Halictidae), but never communal nests, which
were continuously guarded (1981). Similarly, solitary \emph{Ceratina
australensis} nests were more severely parasitized by chalcid wasps
(\emph{Eurytoma} sp.) than social nests of the same species (Rehan et
al., 2011). Importantly, social nesting can also provide insurance
against nest failure in the event of foundress mortality (Gadagkar,
1990; Queller, 1994). In one study of the facultatively eusocial sweat
bee \emph{Megalopta genalis} Meade-Waldo, 1916, adults in the nest
successfully defended brood against raiding ants, but orphaned brood all
succumbed to ant predation (Smith et al., 2003).

Similarly, in environments characterized by strong intraspecific
competition, sociality can provide strategies for securing and
safeguarding limiting resources, especially food and nesting substrate.
Social nests of the facultatively social carpenter bee, \emph{Xylocopa
pubescens} Spinola, 1838, contain a non-reproductive guarding female and
a reproductive forager (Gerling et al., 1981). In one study, the
presence of a guard in the nest prevented pollen robbing by conspecifics
and also allowed the dominant reproductive to complete longer foraging
trips (Hogendoorn and Velthuis, 1993). Importantly, the relative costs
and benefits of tolerating a guard (i.e., a reproductive rival) in the
nest depended on local resource availability and therefore the intensity
of pollen robbing (Hogendoorn and Velthuis, 1993). Competition over
nests is also a driver of social evolution in some bees, especially when
nest substrate is limited or costly to exploit. Shifts in nest substrate
availability may even drive social evolutionary transitions, as for one
stem-nesting allodapine bee, \emph{Braunsapis puangensis} Cockerell,
1929. The recent introduction of \emph{B. puangensis} to Fiji
accompanied by a shift to communal nesting from the ancestral strategy
of reproductive queueing (da Silva et al., 2016). Because Fiji has no
native stem-nesting bees, low competition for nesting substrate in their
introduced environment may have expanded opportunities for egalitarian
sociality (2016). Finally, nests may be limiting not due to a shortage
of substrate, but due to costs of exploiting that substrate. For large
carpenter bees (\emph{Xylocopa} ), the high metabolic costs of wood nest
excavation may favor sociality via nest inheritance strategies (Ostwald
et al., 2021). Indeed, for one population of \emph{Xylocopa
virginica}Linnaeus, 1771, high-density conditions led to an increase in
social nesting due to saturation of available nests (Vickruck and
Richards, 2021). Under climate change, as species are redistributed in
time and space (phenological and geographic shifts), we expect social
evolutionary consequences of these changes in selective pressures
related to parasitism, predation, and intraspecific competition.

\textbf{Conclusions}

Sociality has repeatedly arisen as an adaptive response to extreme and
variable environments. The same features of sociality that have enabled
its evolutionary success in these conditions may render social organisms
particularly resilient to climate change (Blumstein et al., 2022; Fisher
et al., 2021; Komdeur and Ma, 2021; Menzel and Feldmeyer, 2021). In many
contexts, changing climate will favor social bees with long activity
periods, generalist diets, and behavioral adaptations (e.g.,
communication and thermoregulatory strategies) that facilitate survival
in stochastic environments. In other cases, extreme climate conditions
may select for life history patterns and expanded physiological
tolerances common to solitary bees. Our understanding of these effects
is currently data-limited and should be expanded in part through open
sharing of bee functional trait data (especially physiological tolerance
and social behavioral data), which will enable meta-analyses of the
traits co-occurring with sociality and their impacts on climate change
responses. Beyond these differential impacts, climate change can also
shape social evolution itself by shifting the abiotic and biotic
selective pressures that determine the fitness outcomes of different
social strategies. The direction of these shifts will be largely
heterogeneous within and across taxa, depending on such factors as local
climatic variability and the extent to which plasticity governs
responses to environmental change. Experimental studies of socially
polymorphic bee species (e.g., common garden experiments, reciprocal
transplants, and studies manipulating environmental conditions) will
extend our understanding of these evolutionary consequences. Future work
in this area has the potential to clarify interactions between climate
change and sociality at multiple levels and timescales, from shifting
distributions of social bees to evolutionary transitions in social
organization.

\textbf{Acknowledgments}

This work was supported by a National Science Foundation (NSF) award
(DBI-2102006: Extending Anthophila research through image and trait
digitization (Big-Bee)) to KCS and a Macquarie University Research
Fellowship to CRBdS.

\textbf{Author Contributions}

MMO led the writing of the original draft. All authors contributed to
conceptualization and editing.

\textbf{Conflict of Interest}

The authors declare no conflict of interest.

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