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‐Uribe et al., 2019) than
do solitary bees, potentially allowing them to escape resource-depleted
landscapes. Colonies of the African honey bee (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 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 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, Xylocopa sonorinaSmith, 1874, bees that overwintered in groups maintained body
temperatures nearly 1.5°C 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 33-36°C, 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 (32°C) 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; Fründ 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.