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, 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 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 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 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 ofH. 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 Exoneura robustaCockerell, 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 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 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 forHalictus 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 Halictus ligatusSay, 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, Ceratina
australensis Perkins, 1912, unfavorable years (hot, dry years) produced
smaller-bodied brood (Dew et al., 2018). Because C. australensisfemales 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 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.