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.