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.