Introduction
Group living can arise as an adaptive strategy for coping with environmental challenges. For insects, as small-bodied terrestrial organisms, conservation of body water is principal among these challenges [1]. Temporary grouping is known to facilitate water conservation in insects, particularly during seasonal dry periods or vulnerable life history stages [2–4]. These same physiological benefits of grouping may also play a role in the evolution of more stable societies [5,6], though empirical evidence for these effects is scarce. Here, we artificially induce social conditions in a typically solitary bee (Melissodes tepidus timberlakei Cockerell, 1926) to test the hypothesis that grouping can generate water conservation benefits even in the absence of a phylogenetic history of social behavior.
Water availability shapes the distributions of social organisms by shaping the costs and benefits of grouping in a given environmental context [7–11]. Complex social organization can generate novel strategies for regulating nest humidity and collective water balance, particularly among the eusocial insects [12–15]. Likewise in simpler societies, such as ant foundress associations and the facultatively social bees and wasps, water balance considerations can shape social decisions [5,6,16–20]. Particularly for soil-nesting species, these effects may be exacerbated by increased drought under climate change, which is causing rapid drying of soils in many regions [21]. These shifts in water availability and predictability will likely have profound but poorly understood consequences for animal social organization and social evolution [22,23].
Communal nesting strategies are often present at low levels in otherwise solitary populations [24–26], providing a useful empirical context for examining the selective factors underlying transitions from solitary to group living. These societies typically consist of unrelated individuals, wherein mutualistic benefits of grouping compensate for the intrinsic costs of cooperating with non-kin [6,27]. We investigated water conservation in a solitary bee (Melissodes tepdius timberlakei Cockerell, 1926) nesting in water-saturated soil. Communal nesting has not been described for M. tepidus , but is known in the genus [28]. We exposed single and artificially paired bees to a low-humidity stress assay to explore social impacts on water balance. These interactions between grouping and physiological stress resistance have implications for social evolutionary change in changing environments.