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.