Ecological Drivers of Non-kin Cooperation in the Hymenoptera

Abstract

Despite the prominence of kin selection as a framework for understanding the evolution of sociality, many animal groups are comprised of unrelated individuals. These non-kin systems provide valuable models that can illuminate drivers of social evolution beyond indirect fitness benefits. Within the Hymenoptera, whose highly related eusocial groups have long been cornerstones of kin selection theory, groups may form even when indirect fitness benefits for helpers are low or absent. These non-kin groups are widespread and abundant, yet have received relatively little attention. We review the diversity and organization of non-kin sociality across the Hymenoptera, particularly among the communal bees and polygynous ants and wasps. Further, we discuss common drivers of sociality across these groups, with a particular focus on ecological factors. Ecological contexts that favor non-kin sociality include those dominated by resource scarcity or competition, climatic stressors, predation and parasitism, and/or physiological constraints associated with reproduction and resource exploitation. Finally, we situate Hymenopteran non-kin sociality within a broader biological context by extending insights from these systems across diverse taxa, especially the social vertebrates. Non-kin social groups thus provide unique demonstrations of the importance of ecological factors in mediating the evolutionary transition from solitary to group living.

How does climate change impact social bees and bee sociality?

“…Social organization in bees takes a diversity of forms, and includes several independent origins of (and reversals from) eusociality (Danforth et al, 2003;Kocher and Paxton, 2014). Among and within taxa, bee societies vary considerably in group size, in the degree of reproductive skew and morphological specialization, and in the genetic relatedness of group members (Michener, 1974;Ostwald et al, 2022b;Wcislo and Fewell, 2017). Particularly instructive for social evolutionary studies are the many lineages in which social living is a facultative state, which may be determined by environmental conditions.…”

Social conditions facilitate water conservation in a solitary bee

“…Temporary grouping is known to facilitate water conservation in insects, particularly during seasonal dry periods or vulnerable life history stages [2][3][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.…”

“…Complex social organization can generate novel strategies for regulating nest humidity and collective water balance, particularly among the eusocial insects [12][13][14][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][17][18][19][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].…”

“…Communal nesting strategies are often present at low levels in otherwise solitary populations [24][25][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.…”