Desiccation limits recruitment in the pleometrotic desert seed‐harvester ant Veromessor pergandei

Abstract

The desert harvester ant Veromessor pergandei displays geographic variation in colony founding with queens initiating nests singly (haplometrosis) or in groups (pleometrosis). The transition from haplo-to pleometrotic founding is associated with lower rainfall. Numerous hypotheses have been proposed to explain the evolution of cooperative founding in this species, but the ultimate explanation remains unanswered.In laboratory experiments, water level was positively associated with survival, condition, and brood production by single queens. Queen survival also was positively influenced by water level and queen number in a two-factor experiment. Water level also was a significant effect for three measures of queen condition, but queen number was not significant for any measure. Foundress queens excavated after two weeks of desiccating conditions were dehydrated compared to alate queens captured from their natal colony, indicating that desiccation can be a source of queen mortality.Long-term monitoring in central Arizona, USA, documented that recruitment only occurred in four of 20 years. A discriminant analysis using rainfall as a predictor of recruitment correctly predicted recruitment in 17 of 20 years for total rainfall from January to June (the period for mating flights and establishment) and in 19 of 20 years for early plus late rainfall (January-March and April-June, respectively), often with a posterior probability > 0.90. Moreover, recruitment occurred only in years in which both early and late rainfall exceeded the long-term mean. This result also was supported by the discriminant analysis predicting no recruitment when long-term mean early and late rainfall were included as ungrouped periods. These data suggest that pleometrosis in V. pergandei evolved to enhance colony survival in areas with harsh abiotic (desiccating) conditions, facilitating colonization of habitats in which solitary queens could not establish even in wet years. This favorable-year hypothesis supports enhanced worker production as the primary advantage of pleometrosis.

Ecological Drivers of Non-kin Cooperation in the Hymenoptera

“…In desert ants, group founding may be a by-product of the tendency to seek refuge from desiccating conditions in shared belowground spaces (Pfennig, 1995). Under desiccating conditions, groupfounding by the desert seed-harvester ant Veromessor pergandei enhanced queen survival and water content relative to solitary queens, though the mechanism for this advantage is unclear (Johnson, 2021). Shared foraging duties could feasibly reduce risk of desiccation in desert habitats.…”

Tree canopy cover and elevation affect the distribution of red harvester ant nests in a peri-urban setting

“…However, foraging distance from the nest can be directly impacted by surrounding impervious surfaces. Fewer available food sources near sidewalks or parking lots may force foragers to travel farther, increasing individual exposure to heat and dehydration (Friedman et al 2019, Johnson 2021, Stache et al 2022). Extreme weather events – such as droughts and flash floods – are commonplace in the LRGV range of red harvester ants, meaning that each colony will have to determine nest placement to overcome both issues (Friedman et al 2019, Sundaram et al 2022).…”

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].…”