Neotropical stingless bees display a strong response in cold tolerance with changes in elevation

Abstract

Tropical pollinators are expected to experience substantial effects due to climate change, but aspects of their thermal biology remain largely unknown. We investigated the thermal tolerance of stingless honey-making bees, the most ecologically, economically and culturally important group of tropical pollinators. We assessed changes in the lower (CTMin) and upper (CTMax) critical thermal limits of 17 species (12 genera) at two elevations (200 and 1500 m) in the Colombian Andes. In addition, we examined the influence of body size (intertegular distance, ITD), hairiness (thoracic hair length) and coloration (lightness value) on bees’ thermal tolerance. Because stingless beekeepers often relocate their colonies across the altitudinal gradient, as an initial attempt to explore potential social responses to climatic variability, we also tracked for several weeks brood temperature and humidity in nests of three species at both elevations. We found that CTMin decreased with elevation while CTMax was similar between elevations. CTMin and CTMax increased (low cold tolerance and high heat tolerance) with increasing ITD, hair length and lightness value, but these relationships were weak and explained at most 10% of the variance. Neither CTMin nor CTMax displayed significant phylogenetic signal. Brood nest temperature tracked ambient diel variations more closely in the low-elevation site, but it was constant and higher at the high-elevation site. In contrast, brood nest humidity was uniform throughout the day regardless of elevation. The stronger response in CTMin, and a similar CTMax between elevations, follows a pattern of variation documented across a wide range of taxa that is commonly known as the Brett’s heat-invariant hypothesis. Our results indicate differential thermal sensitivities and potential thermal adaptations to local climate, which support ongoing conservation policies to restrict the long-distance relocations of colonies. They also shed light on how malleable nest thermoregulation can be across elevations.

Stingless bee foragers experience more thermally stressful microclimates and have wider thermal tolerance breadths than other worker subcastes

“…This does not follow the trend within colonies of tropical ants for largerbodied workers to have higher heat tolerance and lower cold tolerance (Cerdáand Retana, 1997;Baudier et al, 2018;Roeder et al, 2021). Our results more closely follow the lack of relationship between intraspecific size variation and thermal tolerance in bumblebees (Maebe et al, 2021) and honeybees (Sańchez-Echeverrıá et al, 2019;Barreiro et al, 2024), and are in-line with cross-species comparisons of tropical stingless bees that found no relationship between body size and either upper or lower thermal tolerance (Gonzalez et al, 2022c). However, we report that foragers did have a larger thermal tolerance breadth than hovering guards or standing guards.…”

“…Other studies in temperate climates have found that increasing body size is correlated with decreasing cold tolerance across and within several species (Peters et al, 2016), and a significant effect of increasing body size on increasing heat tolerance and decreasing cold tolerance across three Bombus species (Oyen et al, 2016). In a few examples of tropical bee species, one study reports larger body sizes across four Bombus species were correlated to lowered CT min (when looking within species, this trend was only maintained in one species) while there was no effect on CT max across species (Gonzalez et al, 2022a), and another study reports no strong relationship between size and thermal tolerance across stingless bee species (Gonzalez et al, 2022c). It is not yet clear whether there is a reliable size-tolerance relationship trend in bee species, and it is especially unclear whether any trends exist between polymorphic castes within genetically similar colonies of a single species.…”

“…In many eusocial insects, such as in stingless bees (Tribe: Meliponini), previous studies have used estimates of thermal tolerance to predict responses to climate change (Torres et al, 2007;Macıás-Macıás et al, 2011;da Silva et al, 2017;Hrncir et al, 2019;Souza-Junior et al, 2020;Maia-Silva et al, 2021;Gonzalez et al, 2022c), but division of labor is rarely taken into account. The threat of increasing climate variability to stingless bees is high and these insects are among the most diverse and abundant pollinators in the tropics (Hrncir et al, 2016;Quezada-Euań et al, 2018).…”