Intraspecific variation in non‐flight thermogenesis warming patterns in bumble bees: Effects of social caste, season and body mass

Abstract Bumble bees' ( Bombus spp.) thoracic muscles can generate substantial heat through non‐flight thermogenesis (NFT) to mediate flight under cool conditions and incubate brood. Bumble bee castes vary tremendously in life history, physiology and body size, making them an excellent system to investigate sources of intraspecific variation in NFT. This study measured trends in NFT‐mediated rewarming across castes (queens, workers and males) of a bumble bee species ( Bombus impatiens ), while investigating the effects of body mass and seasonality. We used a thermal imaging camera to quantify changes in thoracic temperature as bees recovered from chill coma and used generalized additive models to compare rewarming curves across groups. All bees showed two distinct phases of rewarming, which we suggest represent passive and active warming, driven by biophysical heat exchange and physiological thermogenesis, respectively. While all castes reached similar equilibrium temperatures, each showed significantly different rewarming patterns. Mass effects were most pronounced for males, with small males (<0.15 g) failing to achieve above‐ambient body temperatures. In addition, fall queens reached higher thoracic temperatures than spring queens, suggesting NFT may also vary seasonally. The adaptive significance of this variation requires further exploration, but our data demonstrate heretofore unknown individual variation and size‐related constraints in thermal physiology in bumble bees.