A honey bee on small flowers. Humoyun Mehridinov, CC BY-SA 4.0, via Wikimedia Commons.
Bumbling about visiting blooms, honey bees are the epitome of a peaceful summer's day. But the enigmatic insects are at the crux of an intense scientific debate. Do honey bees vary their metabolic rate when flying at different air temperatures? After all, insects are at the mercy of the temperature of their surroundings, which affects their metabolism and the amount of power their muscles can produce. But bees shiver to warm up their flight muscles before take-off, so maybe their metabolism isn't affected by the temperature of the air? Jordan Glass (University of Wyoming, USA) and Jon Harrison (Arizona State University, USA) explain that the evidence could point in either direction. Some researchers suspected that the industrious insects drop their metabolic rates in high temperatures, while others suggested that air temperature has no impact on the insects’ metabolic rates. With the jury out, Glass and Harrison decided to revisit the results of experiments dating back to 1980 to resolve the issue of whether honey bees alter their flight metabolism depending on the temperature of the air.
‘Extracting and interpreting metabolic and body temperature data from these older studies was tricky’, says Glass, explaining that some of the measurements had to be obtained from hand-drawn figures, and the bees had all been flown at different temperatures. In Bernd Heinrich's lab (University of Vermont, USA) in 1980, the bees flew at 20 and 42°C, while others in 2005 flew over temperatures ranging from 18 to 39°C. A study from Harrison's own lab in 2001 recorded the metabolic rate of winter bees – which have the lowest metabolic rates – flying at 24°C and, in an additional study in 2022, the bees flew in heliox – which has a lower density than air – forcing them to work their hardest to remain aloft. Glass also performed a new set of experiments, in which he monitored the metabolic rates of 160 bees flying at 20, 30 and 40°C. And, in each experimental series, dating back four decades, the respective researchers had measured the temperature of the insects’ flight muscles to record the temperature at which they were running.
Collating the newly measured metabolic rates and those from Heinrich's lab on a graph, Glass realised that instead of having no effect on the flying bee's metabolism, the air temperature had a definite impact. The bees were clearly working harder at lower air temperatures and made less effort as the air temperature rose. And when Glass reframed the bees’ exertions in the context of the temperature of their muscles, the reason for the disagreement over the impact of air temperature on the bee metabolic rates became apparent. The bees in Heinrich's 2005 study had flight muscle temperatures close to the optimal operating temperature (∼39°C), masking the effect that air temperature has on their metabolic rates.
But why do bees flying in cooler air put in more effort than bees flying in milder conditions? After all, hovering bees carrying no nectar or pollen have 20–30% more power left in the tank – to allow them to carry heavy loads – so they should not be affected by the air temperature. Glass suspects that the answer lies in the bees’ need to run their flight muscles at their optimal temperature. ‘The most obvious explanation is that bees beat their wings faster, but over a shallower angle, when flying at cooler air temperatures to warm their flight muscles to the optimal temperature’, says Glass, adding that he also recorded bees beating their wings more slowly on hot days to prevent their muscles from overheating.
Either way, honey bees adjust their metabolic rate as the temperature of their surroundings changes to ensure that their muscles run smoothly in heat waves and dismal summers.
