— A bumblebee hive's thermostat is controlled by a strict division of labour, a new study has revealed.
Bumblebees warm and cool their hives to keep the brood of the next generation at just the right temperature between 28°C and 32°C. When temperatures get too hot, they cool them by fanning their wings. And when it gets chilly, the bees vibrate their wing muscles to shunt heat down to their abdomen, which they hold up against larvae-containing comb. Watch a video of two incubator bees warming a brood cell (centre of the frame) while other workers move about around them. The bees were tagged for the purpose of the experiment.
"You can see them shiver to transfer the heat," says Sean O'Donnell of the University of Washington in the US.
O'Donnell and his colleagues filmed three Bombus huntii colonies. The team artificially changed the temperature, pushing it up to 38.6°C and down as low as 10.3°C. When temperatures got too hot, the shivering, incubating bees backed away from the brood to leave fanning bees to cool the larvae.
Conversely, the incubating bees compensated for drops in temperature by increasing the time they spent incubating.
The researchers then removed most of the incubating bees. Contrary to expectations, the fanning bees did not switch to incubating. Instead, within 24 hours of their colleagues being removed, the remaining incubating bees took over the temperature regulation on their own by increasing the time they spent shivering heat down onto the brood.
"Task switching was previously thought to be common among bumblebee workers," says O'Donnell. "But this study indicates that there is strong specialisation in labour among individuals."
A bee's body size seems to play a role in determining whether it becomes an incubator or a fanner.
"We expected that larger workers would be incubators, but we found to our surprise the opposite was true," says O'Donnell said. "We don't know whether the smaller bees are really better at warming the nest, or whether the larger bees avoid incubating for other reasons. In general, larger bumblebee workers are foragers for food so they could be committed to that task."
Journal reference: Behavioral Ecology and Sociobiology (DOI: 10.1007/s00265-006-0309-7)