It’s not only human mums who give up precious sleep to care for their newborns. Nursing bees also sacrifice sleep to look after their young – and even offspring that aren’t their own.
That’s one intriguing finding by Guy Bloch’s team from the Hebrew University of Jerusalem. He says it’s “noteworthy in light of Darwin’s theory of evolution”, suggesting that “the remarkable sleep plasticity that we discovered was shaped by the evolution of sociality rather than by individual selection”.
Adding to this, the brood-tending bees gave up sleep to care for pupae that didn’t need to be fed, along with larvae that did.
“The fact that the nursing bees sleep so little, even when caring for pupae that do not need to be fed was the most surprising,” says first author Moshe Nagari.
“Before this study, we assumed that the main functions of activity around the clock without circadian rhythms in nurse bees is to provide improved feeding to the developing larvae, enabling them to grow rapidly.”
Cocoons from which the researchers removed the pupae had a similar effect, but it faded over time. Bloch says this suggests that some substances, like pheromones, that were left on the cocoon were potent enough to trigger the tending bees to continue giving up their sleep.
Also surprisingly, the bumble bee workers (Bombus terrestris) under investigation did not sleep more after their brood was removed, which would be expected if they were sleep deprived.
Sleep deprivation in humans, rodents or flies impacts their health and performance. But the bee study raises questions about the physical and psychological needs for sleep.
“If there is no cost for sleep loss, it means that the brood-tending bees have mechanisms allowing them to significantly reduce sleep without a cost to the brain and other tissue,” says Bloch.
But the purpose of sleep is still a mystery, says Bloch. “We spend much of our time asleep, but still we do not know its biological function.”
Previously, the team had shown that bumble bees adjust their activities depending on their role in the colony. They established that their immobility periods showed all the behavioural signs of sleeping. And while worker bees showed strong circadian rhythms, nurse bees looked after their brood around the clock.
To investigate this, they combined information from video recordings, behavioural analysis, sleep-deprivation experiments and response-threshold tests to explore the effects of brood care on sleep.
Overall, the results are in line with emerging evidence that animals can give up sleep. For instance, the researchers note, birds sleep less during seasonal migrations while some male birds and fruit flies will stay awake to mate, and some cavefish have evolved to sleep less than related species in open water habitats.
But the findings also raise questions about the basic function and evolution of sleep, and mechanisms by which the nurse bees managed to deprive themselves without needing to catch up, says Bloch.
The study was published in the journal Current Biology.