(Jerusalem, October 3, 2019)--It's a phenomenon that most new parents can identify with: chronic exhaustion due to lack of sleep. As it turns out, humans are not alone in this: all animals, including insects, need their sleep. Or do they? That's the question Professor Guy Bloch and his team at Hebrew University of Jerusalem (HUJI)'s Department of Ecology, Evolution & Behavior are looking to answer in the journal Current Biology. They studied brood-tending bumble bee workers and found that those tending the young sleep much less than other bees do, even when caring for offspring that aren't their own.
"Our findings show that sleep is more plastic and less rigid than is commonly accepted," shared Bloch. Insect sleep looks much like sleep in people and other animals. They stop moving, take on a typical sleep posture, and become less responsive to noise or touch. When humans, rodents, or flies are sleep deprived, it compromises their health and performance. But the new study suggests there may be ways around that in some cases.
Bloch and colleagues had earlier shown that bees adjust their activities depending on their role in the colony, with foragers showing a strong circadian rhythm and "nurse" bees tending the brood around the clock. They wondered how activity around the clock affects the way they sleep. To find out, the researchers, spearheaded by postdoctoral fellow Dr. Moshe Nagari, combined video recordings, detailed behavioral analyses, sleep deprivation experiments, and response threshold assessments to characterize the sleep behavior of bumble bee workers.
Their studies show that bumble bees tending young do indeed sleep much less. That's true even when the brood doesn't need to be fed and when the young are not their own offspring. The evidence suggests that substances produced by the pupae drive the reduction in sleep. Surprisingly, however, when the pupae and their substances were removed, those bees did not show the expected sleep rebound. It suggested that they weren't sleep deprived in the expected way.
"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," Nagari says. "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."
The findings add to emerging evidence showing that under certain conditions, animals can give up sleep, the researchers said. For example, they noted that birds sleep less during their seasonal migrations. Some male birds and fruit flies will forgo sleep to give themselves more time to mate. And some cavefish have evolved to sleep less compared with related species of fish that live in open water habitats.
The findings in bees raise questions about whether the sleep loss comes at a cost in terms of health or cognitive performance.
In the past, sleep research focused on the need for sleep and on the damage to small laboratory animals, such as mice and rats, by a lack of sleep. "However, with bees, if there is no cost for sleep loss, it means that the brood-tending bees have a mechanism that allows them to significantly reduce sleep without a cost to their brain or other tissue," Bloch explained. "This, of courses, raises the question about what exactly are these mechanisms and what is the basic function of sleep."
Prof. Bloch says his current research is the first proof that insects change their sleep schedules to care for offspring but adds that now that the precedent has been established he would not be surprised to find similar phenomena with other social insects, such as wasps and ants.
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