Senior bees up all night caring for larvae.

A bleary mom staggering into the nursery at 2 a.m. can tell her troubles to the honeybees.

Foraging worker bees are the first insects known to have a social trigger radically change their biological rhythm, report Guy Bloch and Gene Robinson of the University of Illinois at Urbana-Champaign. That trigger comes from a crisis in the nursery.

Newly hatched honeybee larvae need round-the-clock care, and they typically get it from the youngest adults. Each caretaker fusses around the brood at all hours.

As the nursemaids age, they start venturing outside the hive. Eventually, they switch to full-time foraging and adopt a circadian rhythm. Even when researchers keep bees in constant darkness, the forager-age workers show a regular cycle of activity and stillness spanning 22 to 25 hours.

However, these bees can lose the rhythm under special conditions. If the hive falls short of nursemaids, some of the older bees return to the nursery. Bloch and Robinson moved members of three bee colonies into observation hives. For each hive, the researchers gathered a queen, some 2,000 foragers, and a very young brood, but left out young nursemaids.

In a dramatic display of flexibility, several hundred older bees stopped foraging, lost their circadian rhythms, and pitched in to assume arrhythmic infant care, Bloch and Robinson report in the April 24 NATURE. Such a developmental turnabout "suggests really amazing plasticity," marvels Bloch.

"I find his work really exciting," says neurobiologist Joan C. Hendricks of the University of Pennsylvania in Philadelphia, one of the researchers who last year documented a sleeplike state in the fruit fly Drosophila (SN: 2/19/00, p. 117). "What's powerful is that he was looking at a change in behavior so profound from a seminaturalistic stimulus," removal of nursemaids, says Hendricks. She muses that the bees may offer a prime lesson on how circadian rhythms benefit an organism.

Fly neuroscientist Amita Sehgal, also of Penn, welcomes the new bee study as a unique demonstration of the power of social cues in biological clocks. She points out that Drosophila become arrhythmic in response to constant light and revert to a circadian cycle when light and darkness alternate again.

Could the honeybee sleep clocks switch off without leaving the bees groggy? wonders Paul J. Shaw of the Neurosciences Institute in La Jolla, Calif. Independently of the Penn team, Shaw and his colleagues last year discovered the Drosophila version of sleep.

About "1,400 to 1,500 people fall asleep and die on the road each year," Shaw points out. Yet in the honeybee experiments, bees don't seem to act jet-lagged.

The bees' reversion to the nursemaid role involves much more than sleep, notes Eric Erickson, director of the Carl Hayden Bee Research Center in Tucson. For example, the older bees' secretory glands reactivate. "To make a really rough analogy, bee larvae are fed on 'breast milk,'" Erickson explains. In the experiment, "it's a little like an older woman having babies again," he says.