Within each of our brains is a biological clock. Instead of ticking hands, we have the ebb and flow of hormones, urging us to sleep and to rise. Our internal clock closely follows the 24-hour period of the Earth's rotation, though it is not perfectly synchronous. Human clocks run slightly longer, slower by about 10 minutes on average. Exposure to light and dark realigns the clock with the day. These winding mechanisms are threaded into DNA — and their discovery won a Nobel Prize in October.
Most other organisms' circadian rhythms also center on a 24-hour cycle. Yet a team of biologists recently stumbled upon a wild deviation lurking in the woods. Three species of spiders, called trashline orb-weavers, have incredibly short internal clocks of just 17 to 19 hours.
“We’ve got the shortest naturally occurring period ever recorded,” Darrell Moore, a neurobiologist who studies invertebrates at East Tennessee State University, said during a presentation at the Society for Neuroscience conference in Washington this week. “There must be some ridiculous versatility in spider clocks that allows them to do this. It’s crazy.”
Exposure to light resets the spiders' clocks, as it does ours. But these arachnids go through a much more dramatic shift. Natalia Toporikova, a biology professor at Washington and Lee University in Virginia and a collaborator on the project, described them as “spiders without jet lag.” Put another way, it's as if daylight pushes them through five time zones. Each day. And everything's fine.
Moore, who studied honeybees for the bulk of his career, was investigating why certain orb-weaver spiders switch at different points during a 24-hour cycle from “scaredy cats” to “aggressive.” Orb-weavers spin circular webs, but the trashline spiders add a variation. They build a line of debris — corpses of dead bugs, feces, leaf litter — through the middle of their webs. During the day, they hide in the center of that line, motionless amid the garbage to avoid predators. They awaken after nightfall and become most active three to five hours before sunrise, repairing their webs and refreshing the trash.
The biologists caught wild spiders, took them to a laboratory and put them in small, clear tubes. At the ends of each tube were infrared sensors. Whenever the spiders moved through the sensors, they tripped; this way, the scientists could measure when the spiders were active or resting during a normal light-and-dark cycle.
After several days, the researchers slipped the spiders into weeks of continuous darkness. In total darkness, which is when most organisms revert to their default biological clocks, the trashline spiders revealed their unusually short periods.
Moore has early results for other spider species. A handful have very long internal periods, of up to 29 hours. And black widow spiders seem to have no built-in sense of time. Black widows' clocks are “arrhythmic,” he said, perhaps reflecting their fondness for keeping to the shadows.
A few lab-generated mutant rodents and flies have fast clocks, too, but none that beat this one. “We’re thinking, oh my God, what have we stumbled into?” Moore said.
It is clear that the three species are “extremely sensitive to light at a certain part of their circadian cycle,” he said. But it is unclear what biological mechanisms are resetting the spiders.
Toporikova, a bio-mathematician, has attempted to model the spiders' clocks, with only limited success. These must be constructed from scratch, she said. (The spiders' uncanny flexibility so impressed her, she said, that she is convinced spiders, not roaches, will conquer the world after a “nuclear holocaust.")
Very few invertebrate clocks have been examined in detail. “We’re only barely getting started,” Moore said. Do crickets have strange clocks? he wondered. Do tapeworms? “Someone's going to make a career out of this,” he said. Then he laughed. “It won't be me.”