A desert moonrise. Wasn’t that a Fleetwood Mac album? (Pixabay)
Everyone knows about hibernation, where animals go into a state of deep sleep during the meagre winter months, before emerging in the spring to enjoy the relative time of plenty in the warmer weather. Hey, sounds like a plan to me.
Hibernation’s little-known cousin is estivation, where the opposite is true: animals go into a deep sleep during the summer months, before awakening in the autumn to live it up during the winter. Boy, they must really hate tourists.
But a study in 2017 found that hibernation and aestivation might have another curious cousin to add to their list: lunation is the practice of falling asleep for just a few days each and every lunar month. Though quite why any creature might want to do that is—well, a bit of a mystery.
A team of Oxford University biologists working in southern Africa in the late 2010s discovered an entirely new species of rodent living among the sand dunes of the Namib Desert. They named the creature the ‘great grey pocket-gerbil’ and, excited to have added an entirely new creature to the world’s list of mammals, promptly set about recording and describing its behaviour.
After five months’ study, however, the team began to notice something quite unusual. Roughly every four weeks, the gerbils would routinely find an isolated burrow or hole in an area of scrubland, retreat into it—sometimes in pairs or groups, but typically alone—and not emerge for another three to five days.
Initially, the team presumed that these bizarre rodents might be partly hole-dwelling, and likely had networks of tunnels beneath the desert floor where food could be stored and families could be reared away from predators and the heat of the desert sun. But by capturing and tagging four of the gerbils, the team found that when they retreated below ground the gerbils simply—well, stopped.
There were no underground food stores, nor underground crèches. There were no grand networks of tunnels. Instead, for three or four days each month the gerbils simply retreated into a deep sleep, slowing their heart rate and breathing rate to conserve energy, before waking up and returning to the surface as if nothing had happened a little under a week later. And a month or so after that, the entire process would happen again.
The behaviour amazed and confounded the scientists—and still today remains largely unexplained.
One clue that might help to explain the gerbils’ curious lifestyle, however, is that they tend to coincide their underground holidays with the monthly new moon. Could it be that the relative darkness of a new moon sky makes the gerbils feel unsafe, forcing them to retreat below ground? Could it be that they use the moon to navigate or to get their bearings, and without it in the sky they struggle to find their way around? Or perhaps the timing of the gerbils’ disappearance is entirely unrelated to the phases of the moon, and there’s actually something physiological going on? Do they retreat below ground to conserve energy? Develop fat reserves? Or digest nutritional stores in their bodies?
No one entirely knows. But this process—now known as lunation—seems completely unique to these giant Namibian gerbils. Though a couple of days in bed every do often never did anyone any harm, of course.