Moonstruck – Lunar Phases Influence All Sorts of Creatures

August 20, 2016

The eagle owls began to call at dusk, right around the time the full Moon started to rise in the east. Watching and listening were the ecologist Vincenzo Penteriani and his crew. They would be up all night, trying to learn more about the behaviour of these owls, Europe’s largest, with wings that could spread across a king-size bed.

On this particular full-moon night, Penteriani realised that there would be a lunar eclipse. As soon as the shadow of the Earth covered the Moon, the owls fell silent.

Scientists had long thought that nocturnal animals primarily used sound to communicate. But several years ago, Penteriani caught sight of a male owl through his binoculars, just after sunset. With each call, a previously hidden patch of white feathers flashed along the owl’s throat: a visual cue, the researcher later found, that was key to the species’ mating strategy. And on nights of brightest moonlight, these calls and flashes increase.

Nearly a third of vertebrates, and more than 60 per cent of invertebrates, are active at night; the full moon is the brightest object for most of their waking hours. About two weeks after the Moon’s brightest phase, the new moon will rise at sunrise and set with the Sun, leaving the entire night starlit. The whole cycle runs about 29.5 days from new moon to new moon, and ecologists such as Penteriani, who works at the Doñana biological station in Andalusia, want to know how this shifting pattern of lunar light and darkness shapes animals’ lives.

In our world of street lights and headlamps and blinding motion sensors, we – at least, I – no longer rely on the Moon. Often, I don’t even remember that it’s up there: it seems to surprise me appearing from behind a tree, its light a shock when I wake from a dream.

Yet the Moon has always been in our stories, whether nursery rhymes or darker tales. Clouds unfurl from the face of the full moon and a man becomes a werewolf. Vampires lurk, magic potions burble, bats take wing. Given these tales of night terrors, we might think that predators prefer the full moon’s light to stalk their prey. But when the ecologists Laura Prugh (University of Alaska Fairbanks) and Christopher Golden (Harvard) surveyed the behaviour of 59 nocturnal mammals, they found that most carnivores and insectivores became less active under the brightest moon phases. Primates seemed to be the only group that was consistently more active under the full moon.

Without electric lighting, that applies to us, too. A few centuries ago, anyone who wanted to move about at night depended on the phase of the Moon. By the 17th century, those in cities relied on almanacs to plan their night-time journeys, writes the historian A Roger Ekirch in At Day’s Close: Night in Times Past (2005). In autumn, farmers used the bounty of light from the harvest moon to reap the season’s crops late into the night. Thieves and other human predators sometimes refused to work nights when the ‘tattler’ in the sky might give them away.

For millennia, humans have wondered if the Moon pulls at us with something more than its reflected light. Pliny the Elder observed that, in combination with the Sun, the Moon draws up the tides. Pliny and his compatriots also argued that this lunar force – now known as gravity – pulls on the moisture in our bodies, within our watery brains. The full moon’s drag was thought to cause everything from epilepsy to ‘lunacy’. These days, a few studies have linked human health and behaviour with the full moon, but more have refuted the connection – and gravity’s effect on us is known to be so tiny that it wouldn’t sway our aquatic inner reaches, even during the tidal extremes known as spring tides, around the new and full moons, when the Sun and Moon both exert their gravitational pull in concert.

But it’s hard to shake the feeling that something which looms so large in our night sky, and in the firmament of our imaginations, could be a simple satellite – particularly when its shifting light shapes the lives of so many of our fellow travellers, the creatures of the night.

Although we might be drawn to the showy shine of a full moon, the vanishing of this light as it starts to wane could be just as powerful. At the full moon, the Earth stands between the Moon and the Sun, and the view of our long-time companion is that of a brightly lit coin. During the following nights, as the Moon circles back toward the Sun, that coin slowly shrinks, yet the sky seems much darker than just the dwindling light would allow. And it is: the Moon rises about 50 minutes later each evening, carving a channel of darkness between the Sun dropping below the horizon and the Moon appearing.

It’s this channel that can be travelled by predators – and might have created our lingering fears about the full moon. In Tanzania, the sun sets by 7pm throughout the year. But people still cook outdoors, greet neighbours, walk to get water, often well after nightfall.

The lions here are less likely to hunt under a full moon. But as that moon wanes and the evenings darken, these hungrier lions might encounter not just their traditional prey but people still out in the open. Researchers have found that lion attacks on humans were most likely in the 10 days after the full moon.

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