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Pull of the Moon

September 1, 2006 By Helen Pearson This article courtesy of Nature News.

Tales of the Moon's effects on animal behaviour are not just moonshine.

While werewolves are the stuff of folklore, the light and the gravitational pull of the Moon do have real effects on the behaviour and physiology of a host of earthbound organisms including, perhaps, people.

One of the most obvious ways in which the Moon affects Earth is by pulling on the oceans and creating tides (the gravity of the Moon also creates a phenomenon called 'earth tides', a slight bulge in the Earth's crust that has been linked to volcanic eruptions). And organisms living in or beside the sea have adapted to those cycles.

Studies of fiddler crabs, for example, have shown that even when kept in the lab under constant light and temperature, the animals are still most active at the times that the tide would be out. A similar internal 'circalunar' clock is thought to tick inside many animals, running in synchrony with the Moon and tides, and working in conjunction with the animal's 24-hour circadian clock. This is thought to help animals anticipate tide movements; a skill that might give some creatures an edge. Ecologist Martin Wikelski of Princeton University, New Jersey, has found for example, that Galapagos marine iguanas with the most accurate circalunar clock are more likely to survive tough times, presumably because they are best at reaching feeding spots first1.

Moved by the moonlight

Moonlight can also change animal behaviour. Many marine organisms move up and down in the sea depending on the level of moonlight in order to keep their light levels constant. On land, some nocturnal animals come out on a well-lit night to hunt, others stay hidden to avoid predators.

And African dung beetles, oddly, can walk in a straighter line when the Moon is out: Eric Warrant at the University of Lund, Sweden, and his colleagues reported in 2003 that Scarabaeus zambesianus can detect the pattern of polarized moonlight in the night sky and use it to navigate2. This means they can roll their dung balls in a straight line on a moonlit night.

The effects of the Moon on humans, however, are far from clear.

Tossing and turning

Some healthworkers and police believe that they are busiest on nights when the Moon is full, and a few studies have backed this idea, suggesting that crimes, visits to the emergency room and births are highest at those times of the month.

One plausible reason is that people get less sleep on moonlit nights something that Martin Roosli and his colleagues at the University of Berne, Switzerland, found when they decided (as a joke) to test whether the length of time people sleep is affected by the phase of the Moon. To his surprise, Roosli found that people slept around 20 minutes less on nights with a full moon compared with a new moon. The team studied over 1,100 people over two and a half months3.

But other studies have found no such connections. Teresa Biermann of Friedrich-Alexander-Universität Erlangen-Nürnberg in Germany matched up police records with the lunar cycle between 1998 and 2003 and found no link between suicides, murders or assaults and the phase of the moon4.

Biermann says that earlier studies could have found spurious associations if they looked at small groups of people or over short periods. Over just a few months, the full moon might fall by chance on weekends, when crime rates are usually higher.

Monthly cycle

But what of the best documented of lunar rhythms the female menstrual cycle (the very term 'menses' comes from the Latin for month, mensis)?

There is no convincing evidence that the two are meaningfully connected, says John Palmer of the University of Massachusetts in Amherst. "So far it's best to think of it as a coincidence," he says. Other animals, after all, have menstrual cycles of length radically different from 28 days, and women's menstrual cycles also differ in length.

But the Moon's mysterious allure is likely to persist. There is little research into its effects on the behaviour of humans and other animals, compared with that on the day/night cycle, Wikelski says, although this type of study is growing. "It's disappointing, because it probably has a major influence," he says.

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  1. Wikilski M., Hau M. J. Biol. Rhythms, 10. 335 - 350 (1995).
  2. Dacke M., et al. Nature, 424. 33 (2003).
  3. Roosli M., et al. J. Sleep Res., 15. 149 - 153 (2006).
  4. Biermann T., et al. Chronobiol. Int., 22. 1137 - 1143 (2005).


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