Richard Collins: Moonlight assists night-time hunting success
People, long ago, thought that the sun was a god. They weren’t far wrong; all living things around them depended on the great fiery ball in the sky, so bright you couldn’t even look at it directly. However, sacrificing animals or people, as the Aztecs did, to keep the sun happy, was a bit ‘over the top’.
The moon, controller of the tides, was a lesser deity. Her role as fertility goddess was prophetic; the ancients could not have known it, but she regulates the breeding cycles of many marine creatures.
A full moon on a November night, for example, triggers the explosive release of a quadrillion sperms and eggs along the Great Barrier Reef in a frenetic orgy of coral reproduction.
The moon has little influence on land-based creatures but there are some notable exceptions. Like all deities, she works in mysterious ways.
Do nightjars, those weird birds which ‘churr’ like spinning-wheels in the night, take their cue from the moon when starting to nest? Birds time the fledging of their young so that the food they will need is most available. The light of a full moon helps nightjar fledglings hunt insects at this critical stage of their lives.
To capitalise on the light, a nightjar must begin laying eggs about five weeks in advance of a full moon. But there’s a catch; the lunar cycle is independent of the seasons. In some years, the full moon occurs late in the month and nightjars, facing a journey to Sub-Saharan Africa where they will spend the winter, may not have time to raise a second brood.
Luis San-Jose and colleagues from the University of Lausanne, in a paper just published, discuss another possible influence of the silver goddess.
The barn owl, one of the world’s most widely distributed birds, is found on every continent except Antarctica. There are almost 30 races worldwide.
Although Irish barn owls have creamy-white wings, their faces and bodies are pale. In other parts of the species’ range, barn owls have darker reddish-brown bodies.
Our barn owls are almost unique among night predators in being mainly white. Stealth-and-ambush nocturnal carnivores rely on invisibility when hunting. The barn owl’s pale appearance must surely impair its chances of hunting successfully. If ghosts weren’t white, nobody would see them.
San-Jose investigated the effects of moonlight on the owls’ hunting-success. Using high-resolution GPS tracking, and varying the types of prey item, his team found that white barn owls are actually more successful than red ones in providing for their offspring during bright moonlit nights.
This seems an odd result, since white plumage should render the owls more conspicuous to their prey.
Barn owls target mice and rats, creatures which are averse to light. Rodents tend to freeze motionless under bright light.
San-Jose argues that ‘under full-moon conditions, white plumage triggers longer freezing times in prey, which should facilitate prey catchability’.
The study, its authors say, ‘provides evidence for the long-suspected influence of the moon on the evolution of colouration in nocturnal species’. They are modest in their claims however; ‘whether moonlight shapes the fitness of nocturnal species with distinct colour variants remains unknown’, they concede.
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