Zebra stripes are in for horses, black is a definite nay

The 2016 Ig Nobel prize for physics has gone to Susanne Åkesson of Lund University in Sweden and her colleagues for discovering why blood-sucking horseflies plague dark-coloured horses more than their white neigh-bours. The win means animal physics researchers have swept up this prestigious prize, awarded last week for research that makes people laugh then think, for two years in a row.

“We have found that you’d rather be a white horse than a black one if you like to avoid being bitten by horseflies,” said Åkesson at the awards ceremony at Harvard University, US, “but you can also dress either in stripes like a zebra, or like myself, in a dotted coat.” The webcast of the ceremony indicates that Åkesson remained unplagued by horseflies throughout.

The flies, which are also known as tabanids, detect horizontally polarized light to find their victims. And white coats, unlike the bodies of black or brown horses, reflect depolarised light that flies find hard to spot.

The prize-winning research was kick-started when biologist Åkesson invited physicist Gábor Horváth from Eötvös University in Hungary to join her for an expedition to the North Pole. There they investigated how birds might use the sky’s polarisation pattern to navigate. And they got talking about other types of biology. “We started to discuss … how animals, in particular water-seeking insects, may be attracted to hosts or may be fooled by artificially reflecting surfaces,” Åkesson told me in an email. “The tabanid flies are seeking both water and hosts from which they may extract blood for egg-laying, and they have developed the capacity to see polarized light. This cue is central for them.”

Like dark horse coats, water surfaces reflect horizontally polarised light – light made up of waves that vibrate in just one plane. Once a female horsefly has got a horse in her sights and fed on its blood she’s able to produce more eggs, which she lays near water so that her larvae fall into the liquid when they hatch.

Stripes or spots are even better than a white coat at preventing horsefly attack, the team discovered, probably because they “not only break [up] the…black surface reflecting linearly polarized light, but also introduce a camouflage pattern,” Åkesson says. “A white object will show a large contrast with the surroundings, which may be used to detect it.”

Horseflies aren’t just a nagging – and painful – annoyance, they also transmit diseases. So you can now buy a zebra-striped coat for your pet horse to wear whilst it’s out to graze. Stripes are definitely in this season. For horses, at least.

Flying in the face of fashion

Black, that perennial fashion favourite, has proved a problem for dragonflies as well as ponies. Horváth and his team also received the 2016 physics Ig Nobel for finding that certain dragonflies are drawn to shiny black tombstones. Like horseflies, these insects look for the horizontally polarized light reflected from water so they can lay their eggs nearby. But shiny black tombstones reflect horizontally polarised light too, making them so attractive to some Sympetrum dragonflies that they lay eggs there instead. This makes the tombstones an “ecological trap” for the insects because these graveyard eggs can’t succeed – they don’t have access to the water they need.

Horváth holds a Hungarian patent for an actual insect trap based on polarisation. The horsefly problem could be solved by a trick of the light.

So last season

In 2015, the physics Ig Nobel went to Patricia Yang, David Hu, Jonathan Pham and Jerome Choo for discovering that all mammals weighing more than 3 kg take 21 seconds (plus or minus 13 seconds) to empty their bladders. A larger animal has a bigger bladder that contains more liquid when it’s full. For an elephant, the figure is a colossal 18 litres. You might think it would take longer to get rid of that urine. But a bigger animal also has a longer urethra (the tube to its bladder), which means the pressure on the liquid in that pipe will be higher and it will flow out faster. Hu’s research into the energy it takes a dog to shake itself dry and how mosquitoes avoid death by raindrop collision both feature in Furry Logic: the Physics of Animal Life.

Sadly, in 2014, the Ig Nobel slipped through the paws of animal physics when Kiyoshi Mabuchi, Kensei Tanaka, Daichi Uchijima and Rina Sakai, won the physics prize for measuring the friction when a person steps on a banana skin.

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