Scarlet macaws, peacocks, dart frogs and rainbow fish: the animal kingdom is full of vibrant colors.
But some animals stand out for another reason: the total absence of color in their coat.
Black and white animals are found in every region of the world, from the forests of China to the savannahs of Africa.
But even though the coloring is similar, there are many possible reasons why this happens.
Insect repellent
According to a study from the University of Bristol, England, black and white stripes can prevent insects from biting zebras.
Stripes can help protect zebras from horsefly bites.
“There just doesn’t seem to be a very logical or obvious reason why they are so striped,” said Martin How, associate professor of visual ecology, animal vision expert and study author at the University of Bristol.
“They’re not trying to camouflage themselves in a striped environment…the function of those stripes has always fascinated me.”
Female horseflies bite land vertebrates to feed on blood – and researchers think this could be the key to understanding zebra fur.
“These are really horrible flies, even when they’re not transmitting disease, but if they’re infected and you’re bitten by one of them on the plains of Africa, it could be a matter of life and death,” How said.
“Mosquitoes tend to use their sense of smell more than their vision to find hosts…but in the case of horseflies, they rely heavily on their vision.”
Researchers hypothesize that the black and white stripes prevent horseflies from biting zebras.
To test this, they tracked the flight of horseflies to horses wearing gray blankets and compared them to horses covered in black and white striped blankets.
How do they say that the horseflies landed on the gray blankets. But when the horses wore blankets designed to mimic a zebra’s stripes, the horseflies ran sideways and missed the animal completely. “You can almost see the insect’s decision process: It walks over and thinks, ‘Wait, that’s not what I want,’ and moves on,” How said.
He said researchers performed similar tests with blankets of different patterns. “We found that just about any black-and-white pattern we used, as long as it was about the same scale as the zebra stripes, was very effective at deterring flies,” he said.
According to How, flies’ vision has “terrible resolution,” which could explain the confusion. “When they come from a distance, all they see is a gray blur; when they get about two meters away, the resolution of their eyes starts to pick up the black and white patterns,” he explained. “It stops being a gray object and becomes something with a pattern, and I think that ends up confusing the flies.”
Camouflage
Although black and white coloring is not considered useful for camouflaging zebras, it can help other animals, such as pandas, whose fur can help them hide from predators such as tigers, leopards and dholes.
“In the forests of western China, at certain times of the year, there are black and white patches of snow, rocks and tree trunks,” said animal coloring expert Professor Tim Caro, also of the University of Bristol.
“And if a slow-moving animal like the giant panda is seen 50 or 100 meters away, it is very difficult to distinguish it as an animal from the snowy, rocky background on which it appears.”
Experts say camouflage may also be responsible for the coloring of Gentoo penguins, a species with black backs and wings, but white bellies.
“When the animals are viewed from below, their pale bellies blend into the clear sky,” said Hannah Rowland, a senior lecturer in the Department of Evolution, Ecology and Behavior at the University of Liverpool in northern England.
“When viewed from above against a dark background, especially in water, they blend into that dark background.”
Alert
Another reason some animals are black and white may be the same reason poison dart frogs have such vibrant colors: to serve as a warning.
According to Professor Caro, opossums are capable of releasing anal secretions with an “extremely strong” odor, in addition to being dangerous.
“They tell predators, ‘Look, I’m black and white, you don’t want to try to attack me because I’m too dangerous,'” he says.
He says this theory is supported by the fact that opossums tend to have more prominent stripes in areas where there are many predators.
Social signaling
According to Professor Caro, eye-catching black and white markings can also be a way of signaling to other animals and encouraging group cohesion, as happens for example with ring-tailed lemurs.
The animals themselves are gray, but they have black and white striped tails, which they hold high in the air as they walk.
“We suspect that because they carry their tails vertically when moving in groups, it acts as a marker to say, ‘I’m here, please follow me,'” he said.
There are other examples of this in nature, such as the white spots on the back of tigers’ ears.
“A minority of these signals in mammals are likely related to group cohesion,” he said.
The answer is not simple
There are other theories about black and white coloring.
In the case of Gentoo penguins, for example, in addition to possible camouflage, the black feathers on their backs are rich in a pigment called melanin, which makes the feathers more resistant to environmental damage, according to Rowland.
Different colors can also help regulate body temperature.
“Dark surfaces absorb heat much more easily and heat up much more quickly than white surfaces,” she explained.
“(Penguins) generally turn their backs to the sun or orient their bellies toward the light when the temperature is much higher.”
But the truth is, we’re not sure.
“There’s a very complex combination of factors,” Rowland said.
“Ultimately, science almost never has black and white answers – pun intended.”
Based on an episode of audio program CrowdScience, narrated in English, from the BBC World Service.
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