Dazzling stripes are a way of deterring flies. Photo: Getty
Show Hide image

How the zebra got its stripes

A method for dodging predators? A means of social interaction? Or a way of getting rid of flies?

Zebras' stripes have baffled biologists since Charles Darwin. Many hypotheses have been proposed regarding their purpose but, despite hundreds of years of study, there remains disagreement.

In an attempt to end the debate, researchers have pitted various models against each other and systematically analysed data from past studies. Their results reveal the one reason zebras have stripes: to ward off flies.

A handful of ideas regarding zebras’ stripes have found some support among biologists. One proposed that the dark and light bands change how air flows around a zebra’s body and helps in heat management, which could go a long way in the hot tropical areas that zebras live in.

Another proposed the stripes were used by zebras as a way of social interaction. They may use them to identify other zebras and for bonding as a group in the wild.

A third proposal suggested zebras used the stripes as camouflage. While stripes are clearly visible in the day, there some thought that it helped at dawn, dusk, and in the night.

All these ideas were shot down when tested rigorously. Two others, however, remained intriguing.

The first was that the stripes were used to dodge predators. It is called the “motion dazzle hypothesis”, and it suggests predators are confused by zebras' stripes and cannot understand their movement. Research published in the journal Zoology in 2013 used a simulated visual system to show that zebra stripes do interfere with visual perception. But this is a difficult hypothesis to test in the field.

Martin Stevens at the University of Exeter has researched the motion dazzle hypothesis by getting human subjects to catch moving stripy objects on a computer. “It’s an artificial experimental system,” he admitted.

The second proposal was that stripes helped keep flies at bay. Zebras are especially susceptible to biting flies due to their geographic spread. These flies, which include the tsetse fly, stomoxys stable flies, and tabanid biting flies, are particularly prevalent in areas with high temperature and humidity – exactly the areas where zebras are normally found.

Bites from these flies can be nasty and, quite literally, draining. About thirty flies feeding for six hours on just one horse can draw as much as 100mL of blood. Usually the flies can number in the hundreds around one animal.

Zebras have shorter hair than other equids – the family that includes horses, donkeys and zebras – which may also increase their susceptibility to attack. Also, four diseases which are fatal to equids have been found in Africa. This could mean that investing in anti-biting defenses such as stripes is especially important for zebras compared to non-African equids.

It is possible that the dazzle effect acts on flies, rather than larger predators, and deter them from biting. “Stripes clearly have a number of functions,” Stevens said, “and these could be interacting in zebras.”

Revealing maps

In the new research, just published in Nature Communications, Tim Caro and his colleagues at the University of California in Davis, didn’t perform experiments. Instead they used ecological and observational data on zebras' geographical locations and related factors. It is the first time that a comparative approach has been applied to find the reasons for zebras' characteristic colouration. Caro thinks his findings may have nailed the answer at last.

Caro looked at seven species of equids and scored them for number and intensity of stripes. Just to be sure, they tested all five hypotheses regarding zebra stripes' use: camouflage, predator avoidance, heat management, social interaction, and warding off flies. The extent of overlap between the geographic distribution of striped equids with each of these five measures was calculated.


E. greyvi, E. burchelli and E. zebra have stripes on all their bodies. Other equids don’t. Caro, Izzo, Reiner, Walker and Stankowich

“The results were a shock to me,” said Caro. Of these five proposals, only warding off flies had statistical support. He had not expected such a clear-cut answer to the question. As the map shows, the only places where flies and equids live together are areas that are populated by striped equids.

The exact mechanism by which stripes deter flies remains unknown, but experimental studies performed by researchers at Lund University in 2012 have found support for this proposal. They created striped surfaces and stuck glue on them. Based on the number of flies on the surfaces with different thicknesses of stripes, they concluded that these flies stayed away from stripes as thin as those found on zebras.

“As is normal in science you get a solution that asks more questions,” Caro said. It is time to hand the problem over to vector biologists, who can understand the susceptibility of horses to biting flies.

In Darwin’s days, people didn’t consider animal colouration with respect to fitness advantages. “People thought that animal colouration existed simply to please humans or was caused directly by the environment,” Caro said.

Darwin “would be delighted” that researchers are now considering animal colouration as a functional trait, he said. We might not have all the answers regarding zebra stripes – but it seems we may be looking through the right lens.

The ConversationThis article was originally published on The Conversation. Read the original article.

Flickr: M.o.B 68 / New Statesman
Show Hide image

“I begged him to come home”: Breaking the taboo around texting the dead

Many people text dead loved ones to cope with their grief – but trouble arises when they get an unexpected reply. 

A month after Haley Silvestri’s dad died from a heart attack, she texted him begging him to come home. In the middle of the night Silvestri’s 14-year-old sister had found their father, with his lips and mouth blue, lying on the kitchen floor. “There was nothing there anymore, just a dead body,” Silvestri says. “My father had his first heart attack months before and seemed to be doing OK. Then, this happened.”

In the very first episode of CSI Miami’s seventh season, the protagonist – Horatio Caine – fakes his death. For the first 15 minutes of the episode, the viewer believes the character is truly dead, as the camera lingers on Horatio’s body face down on the tarmac.

Silvestri and her father used to enjoy watching the show together. After he had passed and she realised she would never see her “best friend” again, she picked up her phone. “I texted my dad begging him to come home,” she says. “I begged my dad to please be ‘pulling a Horatio’.”

"My heart was broken and I was bawling as I texted her over and over" 

In texting her father after he had died, Silvestri is by no means unusual. No official figures exist for the number of people who use technology to message their deceased loved ones, but Sara Lindsay, a professional counsellor, clinical supervisor, and trainer, says it is “more common than we think”.

“I see it as a modern and contemporary part of the grieving process,” she says. “I think in a way it's very similar to visiting a graveside, in that the bereaved are reaching out, particularly in the early days, because it takes a long time for people to process the reality that this person has now gone.”

Karlie Jensen, 18, texted her friend immediately after she found out she had died in a car accident. “I texted her as soon as I woke up to the news from my mom that she had passed. My heart was broken and I was bawling as I texted her over and over waiting for a text saying it wasn't her, that my mom didn't know all the facts, and maybe she was just hurt.” Jensen also called her friend and begged her to respond. “I did it because I couldn't let go and couldn't accept she was gone from my life forever,” she says. Karlie continued to text her friend while also calling her voicemail in order to hear the sound of her speaking again. 

Karlie (right) and her friend

After her first text to her deceased father, Silversti also began texting him once a week. She fell into depression, and on her worst days messaged the number. “I think it helped initially because it felt like I was personally writing a note to him, that I knew he only was gonna see,” she says. “I did it because it was my attempt at pretending he was still here and could text me back.”

Lindsay, who has over a decade’s experience of bereavement counselling, emphasises that this behaviour is in no way unhealthy. “I think on the whole it's a very healthy part of grieving, particularly in the first year where the bereaved faces agonising days without their loved ones,” she says. “There is just so much loss and change in their life that’s out of their control, I see this aspect of texting as a small way of being able to reach out and alleviate that pain. That person is suddenly now not there but how they feel about that person hasn't changed.”

"I was going through my phone and I saw his number – I wanted to delete it, but I hesitated I thought maybe I could send a text"

Despite being normal, however, using technology to talk to the dead is a behaviour we rarely – if ever – hear anything about. If the words “texting the dead” make it into the media, they are usually followed by a far more sensationalist “and then they text back!!!!”. Yet although messaging the deceased is popularly seen as the stuff of horror movies and trashy headlines, in reality it is simply a new, modern way to grieve.

Via Mirror.co.uk

“The first time I texted him I was on my bus on the way to school,” says now-20-year-old Dylan Campbell about his cousin Josh, who passed away from leukaemia. “I didn't have many friends so I had no one to talk to. I was going through my phone and I saw his number – I wanted to delete it, but I hesitated I thought maybe I could send a text and someone would reply or I would get something out of it.”

Campbell continued to send his cousin texts for a few weeks, “kind of like a diary”. He says he did so because he regretted not seeing Josh more up until his death, and “had a lot of things to say” that he’d never had the chance to. Linsday says texting in this way is a very healthy way of completing unfinished business. “There might have been something they've never said to their loved one that they want to be able to say and texting is a very normal place to do that.”

"Begging for a dead person to reply to you hurts since you won't ever get what you want in return"

Nonetheless, Lindsay notes that texting the dead can become unhealthy if grief becomes “stuck”, and the texting replaces normal communication or becomes a long term compulsion. Unlike Silvestri and Campbell, Jensen continued to text her friend in the hopes she would text back. She admits now that she was in denial about her death. “Begging for a dead person to reply to you hurts since you won't ever get what you want in return” she says. “I don't know if it helped trying to contact her or hurt worse because I knew I'd never get a reply. I wanted a reply.”

Quite frequently, however, this reply does come. After a few months – but sometimes in as little as 30 days – phone companies will reallocate a deceased person’s phone number. If someone is texting this number to “talk” to their dead loved one, this can be difficult for everyone involved.

“This story doesn't have a happy ending,” says Campbell. “After a few months someone from that number called me and yelled at me to stop bothering them – it was really heart breaking.” When Silvestri texted her father to wish him a happy birthday (“Saying I hoped he was having a great party up in heaven”) someone replied telling her to never text the number again. “I was pissed off,” she says. “Just block my number if it was that serious. This was a form of therapy I needed and it got taken away because someone couldn’t understand my hurt.”

Indeed, behind the sensationalist tabloid headlines of "texting back" is a more mundane - and cruel - reality of pranksters pretending to be the dead relatives come back to life.

"Visiting a grave is a clear recognition that the person visited does not exist in the normal day-to-day state of life, whereas texting allows for a suspension of that reality"

Silvestri, Jensen, and Campbell have never spoken to anyone else about the fact they texted their dead loved ones. Lindsay says that a fear of seeming “mad” combined with cultural phenomena – like the British stiff upper lip – might make people reluctant to speak about it. There is also a stigma around the way much of our modern technology is used in daily life, let alone in death.

This stigma often arises because of the newness of technology, but Christopher Moreman, a philosophy professor and expert on death and dying, emphasises that texting the dead is simply a modern iteration of many historical grieving practices – such as writing letters to the dead or talking to them at their graves. “I don't think the process of grieving is much changed, even if new modes of grieving come about due to new technologies,” he says. In fact, if anything, the differences between old and new ways of grieving can be positive.

“One important difference is in the sense of proximity,” explains Moreman. “I can text a loved one from anywhere in the world, but I can only visit their grave in one specific location. In another way, texting has the same structure whether I am texting someone who is alive or dead, so a sense of proximity also exists in the experience itself.

“Visiting a grave is a clear recognition that the person visited does not exist in the normal day-to-day state of life, whereas texting allows for a suspension of that reality. Some people may complain that new technologies allow us to ignore the reality of death, but there isn't any evidence that one way of grieving is more or less healthy than another.”

Amelia Tait is a technology and digital culture writer at the New Statesman.

0800 7318496