23 April 2018
Why do zebras have stripes?

By Chad Cocking

Africa is blessed with a host of charismatic mega-fauna that call this ancient continent their home; elephants, lions, giraffes and hippos are amongst the most globally recognised of these, but also the most sought after by tourists visiting the Timbavati for the first time. Yet, there is one other animal that might not immediately jump to mind, but as soon as guests at Tanda Tula Safari Camp get to set their eyes on them, their smiles explode, and the cameras start clicking away; this is none other than the zebra.

Zebras are animals that stand out for two main reasons; one being that their much-loved cousins - the horses - are distributed across the world and have become a part of our modern-day domestic animal landscape. Secondly, and no doubt more importantly, for their universal recognition. The fact that zebras are coated in the most unusual black and white striped pattern makes them stand out from the rest of the animal world in a way that few other animals are able to achieve - and this makes them totally memorable. Their stripes make them so recognisable that it is almost inconceivable to think that any safari guide, when approaching a dazzle of zebras (and yes, that is what we call a group of zebras), has ever been asked: what animal is that?

A much more frequently asked question pertains to the presence of these very distinctive stripes, why do zebras have stripes? By coating these animals in such bold, seemingly obvious colours, they appear to make themselves stand out from their environment far more readily than their other four-legged counterparts. So, there must be a pretty good reason for having gone to such extremes of coat colouration. Although the question might be simple, the answer is anything but! Many theories have been put forward, but each one is open to debate.

One of the longest standing theories over the origin of zebra stripes has been that they evolved as a form of camouflage, owing largely to the fact that the predators that prey on them (notably lions and hyenas) do not see in full colour, but rather in very muted pastel-shades. This may make it more difficult to pick zebras out of their environment, as the black and white stripes blend into a grey colour at a distance.  Some studies suggest that this may be as little as 50m during the daytime to only 9m in starry conditions. A strong argument against this is that zebras behaviour does nothing to help disguise them, they are very noisy species that have a partiality for flatulence!

Another oft-proposed theory is that, being such social, herd-dwelling mammals, the stripes of a zebra may help to camouflage individual animals within the herd. It is believed that the dazzle of stripes seen when a herd is on the move may create some form of optical illusion whereby would-be predators have a difficult task in determining the distance, speed and even the direction of movement of a zebra herd. It may also make it difficult for predators to single out one zebra from the mass of stripes of the herd. Counter arguments against this theory range from the fact that, in practice, lions don’t appear to be any more startled or confused by zebra’s fleeing reactions than they do when compared to any other herd-dwelling animals. Additionally, if this was the sole purpose of the stripes, then why is there such geographic variation in the stripe patterns of zebras across Africa, even within one species such as the Burchelle’s zebra (Equus burchelli)?

The latter argument could equally be used against yet another similarly aligned theory which states that the stripes of a zebra offer the ultimate following mechanism. In most large African mammals, one will notice some form of contrasting colouration on their rear ends (the white tails of kudus, the black tip of a lions tail, the white ring on a waterbuck’s behind), these are believed to make the individual animal more visible to its herd members and offspring when they are running away from danger.

Zebras have a unique approach when fleeing from danger, the entire harem will gather into a cluster with the foals in the middle, the mares on the outside and the stallion bringing up the rear, kicking and biting at predators that get too close. In order to stay in this tight group and maintain its effectiveness, a zebra needs to remain clearly visible to it's fellow herd members and what better contrasting patterns to assist with this than coating their entire body in black and white stripes? Perhaps this unique manner of predator avoidance has some link to their unique stripe patterning? However, new theories with more technical approaches to this analysis suggest otherwise.

In 2015, a studied was concluded that an altogether different theory was behind the evolution of zebra stripes. When a team of scientists analysed equid (horse) species across Africa, they found that there was a strong geographical overlap between the most distinctively striped species (both extinct and extant) and the presence of the annoying, vector-carrying parasitic flies such as the tsetse flies and horseflies. It has been proven that these flies are more attracted to dark, uniform colours (which is why one shouldn’t wear dark blue colours when heading to East Africa) than they are to pale colours. However, it appears that the polarising effect of the alternating black and white stripes does a good job in deterring these flies even more than any pale uniform colour, and the study found that there were more stripes on body parts where flies would be most prevalent. One just has to look at the perpetual swishing of a zebra’s tail to realise that flies are clearly an annoyance to them! But if this is such an effective deterent, then one may ask, why is it only zebras that have stripes? The answer may lie in the fact that their hair is much shorter than most of their open-country counterparts, shorter even than the biting mouth-parts of these flies.

Another theory which professes to be the main reason for the age-old conundrum as to why zebras have stripes is based on temperature control, or thermoregulation. Although this idea has long been a potential explanation, the exact mechanism still remains uncertain. The idea is that the light-absorbing black stripes cause a quicker movement of air over their surfaces than the adjoining white stripes, and these air masses moving alongside one another cause eddies or currents that take the heat away from the skin’s surface. A study has indeed shown that the surface temperature of a zebra’s coat may be as much as 3 degrees celsius cooler than other animals grazing in close proximity!

Researchers developed a model to predict stripe presence based on a variety of environmental variables such as temperature, rainfall, leafy tree density, and predator and parasite presence amongst others. The results showed that the most statistically significant environmental factor was indeed temperature. It appears that the consistency of temperature as well as the average coldest winter temperatures are the most influential; the colder the weather, the fewer stripes were present. The fact that zebras spend much of the midday period standing out in the open grasslands, receiving the brunt of the sun’s heat rays, may lend some strong weight to this theory. Although this may seem to contradict earlier findings, it is also likely that the hotter areas are more favourable for biting parasites, and that the last two theories can go hand-in-hand.

Truth be told, these theories are all that we have regarding the evolution of zebra stripes, and there doesn’t appear to be one single theory that completely satisfies a full justification for the presence of stripes on these equids. In the end, the final answer is no doubt a combination of the numerous hypotheses put forward by biologists and ethologists over the years. Each one playing its part in the evolutionary process that has sculpted this stand-out species and they only serve to emphasise the beauty of these stunning creatures.

So, if watching these striped stallions of the bush is high up on your bucket list, perhaps it is time to consider a visit to Tanda Tula Safari Camp, where these plains zebras are a permanent fixture of the landscape.

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