what is with the black stripes on white bodies.

what kind of environment would zebras inhabit that this would cause them to blend in?

or is there some other function?

thanks

This is a question that has puzzled many people over the years. It is worth giving a fairly detailed answer as there have been some ideas thrown out in the past, that while interesting, do not appear to be supported.

First off is the classic that stripes help as a dramatic form of camoflage either to break up the outline of the animal, or make it hard to identify individuals in a herd. However, studies show that predators have no harder time hunting zebras than other prey species so this is clearly not correct.

Next it has been suggested that this distracts flies or other insects, but again there is no evidence for this. Zebras still have to groom regularly and are the target of flies and mosquitos etc.

Number three is the cooling system - the alternating stripes should heat up inconsistently and set up a micro climate of moving air (from cool to hot), but once more there is no evidence for this. Zebras also show a variety of striping patterns (compare a Grevy's zebra with a Hartman's - they are very different).

Finally, we have what appears to be the correct solution - it is a social signal. Stripes help the animals in a herd recognise each other and appear to be a signal for mutual grooming. Many other horse species also have stripes, especially on the neck and lower legs (look at onager, kulan, the extinct quagga - itself a zebra species and others). These places are where most grooming takes place and so it is logical that this is how stripes started and then spread.

Modern zebras and the quagga have stripes most densly packed on the neck and rump so this fits, and observation shows that this is where most grooming takes place between animals. So in short, stripes are there to help bind the animlas together ssocailly by encouraging them to groom each other and stay together as a herd.

Hmm, I can think of a few points in response to Dave's reply...

Point one I'd say that comparison between predator effectiveness in hunting zebra being the same as hunting other herbivores doesn't really contradict the camoflague (or rather pattern-disruption) hypothesis, since other herbivores have also evolved anti-predator systems. The Red Queen hypothesis leads us to expect that all prey species in the predator-prey arms-race will be at a very similar level of adaptation for predator evasion.

Point two I'd say that unless you perform a direct comparison between a striped and unstriped zebra under the same conditions, how do we know that the stripes don't work for deterring insects? Very few mechanisms evolve that are 100% effective, so insect repellance only needs to be slightly better to confer an advantage if there are insect-borne parasites around.

Point three I'd say that there is plenty of evidence for dark areas heating preferentially to light ones (it's basic physics) which leads to convection cells forming which will act to draw air from cooler regions to replace the rising air. I'd never heard this hypothesis before, but it makes perfect sense as a way of maintaining airflow over the surface of something in a hot environment where there is little wind.

The final point you make I won't argue with except to say that a "correct solution" is actually most likely to be a combination of all of the above points and probably some more we haven't thought of yet.

Evolution is, after all, not working on isolated selection criteria.

Well mate, these have all been tested on zebras in the field in comparison both to other herbivores and some of the mutatn 'unstriped' (well, almost all black) zebras. Predators have equal success (as a percentage of hunting efforts) against zebras as the do with other species and fly strike, incidence of trypanosomiasis etc. are as high in zebras as other species.

If these were a genuine defence we would see some kind of benefits (or would see a negative impact on closely related species such as onager) but this is not the case. Mutual grooming is confined to the neck and hindquarters and this also fits with other horse species. I find it all pretty convincing myself.

Alright, fair enough on the flystrike, but I still maintain that the Red Queen means you'd expect near equal predator success on all species. Consider that most other prey species are brown - which you'd expect to be better camoflagued than black and white given the environment - yet zebras are not preyed on preferentially, which suggests that there is some kind of advantage conferred by the pattern disruption produced by stripes (particularly multiple striped objects moving past each other).

The stripes are not "better" than mechanisms that other species use, but they must still confer an advantage otherwise zebra would be first on the menu rather than having equal billing.

As to mutual grooming, if other horse species do this why don't they also have markings in the areas described?

Additionally why do other animals have stripes? Do tigers get involved in much social grooming? What about the numerous striped fish, snakes and insects? Longitudinal stripes are more common than the vertical zebra stripes, but pattern disruption works as a principle for any stripe orientation - why expect the pattern disruption principle to suddenly not work when focussing on just one type of organism when it seems to work for others?

I'm not saying that grooming doesn't have a role, I'm just saying that zebra stripes are probably fulfilling multiple functions, beyond just grooming.

Lots there! OK, Red Queen - you would not necessarily expect equal predator success as all animals are different (lions rarely hunt small gazelle and cheetah don't tackle buffalo). Lions for example take most medium to large prey species and with a few exceptions (like elephans and giraffe) attak all things equally (i.e. the number of hunts of an individual species is typically in proportion to the number of that animal available - wildebeest are the most common and hunted the most often).

If there was a clear benefit to being striped as a defence to predation, lions would initially switch to prey that was easier to hunt (and zebras are not actually than numerous - typically only 20% of the numbers of wildebeest, and then you also have topi, hartebeest, large gazelle, eland, kudu and others) and you would see an increase in zebra numbers. This doesn't appear to happen, to in fact evoke any real effect. If zebra were the only prey (or the dominant species) then I would agree that lions would soon compensate for any camoflage effect, but at low populations and still hunted as effectively as other animals I can't see it.

There are numerous other striped horse species and wild ass etc. (like the onager and kulan) and you also see the classic 'cross' on the shoulders of donkeys which are the target of grooming. Quagga are a very good example (as a zebra species) which was striped only along the shoulders (and lower legs).

As for stripes in other animals, of course these vary. Tigers and mackrel use them for camoflage, some snakes have warning colours etc. but obviously the key feature of the zebra is that they don't appear to have any basis for either of these, being so bold and dramatic. Given the lack of any evidnece to support alternate hypotheses I do think the social / grooming explanation is correct. If nothing else it is the only one supported by analyses of behaviour.

I must say I always thought the stripes would serve a camourflage role by helping to break up the animal's outline, but that may or may not be true! For what it's worth, below is the abstract from a (fairly) recent review. Perhaps the truth is that we aren't too sure, and certainly devising a rigorous (and powerful) experiemnt to properly test these hypotheses in the wild would seem a difficult thing to do.



The possible fitness benefits of striped coat coloration for zebra
Ruxton GD
MAMMAL REVIEW 32 (4): 237-244 DEC 2002
 
The literature addressing evolutionary reasons for the striped patterns of zebra coats is reviewed here. Possible mechanisms, and the evidence for and against them, are discussed. These mechanisms span four general themes: protection from predators; social functions; thermoregulation; and protection from tsetse flies. The last is the only hypothesis that has been tested experimentally, and the results of these tests are inconclusive. Additionally or alternatively, although stripes apparently increase zebra visibility in daylight, it is at least plausible that they provide effective cryptic protection from predators in poor light, although critical testing has not been attempted. Other related evolutionary questions are raised and suggestions made for future research.

I think you missed my point {Dave rather than Alistair - simultaneous posting!} - I was invoking Red Queen to make the point that ALL animals are constantly adapting to their environment (where the "environment" also encompasses the community).

As you say "If there was a clear benefit to being striped as a defence to predation, lions would initially switch to prey that was easier to hunt ", but my point is that the other prey are NOT easier to hunt, because they have also been adapting mechanisms to evade predation. Their mechanisms don't happen to involve stripes. I'm also suggesting that if stripes don't confer an advantage against predators then they would be a disadvantage, since as you say they are bold and distinctive. This makes me wonder why they have not been selected against? Surely patterns for grooming can be less ostentatious? Since pretty much everything else in the same environment is a "safe" colour (brown or grey) why draw attention to yourself? If other species in the same environmet were more flamboyant I wouldn't be arguing so hard for some kind of anti-predator benefit of stripes, but clearly the environment in which zebra live is pretty dangerous to stand out in.

Out of interest what is the average size of a zebra herd compared to other equid herds? The reason I ask is that I would only expect the high-contrast all-over striping of zebras to exist in animals with a fairly large herd size, since the mechanism for the pattern disruption would work effectively where multiple stripy objects are moving past each other. Anecdotally have you ever looked at one high contrast pattern moving past another? It plays havok with your depth perception and focus.

Again I would say that I don't disagree with the grooming hypothesis, I am simply stating that zebra stripes are probably more than a one-trick pony (if you'll excuse the equine pun).

I am interested to see that the above abstract suggests that only the fly strike has been tested. I admit that my information comes from two book on mammals, (though both are textbooks and written by mammal academics) but there it was given that these hypotheses had been tested with the results I outline above.

Obviously if these have been discredited or new results have contradicted previosu studies then these might be far less clear cut than I have made out. Still having worked with zebra in captivity and seen then in the wild, I can say that they groom intensly on the neck, and even when moving quickly and in large numbers, picking out an individual animal is not hard.

Although it has not come up yet, we of course have colour vision while most other mammlas (including lions, leopards etc.) have only black and white. This might make you think that this would help the zebras become camoflaged better, but in fact I would expect the opposite to be true. Most African ungulates are tan, beige, brown or variations on this, often with cryptic markings (like kudu and eland) which would actually match a typical background of 'grey' grass (in black and white vision) than the highly dramatic pure black and white stripes of a zebra.

I got out of phase! Sorry to any other readers, my last post refers to Alistair's and Paolo has managed to put in another in the time it took me to reply.

Right.....

Stripes again. Obviously we have just been talking about black and white vs. colour vision and I see now that I was arguing *against* stripes as a camoflage advantage, whereas Paolo was suggesting that they might be a *disadvantage*, which is rather different.

Still, there need not be a separation here. The bold stripes might make them stand out, but if this helps them keep continuity with the herd and helps then to scout for danger, then despite being conspicious, they would still be advantageous overall. Zebra typically herd with wildebeest and other large animals (especially various hartebeest species and some of the larger gazelle) so the stripes could (I don't know, this is just a suggestion) also act as way of keeping the bigger herd together. Again, this would act as an overall benefit for keeping them safe with more animals on the look out for predators.

Herds in horses vary massively, both within and between species. Grevy's zebra (the very stripey ones) tend to live in small groups dominated by a single stallion, whereas the common zebra tends to mix with other species. Individual family groups will operate within this, but the zebra 'herd' could be hundreds strong but then spread out over a very large area (hence my suggestion of herd coherency) with other animals in between.

As for moving stripes, I agree that this can make life difficult on the eye. However, i would expect a hunting lion to have the choice of individuals that are more or less stationary when picking a target. By the time the lion goes for the kill it should be at close range (typically under 50m, and often more like 10-20m) so it should be realtively easy for it to keep its eyes on the target as it will be fixed in its vision and very close by. Other zebra will be running in the background, but the target anial should dominate and I don't think this would be a big probelm. Again, the data I have suggests that it is not a big probelm for lions at least.

I think that we may be talking at slight cross-purposes. I'm considering evolutionary-scale adaptation of the feature rather than the nitty-gritty of individual interactions with lions.

I just don't see the selective advantage to bold stripes if they don't confer advantages beyond group cohesion and social interactions. Other animals manage to maintain herds without strong visual signals - wildebeest etc. seem to do pretty well en masse. It strikes me that natural selection will trend the population towards the regions of greatest survivorship, which would be towards the least conspicuous members if in a predator dense habitat (your group-cohesive stripy fellows would be in the population - but not for many generations as they would be selected against because they weren't as well camoflagued as their less contrasty conspecifics). Unless there is some kind of sexual selection at work on stripes to override this it just doesn't make sense to me without stripes being good for something else as well.

What about baby zebras? Most young animals are born with some kind of camoflague to help evade predators - growing into adult colours later. From my experience with taxidermy zebra the young are striped like the adults. This suggests to me that the stripes do actually confer some kind of camoflague, since new-born foals would be at a substantially higher risk of predation than adults - surely enough to merit a different coat in the first few weeks or months.

Speaking of baby animals I am currently cataloguing a juvenile great crested grebe which appears to have bold black and white stripes much like a zebra...

Well, I would like to try and wrap up zebras, if only to try and avoid boring any readers. First off on the grebe, these live in very dense reedy areas where this kind of pattern would (I suspect) provide better camoflage than a baby zebra on the plains (despite some tall grasses, its not really the same thing). Also the grebe would be left alone for long periods where a foal would be with the mother.

Anyway, I do accept your points, but I do think that social (as opposed to sexual as you suggest) signalling to maintain herd coherency and social relationships could be sufficiently important to a zebra's survival to be worthwhile even if it did act as a disadvantage as making it more conspicious to predators.

Young animals would need to recognise and be recognised by other animals in the herd and this would therefore require them to be striped from birth. Finally, the mutuant zebras i referred to earlier (some are almost all black, others more like dappled) are almost compeltely shunned by other zebras and thus fall prey to predators quickly when denied the shelter of the herd.

Although other ungulates do not show this level of patterning but their herd structure is often very different. Sadly comparing zebras to other horse species is difficult as they live in very different environments (more open, drier and with far few predators, or other large animals for that matter), although there are clearly strong difference between the three extant species in striping pattern and the quagga. The common zebra also shows large differences from the north to south of its range (the southern ones are much less stripey on the legs) and if there was a camofalge advantage to stripes I would expect them to be more common in other species (kudu are striped as are eland, but far less so) and more unifrom across zebras themsselves.

Still, after Alistair's post, I think the key thing here is that more evidence is needed really. That's science though, and right now I do think the soacil signally etc. explanation is by far the best explanation.

Can we leave it there? :-)

Ok, I think it's fair to say that we all agree that more research is needed!

Fun discussion to be continued in a pub perhaps?

Dave,

Not convinced about anything you've said about GCG!!! Hatchlings spend the first 3-4 weeks either riding on the backs of the parents or in very close proximity. They aren't really left in reeds either! Great crested grebes are precocial (that is, the young are active a day or two after hatching) and fledge in around 70 to 80 days. Immatures have adult plumage but retain black and white stripes on the head and neck only.

I'd be extremely wary about making inferences about GCG young having striped patternation for camouflage, as for the above reasons, they are not (usually) left anywhere on their own.

My two cents...

Dave.

Whoops, my bad. Thanks for the correction. I though they were more of a 'reed' bird. I think I was thinking of chicks rather than fledged youngsters, but clearly that was a mistake! Peer review rules.....