by Matthew Cobb
Hello everyone, Jerry has kindly (or foolishly) handed over the reins of the WEIT blog to me for the next 10 days or so. See if you can spot the difference!
Some of the most spectacular signs of evolutionary adaptation are the many examples of camouflage shown by animals. Although many examples of camouflage are shown by prey animals seeking to avoid being eaten, predators also use camouflage to avoid detection. My good friend Professor Innes Cuthill of Bristol University, UK, studies animal camouflage, and has just posted this excellent audio slideshow on the BBC website.
Innes describes various cases of how camouflage works in different animal species, and there are some great pictures to go with it. Sometimes, changing color is not actually to do with camouflage – this is the case in chameleons, and also in octopuses and squid, which can use rapidly changing patterns of skin color to communicate in ways we do not fully understand. And by ‘ways’ I mean both what they are communicating and how they change their color so quickly.
One of the most spectacular examples of cryptic camouflage can be seen in the octopus, in this video:
This is taken (without credit!) from a fantastic five-minute talk on underwater animals by David Gallo at Ted.com, which you can find here and which includes some great interactions between squid at around 2m 40s and some cuttlefish showing fantastic rapidly changing color patterns.
One example of camouflage given by Innes Cuthill is the zebra, which he suggests may have stripes because it disrupts their outline, making it more difficult for predators to decide where the zebra begins and ends. This may be true – but in reality we simply do not know what the adaptive advantage is. Indeed, it is possible that the stripes have nothing to do with what is really going on (they may be simply a side-effect of the true advantage), although that seems unlikely. A non-camouflage explanation is that zebra foals are born into a world of stripes, and that the stripes on their parents help to enable them to identify their fellow-zebras, and reinforce their herd identity.
The problem with all these explanations is that they are what the late Stephen Jay Gould called ‘Just So Stories’, after the children’s fables written by the British author Rudyard Kipling (‘How the elephant got his trunk’, and so on). They fit the facts, and they may be true, but they lack the decisive support that science alone can provide: experimental evidence.
In discussing this with Jerry last week, he pointed out that a simple test of the ‘disruption’ hypothesis to explain the zebra’s stripes would be to paint some all black or all white, and see what happens to the predation rate. I suspect that would not be possible, either ethically or practically, but some kind of experiments on zebras, lions, or both, will be required before we can be really sure why zebras have stripes. Post your explanations – and above all, think of a doable experiment that could test your hypothesis!
I think I’d paint them checkered – or at least turn the stripes the other way (even if it isn’t actually doable).
How do lions prey? Does it really matter where one animal ends and another begins? Can’t they just charge in and have a bite out of one? Actually, I thought they picked off the stragglers, that then can’t have the benefit of fitting into the herd.
What about striped predators, such as Tigers? I would think the reason for their stripes would be different than a Zebra’s.
I think I saw on some documentary somewhere that a zebra’s stripes help to cool the beastie by creating air currents.
@SeanK – There’s a big difference between the tiger’s stripes and those on the zebra. The tigers’ ones are easy to explain because they do actually contribute to blending in with the background and breaking up the outline, which of course makes them harder to spot. The problem with zebras is that their stripes make an individual apparently very conspicuous. Because this is counter-intuitive, it demands a more creative explanation.
I think a good preliminary experiment to test some explanations for it could be done using Matthew’s cats and a projector, possibly eliminating the need to paint horses and set them out amongst the lions.
I think zebra stripes may seem conspicuous to us, but to lions, who only see in B&W, they produce a flurry of indistinguishable streaks of color, particularly when a herd is on the move, that ultimately makes it harder to pinpoint and attack a single animal.
SeanK:
Go view the audio slideshow. It mentions “tigers have stripes, like verticle grass stains”. The end mentions the zebras.
oops: vertical grass stains.
To test the ‘disruption’ hypothesis, a computer could be programmed to locate individual zebras that were filmed while grazing. Then have the same program try and locate individuals that were filmed during a stampede.
Thanks for posting. I’ve seen that video before in biology classes, but I always enjoy watching it. The animal kingdom never ceases to amaze me.
I’m not sure whether or not I buy the theory that stripes help zebras fit into a herd. Why don’t we see stripes on more herd animals if they are advantageous in this regard?
What about okapi, which are a solitary, forest-dwelling ruminant related to the giraffe? They have stripes only on their back end.
Perhaps stripes evolved through sexual selection? Maybe zebras think stripes are really attractive? 🙂
The thing I always try to remember about these types of speculations is that they’re just that–speculations. My favorite example is the giraffe–some work has show the long giraffe neck likely evolved from sexual selection, not for increased foraging capabilities.
cheers,
Mary H.
http://stalecheerios.com–a serial for positive animal training.
okay…here’s an interesting paper I just found. (I’m not at school, so I couldn’t access the paper, just the abstract.)
Another theory for zebra stripes:
How the Zebra Got its Stripes-Biting Flies as Selective Agents in the Evolution of Zebra Coloration.
Waage, JK
J. ENTOMOL. SOC. SOUTH. AFR. Vol. 44, no. 2, pp. 351-358. 1981.
Zebra Equus zebra stripes have traditionally been thought of as an adaptation against detection by vertebrate predators, such as lions and hyaenas. A different hypothesis is suggested: that the stripes are an adaptation against visually orienting biting flies and act by obliterating the stimulus presented by a large dark form, which is important in host-finding by many diptera. This hypothesis is supported by some indirect evidence and by a field experiment which compares fly catches on moving and stationary black, white and striped models. Striped models caught significantly fewer tsetes and other flies than solid black or white models, but this difference was much reduced in the presence of olfactory attractants.
Though, several other papers I found seemed to indicate that stripes, in general, function for camouflage.
Mary H.
http://stalecheerios.com/blog
The problem with the biting fly argument is why aren’t ALL animals living in the tsetse area striped? Waage’s paper does have the great advantage of actually having done an experiment, rather than sitting around speculating about herd behavior, like me!
The reason why all animals don’t have these stripes is that there is a considerable price to pay for having them, they are very conspicuous. So the question is to prove that the trade off is worth it for zebras and not for other animals in the same area. Dunno how you’re gonna do it mind you.
And you’ll need to paint some zebras with spots as well! Reminds me of Alice in Wonderland, ‘We’re painting the roses red, we’re painting the roses red …’
How does natural selection bring about such animals as the leafy sea dragon and the pygmy sea horse. There would have to be billions of variations before it would mimic a plant or coral so perfectly. Even then how would the creature know to live within that camouflage. If we spent all our time as humans hiding in particular bush evading some predator, would we over time mimic the look of that bush? What tie is there to plant and animal to bring this about is it just chance?
Being a bit camouflaged is better than being not at all, you may avoid detection by a predator who isn’t paying attention. Predators aren’t perfect, they lose concentration too.