NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/sci/bio/4750

Internet Message Format | 1993-01-02 | 9.7 KB

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From: [e-mail] (Kathleen Hunt)
Newsgroups: sci.bio
Subject: Re: why (evolutionarily) are zebras striped?
Date: 3 Jan 1993 01:45:06 GMT
Organization: University of Washington, Seattle
Lines: 167
Distribution: usa
Message-ID: <[e-mail]>
References: <[e-mail]> <crystal.725914988@glia> <[e-mail]>
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Summary: more than you wanted to knowe

>>In <[e-mail]> [e-mail] (Michael Levin) writes:
>>>    Does anyone have any ideas why zebras are striped? Surely in the
>>>sense of camouflage, those yellowish african plains call for a
>>>monotone antelope-like color. Why are they striped? Please reply to
>>>[e-mail].
>>
>>>Mike Levin

Coincidentally I have just been doing some research on the natural history of
zebras.  There are several hypotheses as to why the stripes evolved, but none
has much supporting evidence  (it's hard to get research funding for the
pressing question of the function of zebra stripes...:-)

Some of the hypotheses have already been mentioned by others:

1. Camouflage -- stripes blend with grasses
>In article <crystal.725914988@glia>, [e-mail] (Crystal) writes...
>>Tall grasses have light and dark shadows.  Try to find a zebra in the tall
>>grasses of some of those plains. :>

This has been a common answer since about the turn of the century, but the
problems that leap to mind are that zebras mostly live in short grass plains
(or the plains zebra does, anyway), and that zebras always seem to be
*extremely* obvious when seen on TV nature programs (which is where most of
see them :-)  As _The Encyclopedia of Mammals_ says, "Zebras are active,
noisy, and alert.  They never attempt to conceal themselves or to "freeze"
in response to predators and they prefer to rest, grouped, in exposed
localities where they have the advantage of a good view at the cost of being
conspicuous themselves.  The widespread theory that their stripes are
camouflage is therefore contradicted by the zebra's behavior."

"The Natural History of Zebras" concurs:  "It is hard to imagine a
concealing function for the zebra's bold stripe patterns...Concealing
coloration is of doubtful value among social animals like zebras, whose
nervous behavior probably relates to the fact that their stripes are
conspicuous.  Instead of freezing when alarmed, as do most animals whose
markings blend with their surroundings, zebras bolt away."

2. Camouflage -- stripes blur at a distance
Field biologists uniformly report that stripes don't blend in noticably
with grasses, but at a *distance* they blur to camouflage the zebra.  For
instance, in mixed herds viewed at a distance, non-striped animals like
wildebeest appear as distinct dark blobs, while zebras blur into a formless
gray mass.  Conceivably a predator in the initial stages of picking out a
prey animal might overlook the zebra and head for the wildebeest.  This
blurring effect is reportedly most pronounced at dawn, dusk, and at night,
when zebras are particularly vulnerable to predators.  The blurring effect
works less well in the heat of midday, when instead the stripes can seem
to vibrate.

The stripe pattern -- thin vertical stripes changing to wide horizontal
stripes on the rump -- reportedly accentuates the blurring effect so that
a real zebra appears almost invisible at night, while a "pseudozebra" of
uniform horizontal stripes can be seen much better.

3. Camouflage -- insects don't recognize a striped animal as an animal
Biting insects use a variety of cues to find prey animals, including heat,
but at least some insects rely to some extent on vision, and selectively
land on large, solid-colored objects.  Thus a striped animal might confuse
biting insects.  This hypothesis has been tested at least once -- there's
an abstract in Cambridge Life Sciences from a few years ago, though of course
I don't remember the authors -- they put up large solid-colored sheets and
large striped sheets, and confirmed that insects land preferentially on
the solid-colored sheets.  (I can't remember if the solid-colored sheets were
light or dark, or if the authors addressed the issue of other cues used by
insects.)

4. Predator Confusion -- can't pick out individual from herd.
It's been suggested that in a mass of fleeing zebras, the stripes might
confuse a predator so that it can't pick out an individual zebra.  In theory
the dancing stripes would dazzle the predator and make it misjudge its leap,
and the vertical-horizontal stripe transition would disrupt the form of a
single zebra.  I'm unconvinced that that the stripe transition would have
that effect (to me the horizontal stripes just yell out "this is a zebra's
rump!")  Also, when viewed from *behind* the stripes practically make a
bull's-eye.

"The Encyclopedia of Mammals" adds:  "These theories founder on the
observable confidence with which lions kill zebras and on the fact that in
those places for which there are records, zebras are killed broadly in
proportion to their relative abundance."

5. Sexual Selection
>I would imagine someone has proposed stripes as a form of sexual
>selection, though the lack of sexual dimorphism would seem to
>cast doubt on that hypothesis. Anyone?
I don't think this will work because, as you say, there is very little
sexual dimorphism (true of equines in general) in *any* morphological
trait -- certainly none in the stripes that I'm aware of.

6. Individual Recognition
Zebras have individually distinct stripes, so these social animals may
be able to recognize individuals by stripes.  I imagine this might be one
function of stripes, but not the function they originally evolved for.
(teleologically speaking. Of course stripes only evolved because of some
increase in fitness enjoyed by zebras with stripes, not with any goal in
mind...let's nip that "Evolution Is Not Goal Oriented" flame war in the bud...)

7. Species Recognition
Certainly zebras must be able to notice each other immediately because of
the stripes.  They can undoubtedly also tell if the other zebras are the
right species (the two sympatric zebra species (Grevy's and Plains) have
very distinct stripe patterns, and have very distinct conformation anyway,
and also all 3 zebra species have unique butt patterns which might be of
help to a solitary zebra trying to figure out which herd to follow.)
There is some evidence that zebras are generally attracted toward stripes,
even striped panels.

8. Some vague thermoregulatory function
Unfortunately for this fuzzy theory, zebra stripes do not vary in accordance
with the wide variety of climates and elevations that zebras live in.

9. Other intraspecific social functions...
"The Enc. of Mam." describes a theory that links the evolutionary origin
of stripes with social grooming, an important social behavior in equines:
"Many animals have visual 'markers' to direct companions to particular
parts of the body.  In horses and zebras the preferred area is the mane and
the withers.  Extreme bending at the base of the neck causes skin wrinkles,
so it is possible that the evolutionary origin of stripes lie in enhancement
of this natural characteristic.  Once the optical mechanism was established
on this small target area, its effectiveness would have been enhanced by
spreading over the entire animal.  Significantly, the three contemporary
species are most alike in the flat panel area of neck and shoulder..."

In addition, *all* living equines show a tendency for striping across the
shoulder area.  The minimal striping for a wild equine is a dark dorsal
stripe down the spine (on all wild equines and many domestic horses), often
accompanied by a few vertical shoulder stripes and a couple horizontal
stripes on the legs.  Shoulder stripes are the last to go in the southern
subspecies of plains zebras that have lost some of their striping (for
example the quagga, the now-extinct southernmost subspecies, had a solid-
colored trunk and legs but retained a striped neck and head.)  So maybe there
is something about the neck & shoulder in particular that makes stripes
useful there??

10. Shadows from Grasses stuck on the zebra
When the Giraffe and the Zebra left the high veldt, they scuttled for days
and days till they came to a great forest, 'sclusively full of trees and
bushes and stripy, speckly, patchy-blatchy shadows...and after another long
time, what with standing half in the shade and half out of it, and what with
the slippery-slidy shadows of the trees falling on them, the Giraffe grew
blotchy, and the Zebra grew stripy...
  Oops!  That's a Rudyard Kipling Just So Story.  Sorry.  :-)

Just as a side note, stripe extent and color vary in different subspecies.
(and of course in species too).  After having spent many years seeing the
Damara plains zebras at the local zoo, with their slightly brownish stripes
on a creamy background, I was stunned when I saw some Grant's plains zebras
with absolutely jet-black stripes on a stark white background -- and the
stripes went all the way down the leg right to the hoof.  Beautiful. (Though I
am fond of the Damara's "shadow stripes", faint brown stripes just visible
between the big dark rump stripes...)

Refs:
MacClintock, D. 1976. A Natural History of Zebras.  Scribners, NY.

MacDonald, D. 1984. The Encyclopedia of Mammals. Facts on File, NY.
(particulary pp. 486-487, "The Zebra's Stripes" by contributor J. Kingdon.)


Kathleen

--
Halfway round the course, up spoke the noble rider:
 "I fear we must fall back, for she's going like a tiger."
Up spoke the noble horse:  "Ride on, my noble master,
  For we're halfway round the course, and now we'll see who's faster."