Re: Zebra stripes evolved to keep biting flies at bay

On Fri, 10 Feb 2012 23:06:08 -0700, Richard Norman
<r_s_norman@xxxxxxxxxxx> wrote:

On Fri, 10 Feb 2012 23:31:15 -0600, Don Cates <caHORMELtes@xxxxxxx>
wrote:

On 10/02/2012 7:46 PM, Richard Norman wrote:
On Fri, 10 Feb 2012 15:48:01 -0800 (PST), chris thompson
<chris.linthompson@xxxxxxxxx> wrote:

On Feb 10, 6:33 pm, Metspitzer<kilow...@xxxxxxxxxxx> wrote:
Why zebras evolved their characteristic black-and-white stripes has
been the subject of decades of debate among scientists.

Now researchers from Hungary and Sweden claim to have solved the
mystery.

The stripes, they say, came about to keep away blood-sucking flies.

They report in the Journal of Experimental Biology that this pattern
of narrow stripes makes zebras "unattractive" to the flies.

They key to this effect is in how the striped patterns reflect light.
Continue reading the main story

"We started off studying horses with black, brown or white coats,"
explained Susanne Akesson from Lund University, a member of the
international research team that carried out the study.

"We found that in the black and brown horses, we get horizontally
polarised light." This effect made the dark-coloured horses very
attractive to flies.

It means that the light that bounces off the horse's dark coat - and
travels in waves to the eyes of a hungry fly - moves along a
horizontal plane, like a snake slithering along with its body flat to
the floor.

Dr Akesson and her colleagues found that horseflies, or tabanids, were
very attracted by these "flat" waves of light.

"From a white coat, you get unpolarised light [reflected]," she
explained. Unpolarised light waves travel along any and every plane,
and are much less attractive to flies. As a result, white-coated
horses are much less troubled by horseflies than their dark-coloured
relatives.

Having discovered the flies' preference for dark coats, the team then
became interested in zebras. They wanted to know what kind of light
would bounce off the striped body of a zebra, and how this would
affect the biting flies that are a horse's most irritating enemy.

"We created an experimental set-up where we painted the different
patterns onto boards," Dr Akesson told BBC Nature.

http://www.bbc.co.uk/nature/16944753

It would be interesting to test whether different kinds of biting
Dipterans were attracted to differing light patterns, and if that
matched up with the different widths of stripe patterns on the
different species of zebras.


Insects in general can detect the polarization of light. The
behavioral studies in the actual paper show that the zebra striped
pattern is less attactive to horse flies, Tabanids, than either solid
black or white. The abstract to the real paper doesn't emphasize the
polarized light aspect and the title is about brightness/polarization
patterns. I haven't yet checked the text to see just what role
polarized light plays other than as a possibility.

The paper itself is

Polarotactic tabanids find striped patterns with brightness and/or
polarization modulation least attractive: an advantage of zebra
stripes
A Egri et al.
J Exp Bio l215, 736-745 (2012)
http://jeb.biologists.org/content/215/5/736

To be specific, the abstract claims:
"Here, we demonstrate that a zebra-striped horse model attracts far
fewer horseflies (tabanids) than either homogeneous black, brown, grey
or white equivalents...Besides brightness, one of the likely
mechanisms underlying this protection is the polarization of reflected
light from the host animal. We show that the attractiveness of striped
patterns to tabanids is also reduced if only polarization modulations
(parallel stripes with alternating orthogonal directions of
polarization) occur in horizontal or vertical homogeneous grey
surfaces."

Might it not be a good idea to find somr connection between the
attention of these flies and zebra reproductive success before claiming
any evolutionary connection?


Horse flies are truly horrible blood sucking pests. Anyone aquainted
with their bites or has seen animals trying to escape them doesn't
doubt that protection from their attack is a Good Thing. Animals
attacked by a large number of flies are often severely weakened and
can die from blood loss, not to mention the diseases spread by the
flies.


I remember a particular video about Canadian caribou. One scene that
was focused on a particular animal which was standing in one place
grazing in the distance, apparently peacefully. Then it gave a mighty
shake, and it looked as if the animal's outline has suddenly ballooned
outward by about a half a diameter, and then settled back onto itself.
In fact it was a mass of flies covering the animal from head to tail.
The narrator mentioned that the caribou go into deep water to
temporarily hide from the flies. But then caribou can't eat either,
and so face a Hobson's choice.

I can only imagine what that must feel like.


In thinking about the effect of stripes, it occurs to me that the
flies (actually only the females bite and suck blood) might use a
search pattern to find prey locating very large dark objects
preferentially or alternatively large white objects. Objects that are
variegated or striped might not be seen as "large objects". The
pattern breaks up the animal's outline so a predator gets confused
about what is is looking at. That is an example of disruptive
coloration, usually the explanation for zebra stripes in confusing
predators like the big cats or hyenas. When zebras run together as a
herd it is very difficult to tell just where one particular animal
begins or ends. For insects, this effect would work equally
effectively for intensity (black vs white) and polarization (polarized
vs not) variation in the object viewed.


.

Relevant Pages

  • Re: Zebra stripes evolved to keep biting flies at bay
    ... The stripes, they say, came about to keep away blood-sucking flies. ... They key to this effect is in how the striped patterns reflect light. ... Insects in general can detect the polarization of light. ...
    (talk.origins)
  • Re: Zebra stripes evolved to keep biting flies at bay
    ... The stripes, they say, came about to keep away blood-sucking flies. ... of narrow stripes makes zebras "unattractive" to the flies. ... They key to this effect is in how the striped patterns reflect light. ...
    (talk.origins)
  • Re: Zebra stripes evolved to keep biting flies at bay
    ... The stripes, they say, came about to keep away blood-sucking flies. ... They key to this effect is in how the striped patterns reflect ... behavioral studies in the actual paper show that the zebra striped ...
    (talk.origins)
  • Re: Zebra stripes evolved to keep biting flies at bay
    ... The stripes, they say, came about to keep away blood-sucking flies. ... They key to this effect is in how the striped patterns reflect light. ... behavioral studies in the actual paper show that the zebra striped ...
    (talk.origins)
  • Re: Zebra stripes evolved to keep biting flies at bay
    ... The stripes, they say, came about to keep away blood-sucking flies. ... They key to this effect is in how the striped patterns reflect ... behavioral studies in the actual paper show that the zebra striped ...
    (talk.origins)