Zebras have stripes to
deter the tsetse and other blood-sucking flies, according to a
fresh bid to settle a debate that has raged among biologists for
over 140 years.
Since the 1870s,
in a dispute sparked by the founders of evolutionary theory Charles
Darwin and Alfred Russel Wallace, scientists have squabbled over
how the zebra got its trademark look.
Are its stripes
for camouflage, protecting the zebra with a "motion dazzle
confusion effect" against hyenas, lions and other predators in the
savannah?
Do the stripes
radiate heat to keep the zebra cool?
Or do they have a
social role -- for group identity, perhaps, or mating?
But a new study,
published in the journal Nature Communications on Tuesday, says the
strongest likelihood is that the stripes discourage parasitic
flies.
The finding was
intriguingly thrown up by lab experiments in 2012 that showed how
blood-feeding flies shun stripey surfaces and prefer instead to
land on uniform colours.
Researchers led by
Tim Caro of the University of California at Davis, say there is no
black-and-white answer to the Great Stripe Riddle -- but the insect
theory is by far the best bet.
"A solution to the
riddle of zebra stripes, discussed by Wallace and Darwin, is at
hand," they write.
The team found a
strong geographical overlap between zebras and the two groups of
biting flies, Tabanus and Glossina, that feed on equid species,
which explains why zebras would need a shield against this
pest.
There is also
plenty of indirect evidence, they say.
Other equid
species, such as wild horses, are far more likely to be plagued by
biting insects.
Researchers find
comparatively little blood from zebras in tsetse flies, even though
the zebra has a thin coat with hair strands that are shorter and
finer than those of giraffes and antelopes.
At the same time,
zebras are far less susceptible to sleeping sickness, a
tsetse-borne disease that is widespread among other African
equids.
The correlation
between reduced biting-fly nuisance and stripes is "significant,"
says the study.
"Conversely, there
is no consistent support for camouflage, predator avoidance, heat
management or social interaction hypothesis."
Parasitic flies
can hand on a range of diseases when they bite their prey, and
their appetite can be enormous.
Experiments with
horse-flies carried out in the United States found that cows can
lose between 200 and 500 cubic centimetres (0.4 and 1.05 pints) of
blood per day to the insects, and as much as 16.9 kilos (37.2
pounds) in weight over eight weeks.