Researchers on Wednesday described experiments demonstrating
that horse flies have a difficult time landing on zebras while
easily landing on uniformly colored horses. In one experiment,
the researchers put cloth coats bearing striped patterns on
horses and observed that fewer flies landed on them than when
the same horses wore single-color coats.
"We showed that horse flies approach zebras and uniformly
colored horses at similar rates but that they fail to land on
zebras - or striped horse coats - because they fail to
decelerate properly, and so fly past them or literally bump into
them and bounce off," said behavioral ecologist Tim Caro of the
University of California-Davis, lead author of the research
published in the journal PLOS ONE.
Close cousins to horses and donkeys, the world's three zebra
species, known for their black-and-white striped bodies, roam
Africa's savannas eating a variety of grasses. Their stripe
patterns vary among individuals, with no two alike.
There had been four main hypotheses about the advantages zebras
accrued by evolving stripes: camouflage to avoid large
predators; a social function like individual recognition;
thermoregulation, with stripes setting up convection currents
along the animal's back; and thwarting biting fly attacks.
"Only the last stands up to scrutiny," Caro said. "Most
biologists involved with research on mammal coloration accept
that this is the reason that zebras have stripes."
African horse flies carry diseases such as trypanosomiasis and
African horse sickness that cause wasting and can be fatal.
The researchers videoed horse flies as they tried to prey on
captive zebras and domestic horses at a livery in North
Somerset, England. Stripes did not deter flies from a distance,
as they circled horses and zebras at similar rates. But the
flies managed to land on zebras less than a quarter as often.
University of Bristol biologist and study co-author Martin How
said stripes may dazzle flies somehow once the insects venture
close enough to see them with their low-resolution eyes.
"In addition to stripes that prevent controlled landings by
horse flies, zebras are constantly swishing their tail and may
run off if horse flies do land successfully, so they are also
using behavioral means to prevent flies probing for blood," Caro
(Reporting by Will Dunham; Editing by Sandra Maler)
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