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New research may help answer the question of why zebras have stripes while offering clues to new ways of protecting horses from flies.

For their study, an international research team from the University of California, Davis, and the University of Bristol and University of Exeter in England observed three captive plains zebras and nine solid-colored horses kept in adjacent fields.

Initially, the researchers documented the number of horseflies that approached and/or landed on the animals---as well as how many of the insects attempted to feed.

In the second phase of the study, the researchers repeatedly covered seven of the horses with sheets that were one of three colors ---black, white or black-andwhite striped---and observed subsequent fly behavior for 30 minutes. Once again, the number of landings were documented.

In addition, the zebras and horses were filmed for several hours, and the researchers used the resulting footage to extract data about the flight patterns of flies as they came close to each animal. The flies’ flight trajectories were divided into three segments: an “approach” as the fly entered the camera’s focal area and slowed down as it came near to the animal, a “leave”---direct and accelerating flight away from the animal and out of the range of the camera, and “investigation,” the period between the other segments when the fly hovered close to the animal. The researchers then calculated the speed, duration and tortuosity (the distance traveled divided by the distance between the start and finish points) of each fly’s flight segments.

The researchers determined that roughly the same number of flies approached and circled the horses and zebras, but the insects had a much more difficult time landing on the zebras. They failed to decelerate and instead flew over or bumped into the zebras and departed without attempting to feed. In fact, the researchers did not see a single fly probe a zebra’s skin during more than five hours of observation.

On the other hand, 239 instances of attempted feeding on horses were observed during an 11-hour period. Similar differences in fly behavior were observed when horses wore the solid-colored or striped sheets: During a 30-minute period five flies landed but did not feed on horses wearing zebra-striped sheets, while during the same period more than 60 landed on horses wearing solid black or solid white sheets.

The researchers speculate that stripes disrupt the visual systems of the flies, so when they get close to zebras---or horses wearing striped sheets--they have trouble navigating around and landing on the potential hosts.

Reference: “Benefits of zebra stripes: Behaviour of tabanid flies around zebras and horses,” PLOS One, February 2019

OUT OF SIGHT: A zebra’s stripes may disrupt the visual systems of flies, making it difficult for the pests to navigate near the animal and land.

FLIGHT PATTERNS: When researchers covered horses with white, black or black-and-white-striped sheets, they discovered that flies were far more likely to land on the solid-colored surfaces than on those with stripes.