A “Costume Change” for Zebras and Horses Reveals How Stripes Thwart Horsefly Landings

By Kat Kerlin, Human & Animal Health, UC Davis

Scientists learned in recent years why zebras have black and white stripes — to avoid biting flies. But a study published on February 20, 2019 in the journal PLOS ONE probes the question further: What is it about stripes that actually disrupts a biting fly’s ability to land on a zebra and suck its blood?

Professor Tim Caro of the University of California, Davis, and University of Bristol’s Martin How led a series of new experiments to better understand how stripes manipulate the behaviour of biting flies as they attempt to come in to land on zebras.

Joren Bruggink of Aeres University of Applied Sciences (left) and Jai Lake of the University of Bristol investigate how horseflies behave around horses wearing different coloured coats. This was part of an experiment led by UC Davis, focused on why zebra stripes are so good at warding off biting flies. Photo: Tim Caro/UC Davis

The experiments took place on a horse farm in Great Britain that kept both zebras and horses, and entailed the following: 

• Close-up observation of zebras as flies attempted to land on them;
• Detailed videos to record flight trajectories as the flies cruised close to the zebras;
• Dressing the horses and zebras sequentially in black, white and then black-and-white striped coats.

Stripes make lousy landing strips

In the study, flies were just as attracted to zebras as they were to horses, indicating that stripes do not deter flies at a distance.

“Once they get close to the zebras, however, they tend to fly past or bump into them,” said Caro. “This indicates that stripes may disrupt the flies’ abilities to have a controlled landing.”

Compared to rates at which flies landed on the white and the black coats, hardly any landed on the striped coats.

“Stripes may dazzle flies in some way once they are close enough to see them with their low-resolution eyes,” said How.

UC Davis wildlife biologist, Tim Caro, observes zebra behaviour in response to biting fly annoyance. Photo: Joren Bruggink/Aeres University of Applied Sciences

Zebras swish and run, horses twitch  

The study also noted that zebras and horses respond very differently to the presence of flies. Zebras swish their tails almost continuously during the day to keep flies off. They stop feeding if bothered by them, and if the flies are particularly persistent, the zebras will run from them. On the other hand, horses primarily twitch and occasionally swish to ward off flies. As a result, any flies that actually contacted zebras were soon dislodged compared to horses.

Researchers do not yet understand why zebras evolved these sophisticated defense mechanisms. A possible explanation is that zebras may be highly prone to infectious diseases carried by African biting flies, although that hypothesis requires further study.

The study’s coauthors include Yvette Argueta from UC Davis; Emmanuelle Sophie Briolat, Maurice Kasprowsky, Matthew Mitchell, and Sarah Richardson of the University of Exeter; Joren Bruggink of the Netherlands’ Aeres University of Applied Sciences; and Jai Lake from the University of Bristol.

Main photo: Shutterstock/Sarel