Why do zebras have stripes? To confuse flies - Cosmos

Zebra stripes deter biting flies, new research reveals.

The zebra’s iconic striped pattern has been a source of scientific interest for hundreds of years, with hypotheses such as camouflage, predator avoidance and, most recently, thermoregulation attempting to explain their function.

The latest study, led by Tim Caro from the University of California Davis, US, closely examined the behaviour of horse flies (from the family Tabanidae) in a small population of captive zebras and horses – and the results may be the definitive answer to this long-standing mystery.

Previous studies have noted that zebras (Equus burchelli) suffer less from horse fly and tsetse fly attack than non-striped mammals such as antelope or cattle, but until now the reason why has been unclear.

The scientists used high-speed video footage of flight and the biting behaviours of horse flies on striped versus plain fur. The flies circled and flew near zebras and horses at similar rates, but landed less than one-quarter as often on the former.

Video analysis showed the flies’ landing behaviour was adversely affected by the zebra stripes. The insects failed to decelerate in a controlled fashion, and then veered away at high speed. Many simply “bumped into zebras but failed to land”.

Fly behaviour was also assessed in the vicinity of horses wearing black, white and striped coats – with the same result. The flies failed to land on the bodies of horses wearing a zebra onesie, but tellingly, the animals were bitten just as much as usual on their heads.

Caro and colleagues also carefully described zebra and horse behavioural responses to flies. If a horse fly, also known as a tabanid, did manage to land on a zebra, the animal would respond rapidly by tail swishing or simply running away. These behavioural measures were employed at a far higher rate than by horses.

“As a consequence of both of these morphological and behavioural defences, very few tabanids are able to probe for a zebra blood meal,” the researchers note.

The findings, published in the journal PLOS One, conclude that “zebra stripes are now believed to have evolved to thwart attack by biting flies.”

The mechanism by which the stripes confuse fly visual systems has been studied in both field and laboratory, but Caro and team still believe that “interspersed black and white stripes are likely to prevent accurate assessments of angular velocity of looming objects (such as a zebra) in ways that demand further investigation”.

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