Why do zebras have stripes?

For a long time, this was one of those questions where biology textbooks listed five plausible theories and shrugged. Over the last decade the evidence has actually started to settle, and one answer is clearly winning:

Fly deterrence. Specifically, biting flies — horseflies, tsetse, stable flies. The strongest evidence:

• A 2014 study (Caro et al.) mapped striping across all extant equid species and found the tightest geographic correlation was with biting-fly abundance, not with predator presence, not with heat, not with social behaviour.
• A 2019 experiment dressed regular horses in zebra-striped coats and filmed the flies. Flies approached the striped horses at the same rate as the plain ones, but they couldn't land properly — they decelerated badly, bounced off, or overshot. The stripes seem to disrupt the optic flow they use for landing.
• Zebras are unusually vulnerable to fly-borne diseases (trypanosomiasis, African horse sickness) and have unusually short hair compared to other African grazers, which leaves them more exposed. The selection pressure to evolve a fly defence would have been strong.

The older theories have mostly weakened under scrutiny:

• Camouflage — lions can't actually resolve stripes at typical hunting distances, and they ambush anyway. Mostly dead.
• Motion dazzle / predator confusion — both computer-vision simulations and live predator tests give it weak support at best.
• Thermoregulation (the idea that black and white surfaces at different temperatures set up cooling air currents) — promising early studies, patchy replication.
• Individual recognition — probably real but minor; most social mammals manage this fine without stripes.

So the modern short answer is: zebras have stripes because flies can't land on them properly, and in equatorial Africa — where the flies carry diseases that can kill an equid — that mattered enormously.