What's the difference between a llama and an alpaca?

A two-paneled image with a llama on the left and an alpaca on the right.

It's easy to mistake llamas and alpacas for each other: Both are slender fluffy four-legged mammals with long necks that live in herds. So what's the difference between these two species?

"That is the most common question the public asks us," Rebecca Gill, co-owner of Cotton Creek Farms, a Michigan farm that is home to both animals, told Live Science.