Photos of adorable white bison appear in Yellowstone National Park

Photos of a white bison in Yellowstone National Park have sparked excitement as well as questions: How does this happen?

A park visitor said she took photos in the park earlier this month, showing a fluffy white calf snuggling up to its dark brown mother. Park officials said this week that they have yet to spot a white calf in this sprawling Wyoming-Montana park, which is home to about 5,000 bison, also called American buffalo.

The birth fulfills a Lakota prophecy that portends better times, according to members of the Native American tribe who cautioned that it is also a warning that more must be done to protect the land and its animals.

In nature, there are two genetic variations that can give rise to unusually light-colored animals: leucism and albinism. In both cases, the animal inherits two copies of the genetic mutation – one from each parent, which usually appears normally colored itself.

Leucistic animals lack certain cells that otherwise produce melanin, a natural pigment that gives color to fur, eyes, feathers and skin. Their bodies may appear almost entirely white, or only in patches of white, and they usually have normal or dark eyes.

Albinism, which is rarer, results from a total absence of melanin. Albino animals are almost entirely white and may have light pink or orange eyes and reduced vision. Albino bison will not have dark colors in their eyes, noses and hooves, said James Derr, a geneticist at Texas A&M University.

The Yellowstone calf, with its black nose and eyes, is not albino, said Jim Matheson, executive director of the National Bison Association.

There is a third possibility: a light-colored calf could be the result of a bison crossed with a white domestic cow. In this case, the calf may be light tan in color, with brown eyes and a black or brown nose, Derr explained.

Matheson said it’s unclear how often baby white bison are born in the wild.

“We just don’t know how often this happens because we’ve never tracked this historically,” he said.