A common claim on many “Unbelievable Fact” lists is that the claim the hippopotamus is the only animal that produces naturally pink milk. Although this factoid has been circulating online for several years, it received a large bump in credibility when it was posted the Facebook by a National Geographic account on 26 July 2013:

The hippopotamus, however, does not produce pink milk. This claim is based on the combination of two unrelated facts:

Hippos secrete hippusudoric acid, which has a reddish pigmentation. When white (the color of milk) and red (the color of hippusudoric acid) combine, the resultant mixture is pink.

But according to biologist David Wynick, there’s no evidence suggesting that hippos secrete hippusudoric acid into their milk:

I think this is an Internet legend that is oft repeated but without any evidence for it that I can find. It is true that hippos secrete a red pigment in their sweat that acts as a natural suntan lotion, but nowhere can i find evidence it is secreted in breast milk and thus turns it pink. Further, since the pigment is acidic I would guess that would not combine well with the milk.

Hippos produce white- or beige-colored milk similar to that of other mammals. While it’s true that the exterior of a hippo could sometimes appear pinkish due to the animal’s secretion of hippusudoric acid, this phenomenon does not produce pink milk:

Like all mammals, hippos produce white or off-white milk for their young. Despite this fact, it’s easy to see where some of the color confusion comes from. Hippos have no actual sweat glands, but they do have mucus glands, which release an oily secretion frequently referred to as “blood sweat.” Despite its name, this secretion is neither blood nor sweat; instead, it’s a blend of hipposudoric acid and norhipposudoric acid. Combined, these two acids play an important role in the hippo’s health. They not only serve as a natural form of sunscreen and moisturizer for the animal’s sensitive skin, but also offer tremendous antibiotic properties to protect hippos from harmful bacteria when they’re in the water Now here’s where it gets weird; this special secretion comes out colorless like human sweat, but turns to a bright orange-red in the sun so that it looks like blood. A few hours later, it loses its blood-like luster and shifts into a dirty brown color.

Social media posts claiming that hippos produce pink milk are often accompanied by a photograph purportedly showing this mythical product: This image, however, does not show five bottles of pink hippo milk. The photograph actually displays the product a dairy-free Strawberry milkshake recipe called “Strawberry Unmilkshake” that was originally posted by the web site Veggie Wedgie in 2010.

National Geographic online may have perpetuated the rumor that hippos produce pink milk with a Facebook post in 2013, but the magazine’s page on the hippopotamus and its fact sheet on National Geographic Kids make no mention of this debunked legend.