Black cats bring bad luck, according to superstition. But the same quirks of biology that make some cats black might also have protected the dark-haired felines from diseases a long time ago.

In a range of animals, from mice to sheep, scientists have already identified two genes that play a role in coat color. Depending on how they work together, the two genes make an animal's fur look a range of colors, from reddish-yellow to blackish-brown.

Now, a new study shows that solid-black house cats have a certain mutation in one of those genes. Black jaguars have a distinctive mutation in the other gene, and this defect is missing in jaguars that are more typically yellowish-brown in color. Meanwhile, dark-brown jaguarundis—felines native to South and Central America—have their own particular mutation in the second gene.

The new findings made the researchers wonder why some cats are black in the first place. Camouflage at night is one explanation. Other research on coat-color genes suggests that the same mutation that makes some cats black might also have helped them resist a deadly infection thousands of years ago.

So black cats may actually be the lucky ones after all.—E. Sohn