Roto and Copper, two Gentoo penguins at Loveland Living Planet Aquarium in Utah, cared for three children together, taking turns feeding them. They’re a social pair, just like Coco and Gossamer, a neighboring penguin couple that raised their own chicks.

Or did they?

We tend to think of penguins as monogamous, with social bonds formed between two parents for life. But researchers have discovered that penguins in captivity, like some species in the wild, sometimes stray. After sampling the DNA of 19 Gentoo penguins at the aquarium, researchers revealed last month in the journal ZooBiology that Roto is the father of two chicks believed to be Gossamer’s offspring.

“We’ll go back to the classic movies where the male gives the female a rock and they start to build their nest with it, and they are totally monogamous,” Steve Vogel, the aquarium’s zoological operations director said, “and that is not true 20 percent of the time.” At least not at this aquarium.