One visitor to the Inokashira Park Zoo outside of Tokyo got an unusual sight early this month: A squirrel with what appears to be large mammary glands.
The photo, posted on Twitter earlier this month, has been shared more than 47,000 times and received 141,000 likes.
But what’s going on in this photo?
It’s not what it looks like, says Jessica Haines, an ecologist at MacEwan University in Edmonton, Canada. Squirrels, you see, “have nipples that go all the way up and down their chest,” says Haines, who researches red squirrels in Alberta. In other words, they’re not centered in the pectoral region like they are in humans.
And they don’t just have two. Mammary gland count differs by species but can reach up to ten for squirrels.
And for those of you who think the squirrel—which does appear to be a female—might be lactating, Haines ruled out that hypothesis, too. This is because when squirrels are nursing pups, their nipples elongate, and the skin around them loses its fur, all of which make a lactating female recognizable from a distance.
For scientists studying squirrels, “most of us are so trained that we can see large nipples with the naked eye when a squirrel is about 30 to 40 feet into a tree,” says Ben Dantzer, an integrative biologist at the University of Michigan.
Dantzer’s best guess is that what’s going on in this photo is a combination of an unusual pose mixed with an excess of fat. But this is strange, too, because tree squirrels like this species don’t really get fat in the wild.
Instead of storing fat on their bodies, squirrels hide their seeds and nuts—and the calories they contain—belowground or in tree crevices. (Read: 5 Surprising Facts About Squirrels)
“I suspect that this squirrel, just by looking at the picture, probably lives around humans and eats a lot of garbage and fatty foods,” says Dantzer.
While it probably feels a little juvenile to focus so much attention on the chest of a squirrel, the viral photo might as well be an opportunity to celebrate the many strategies mammals have developed for feeding their young.
Female humans have two mammary glands located on the pectorals, of course. But did you know this is also where elephants, hyraxes, and manatees feed their offspring?
Many other mammals keep their mammary glands further down the belly, such as dogs, cats, and horses. Cows also have mammary glands near the pelvis, but each of their four separate glands has fused into a single structure called an udder.
“Each quarter has its own teat and ductal system that's entirely separate from the other quarters,” says Amy Skibiel, a lactation physiologist at the University of Idaho.
Believe it not, some species of bats seem to have the best of both worlds, sporting both pectoral and pelvic nipples. However, only the top pair produce milk. The bottom ones are called false nipples, and are actually used as handles for baby bats to hold onto while their mother is in flight.
And then there are the monotremes, like the duck-billed platypus and echidna. They don’t have nipples at all.
“Monotremes have what’s called an areola, which is a mammary patch,” says Skibiel. “The milk kind of oozes out onto the patch, and the young lap it up.”
Editor's note: This story has been updated.