It’s worth pausing here to reflect on how bizarre naked mole rats are in so many other ways. They are underground mammals that have lost their hair and sight. They don’t feel pain. They can survive 18 minutes without oxygen. They’re unusually immune to cancer. It might not be so far-fetched that their queens communicate through poo. Even if they weren’t making sex hormones themselves, the female subordinates’ attentive parenting would imply that they get the hormones somewhere, Mogi’s team reasoned. Plus, naked mole rats are known to eat feces. They decided to test the idea.
The team collected fecal pellets from pregnant queens and gave them to a handful of subordinate females, which soon became much more responsive to the cries of pups. Then they repeated the experiment to make sure the hormones were really the key component of the poop. This time, they took fecal pellets from nonpregnant queens and added estradiol—a type of estrogen—to only half of the pellets. Only the naked mole rats that ate the estradiol-supplement poop became more responsive to pup cries.
Mogi was excited. He had never seen hormones work like this before. Hormones are powerful mediators of behavior, but their effects are normally limited to the body of the animal making them. Here the queen seems to be making hormones to alter the bodies of totally separate animals. Insect colonies have sometimes been called superorganisms for the way thousands of individuals behave as one unit; in this case, hormones seem to be acting on naked-mole-rat colonies as a single superorganism.
Sue Carter, an endocrinologist at Indiana University, points out that research from the ’80s has suggested mother mice regulate their babies’ development through feces as well. “The whole idea that one organism would voluntarily ingest and be regulated by the fecal by-products of another,” she says, “makes us wonder what is going on,” especially since we humans are conditioned to be disgusted by waste.
Chris Faulkes, an evolutionary ecologist at Queen Mary University of London who studies naked mole rats, says he wishes the study had presented more evidence that female subordinates naturally eat the feces of queens. Naked mole rats definitely eat their own poop; it helps them get more nutrients out of their food. And queens have been documented to eat poop from other adults. Faulkes has never personally observed female subordinates begging or eating feces from a queen, though he also hadn’t been specifically looking for it. (Mogi told me his team did observe adults eating the queen’s feces out of habit.)
The paper also references in passing that, especially after the queen gives birth, female subordinates’ nipples sometimes become enlarged—despite the fact that they make no sex hormones of their own. Perhaps the hormones in the queen’s poop explain it. But Faulkes says he has also noticed whole colonies with “massive” nipples and others where the nipples of non-breeders never really develop. “That’s something that has really fascinated all of us mole-ratters for decades,” he says. And the mystery endures for now.