Monogamy is a pretty strange thing in the animal world. Some animals do it, but most don’t. Sometimes, it brings clear practical advantages, but other times, it’s not apparent why some animals are monogamous. This also goes for mice.

The new study looked at the deer mouse (Peromyscus maniculatus), which ranges all across North America, and the oldfield mouse (Peromyscus polionotus), which mostly lives in Florida and Georgia. The mice are similar in many ways and are closely related biologically. However, their love life is vastly different.

Whereas the oldfield mouse mates for life, the deer mouse will get it on as much as possible. Researchers wanted to see why two similar species can behave so differently when it comes to promiscuity.

It’s not the first time the two species have been compared. Researchers looked at their skulls, teeth, overall anatomy, and even their genetics — and didn’t find big differences. If there was one thing causing the monogamy difference, it would likely be a hormone.

So researchers looked more closely at their adrenal glands.

The size difference of these organs between the two species is stunning. Gram per gram, the adrenals of the monogamous mice are roughly six times heavier than those of promiscuous mice. They also produce a lot of hormones, including ones that are quite likely linked to sexual behavior.

“This pair of organs, located in the abdomen, produces many hormones important for behavior,” said Andrés Bendesky, MD, PhD, a principal investigator at Columbia’s Zuckerman Institute. “These include stress hormones such as adrenaline, but also a number of sex hormones.”

“This extraordinary difference in the size of an internal organ between such closely related species is unprecedented,” Bendesky said.

The researchers note that while these glands are divided into three zones, the monogamous mouse has a fourth zone.

“We called this the zona inaudita, which is Latin for ‘previously unheard-of zone,’ because no one has ever observed this type of cell in another animal,” said Natalie Niepoth, Ph.D., a co-first author on the study who is now a senior scientist at Regeneron.

In this “zona inaudita”, the researchers uncovered 194 genes that were very active. In particular, one gene, called Akr1c18, was much more active in the monogamous mice. This enzyme helps create a hormone called 20⍺-OHP. The hormone is poorly understood, even though it is found in many more mammal species, including humans.

“The hormone from these cells was actually first discovered in humans many decades ago, but nobody really knew what it did,” said “We’ve discovered that it can promote nurturing in mice, which gives us an idea of what it might be doing in humans.”

The researchers suspect that one of the key differences leading to this unexpected monogamy may be linked to this hormone, although this is still speculative at the moment. Ultimately, the team hopes that their findings will spur more research, not just in mice but also in humans.

“I hope that our study motivates further investigation into the link between 20⍺-OHP and parenting in humans,” said Jennifer R. Merritt, Ph.D., a co-first author on the study and postdoctoral researcher in the Bendesky lab. “We have so much to learn about the role this hormone plays in human parental behavior.”

Journal Reference: Andres Bendesky, Evolution of a novel adrenal cell type that promotes parental care, Nature (2024). DOI: 10.1038/s41586-024-07423-y. www.nature.com/articles/s41586-024-07423-y