Some Mice May Owe Their Monogamy to a Newly Evolved Type of Cell in the adrenal gland, which may have evolved only 20,000 years ago

Amazing stuff!

Unfortunately, Columbia University is an ideology driven elite university. The lead researcher on this study (assistant professor Bendesky) uses the cringy term "people" to refer to women! Quote: "... a drug to help treat the postpartum depression that people often experience after childbirth ..."

"What makes the oldfield mouse steadfastly monogamous throughout its life while its closest rodent relatives are promiscuous? The answer may be a previously unknown hormone-generating cell ...

More than 100 years of previous research has shown that the mice species behave in strikingly different ways. Whereas the deer mouse is promiscuous — even a single litter of pups can have four different fathers — the oldfield mouse mates for life. ...

To find out why these close mouse relatives behave so differently, the scientists examined their adrenal glands. 

"This pair of organs, located in the abdomen, produces many hormones important for behavior," ... "These include stress hormones such as adrenaline, but also a number of sex hormones." ...

The adrenal glands of these mice proved startlingly different in size. In adults, the adrenals of the monogamous mice are roughly six times heavier than those of promiscuous mice (after adjusting for differences in the body weight between the species).

"This extraordinary difference in the size of an internal organ between such closely related species is unprecedented," ...

Genetic analysis of the adrenal cells revealed that one gene, Akr1c18, saw far more activity in the monogamous mice than in the promiscuous rodents. The enzyme this gene encodes helps create a little-studied hormone known as 20⍺-OHP, which is also found in humans and other mammals. 

The researchers observed that increasing 20⍺-OHP hormone levels boosted nurturing behavior in both mouse species.  ...
"This marks the first time we found anything that could increase parental care in the promiscuous group," ...

Normally these glands are divided into three zones. But the scientists discovered that the adrenals of the monogamous mice possessed a fourth zone. ...
In zona inaudita [fourth zone] cells, the researchers found that 194 genes, including Akr1c18, were far more active compared with the same genes in other adrenal cells. ..."

From the abstract:

"Cell types with specialized functions fundamentally regulate animal behaviour, and yet the genetic mechanisms that underlie the emergence of novel cell types and their consequences for behaviour are not well understood1. Here we show that the monogamous oldfield mouse (Peromyscus polionotus) has recently evolved a novel cell type in the adrenal gland that expresses the enzyme AKR1C18, which converts progesterone into 20α-hydroxyprogesterone. We then demonstrate that 20α-hydroxyprogesterone is more abundant in oldfield mice, where it induces monogamous-typical parental behaviours, than in the closely related promiscuous deer mice (Peromyscus maniculatus). Using quantitative trait locus mapping in a cross between these species, we ultimately find interspecific genetic variation that drives expression of the nuclear protein GADD45A and the glycoprotein tenascin N, which contribute to the emergence and function of this cell type in oldfield mice. Our results provide an example by which the recent evolution of a new cell type in a gland outside the brain contributes to the evolution of social behaviour. ...
By measuring the number of mutations distinguishing these species, the scientists estimated this novel cell type evolved within the past 20,000 years, "which is just an eyeblink when it comes to evolution," ..."

Some Mice May Owe Their Monogamy to a Newly Evolved Type of Cell | Columbia | Zuckerman Institute Scientists discover the cells and hormones that inspire mice to nurture their young; the same hormones are also present in humans

Evolution of a novel adrenal cell type that promotes parental care (no public access)

The adrenal glands of a deer mouse (left) and oldfield mouse (right), showing the relative size of the zona fasciculata (green) and the novel zona inaudita (red).