Some mice have a cheating heart. It’s a hormonal thing, scientists find.

The deer mouse, believed to be the most common mammal in North America, has a very different view of family values ​​than its evolutionary sister, the field mouse.

Oldfield mice are monogamous. Fathers groom their young, keep them warm and make sure they don’t stray far from the nest. The deer mouse prefers the swinging lifestyle when it comes to sexual partners. It is not unusual for the pups in one litter to come from four different fathers. As for the deer mouse fathers, they are negligent. Nothing, it seemed, could bring out dad’s warm and fuzzy behavior.

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Until now. Researchers at Columbia University who investigated the two species of mice found what appears to be a crucial difference: Oldfield produces an adrenal cell not found in other mice. The cell makes a hormone, which when inserted into virgin deer mice of both sexes prompted 17 percent – even males – to stick their young and keep them close to the nest.

Alas, it had no effect when it came to the deer mouse’s choice to play the field with multiple female partners.

He didn’t want them to spend more time with their colleagues, said Andrés Bendesky, one of the authors of a paper published Wednesday in the journal Nature describing the research.

By examining other mouse species, Bendesky and his team determined that the newly discovered cell type had evolved in old field mice about 20,000 years ago, essentially a “blink of an eye” on the evolutionary time scale.

Although parenting and monogamy are separate traits, they are linked in biology, said Bendesky, principal investigator at Columbia’s Zuckerman Institute.

The vast majority of mammals – 92 percent, according to Bendesky – are promiscuous like the deer mouse. When female deer mice are in heat, they sometimes mate with multiple males on the same night, allowing different fathers to fertilize different eggs.

In most promiscuous species, males do not participate in the care of the young. Bendesky said there are only three outliers; obvious species where the males actually help with parenting: the banded mongoose, the gray bamboo lemur and the Goeldi monkey.

“All three are derived from a recent monogamous ancestor,” Bendesky said, “underscoring the close and enduring link” between monogamy and shared parenting.

Monogamy is still a controversial topic in the animal kingdom, with some scientists saying that between 3 and 5 percent of mammals are monogamous.

Researchers refer to two distinct types of monogamy: social monogamy, in which partners are married and live together for one or more breeding seasons; and genetic monogamy, in exclusively married couples.

There are various theories about the evolutionary advantage that monogamy gives to men. Some scientists say that staying home with a mate, rather than prowling for other females, may be a way to keep competing males from snacking on the offspring. Another explanation is that it was much easier for males to keep competing males away from one female than from other females.

Bendesky, who has been studying the difference between oldfield mice and deer for 12 years, said he found an unexpected clue in the anatomy of the two species. Each of the two adrenal glands of an old mouse weighs 7 milligrams – more than four times heavier than deer mice.

“It’s huge,” Bendesky said of the difference. When scientists bred mice to show more or less anxiety – a feeling derived from hormones manufactured in the adrenal gland – they never found a difference in gland size of more than 20 percent.

The adrenal glands are one of the main sources of steroid hormones, which act as important regulators of behavior, including parental care. The large difference in adrenal size suggested that the oldfield mice were producing more of at least some of the steroid hormones.

When the scientists looked more closely at the differences between the species, they discovered that each adrenal gland in the Oldfield mouse has four rows or zones, instead of the three in the deer mouse. The fourth is called the zona inspection (Latin for “zone not heard before”) in which the new adrenal cell is located.

Scientists discovered that the newly discovered cell was different from other adrenal cells through genetic analysis. They found that 194 genes were turned up higher in the newly discovered cells than in other cells. The activity level of the genes can be turned up or down, just as light can be adjusted with a dimmer switch.

In the newly discovered cells, oldfield mice make a hormone called 20 alpha-hydroxyprogesterone (20alpha-OHP), which was discovered in humans in 1958.

“But nobody really knew what it does in people,” Bendesky said.

In that sense, the hormone was much like the organ that produced it. The adrenal gland, first described in 1564, was so important to scientists that in 1716 the Bordeaux Academy of Sciences sponsored an essay competition to determine the organ’s purpose. No entry was deemed deserving of the prize.

Much later, the discovery of diseases such as adrenal insufficiency helped to regulate the production of hormones related to metabolism, immunity, blood pressure and response to stress.

Bendesky and his colleagues’ research that revealed the new type of cell surprised other scientists.

“It’s amazing,” said Steven M. Phelps, a professor of integrative biology at the University of Texas at Austin, who was not involved in the study but has been following Bendesky’s work on deer and ancient cloned mice for some time. “The most exciting piece is the origin of the appearance of a new type of cell.”

Phelps said it marked the first time in his 30 years in the field that he could remember such a discovery of a new type of cell.

“What was really exciting to me about the paper was the idea that this hormone produced in the adrenal gland” is broken down and used in the brain to influence caregiving behavior, said Jessica Tollkuhn, associate professor at Cold Spring Harbor Laboratory, who were not involved in the study.

“This is really a new aspect of biology that hasn’t been described before,” Tollkuhn said.

Margaret M. McCarthy, professor of pharmacology at the University of Maryland School of Medicine, expressed surprise that evolution has driven parental behavior in such a complex way. Regulating the brain with a hormone forged in the adrenal gland was less straightforward, she said, than developing a new neural circuit.

“That’s what happened in voles where you have the monogamous and non-monogamous voles,” McCarthy said, referring to the small rodents that are sometimes mistaken for mice. “Evolution always surprises. There are a million ways to solve a problem.”

The mice findings could lead to insights into parenting behavior in humans, scientists said.

In mice, the parenting hormone is often converted into a compound that closely resembles allopregnanolone, which was approved by the Food and Drug Administration in 2019 to treat postpartum depression. The medication is called brexanolone and is sold under the brand name Zulresso.

Tali Kimchi, an associate professor at the Weizmann Institute of Science in Israel, said by email that the Nature paper raises the possibilities for deeper research into postpartum depression, “one of the most devastating, incurable psychopathologies we know, with long lasting and sometimes even. fatal effects on both parents and offspring.”

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