The deer mouse, considered the most common mammal in North America, has a very different view of family values than its evolutionary sibling, the sand mouse.
Sand mice are monogamous. Fathers groom their young, keep them warm and ensure that they do not stray far from the nest. The deer mouse prefers variety when it comes to sexual partners. It is not uncommon for babies from the same litter to come from four different fathers. As for male deer mice, they are downright careless. Nothing, it seems, can bring out warm behavior in fathers.
So far. Because researchers at Columbia University who studied the two species of mice have discovered what appears to be a crucial difference: Sand mice produce an adrenal cell that is not found in other mice. This cell produces a hormone that, when injected into virgin deer mice of both sexes, causes 17 percent of them — even the males — to groom their young and keep them close to the nest.
However, this had no effect on the deer mouse’s tendency to pursue female conquests. This did not make them want to spend more time with their partner, observed Andrés Bendesky, one of the authors of an article published last Wednesday in the journal Nature describing the research.
By examining other mouse species, Bendesky and his team determined that the newly discovered adrenal cell evolved in sand mice about 20,000 years ago, the equivalent of a “wink of an eye.” » on the evolutionary time scale.
Although child-rearing and monogamy are distinct characteristics, they are biologically linked, said Mr. Bendesky, a senior researcher at Columbia’s Zuckerman Institute.
The vast majority of mammals — 92%, according to Bendesky — are as sexually promiscuous as 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 of these species, males do not participate in raising the young. According to Mr. Bendesky, there are only three species in which males participate in raising children: the striped mongoose, the gray bamboo lemur and the Goeldi’s tamarin.
“All three came from a recently monogamous ancestor,” Bendesky said, “which supports the close and enduring connection” between monogamy and shared parenting.
The issue of monogamy in the animal kingdom remains controversial, with some scientists claiming that only 3 to 5 percent of mammals are monogamous.
Researchers distinguish between two types of monogamy: social monogamy, in which partners mate and live together for one or more breeding seasons, and genetic monogamy, in which couples mate exclusively with each other .
There are different theories about the evolutionary advantage that monogamy confers on males. Some scientists argue that staying home with a mate, rather than prowling in search of other females, may have been a way to prevent competing males from devouring their young. Another explanation is that males simply found it easier to keep rival males away from a single female, rather than several.
Mr. Bendesky, who has been studying the difference between sand mice and deer mice for 12 years, found an unexpected clue in the anatomy of the two species. Each of the sand mouse’s two adrenal glands weighs 7 milligrams, more than four times the weight of those of the deer mouse.
“It’s huge,” Bendesky said of the difference. When scientists bred mice to exhibit more or less anxiety — a feeling derived from hormones made in the adrenal gland — they never found a difference in gland size greater than 20 percent.
The adrenal glands are one of the main sources of steroid hormones, which play an important role in regulating behavior, including parental care. The large difference in adrenal gland size suggests that sand mice produce higher levels of steroid hormones, at least some of them.
Taking a closer look at the differences between species, scientists discovered that each sand mouse adrenal gland has four layers, or zones, instead of three in the deer mouse. This is the fourth, called zona inauditawhich contains the new adrenal cell.
By performing genetic analysis, scientists established that the newly discovered cell was different from other adrenal cells. They found that 194 genes were more active in the newly discovered cells than in other cells. The level of gene activity can be increased or decreased, just as a light bulb can be dimmed using a dimmer switch.
In the newly discovered cells, sand mice produce a hormone called 20a-hydroxyprogesterone, which was discovered in humans in 1958.
“But no one knew what this hormone actually did in humans,” Bendesky said.
In this sense, the hormone is very similar to the organ that produces it. The adrenal gland, first described in 1564, was such an enigma to scientists that in 1716 the Bordeaux Academy of Sciences held an essay competition to determine the usefulness of this organ. No submissions were deemed worthy of the prize.
It was only much later that the discovery of diseases such as adrenal insufficiency clarified its role in the production of hormones involved in the regulation of metabolism, immunity, blood pressure and stress response.
Bendesky and his colleagues’ research revealing the new cell type surprised other scientists.
“It’s extraordinary,” 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 for some time. “The most interesting element is the origin of what appears to be a new type of cell. »
Phelps noted that in his 30-year career, he could not recall such a discovery ever occurring.
“What really excited me about this paper is the idea that this hormone produced in the adrenal gland” is then broken down and used in the brain to influence caregiving behavior, said Jessica Tollkuhn, associate professor. at Cold Spring Harbor Laboratory, which was not involved in the study.
“This is a new aspect of biology that has never been described before,” said M.me Tollkuhn.
Margaret M. McCarthy, a professor of pharmacology at the University of Maryland School of Medicine, expressed surprise that evolution had instilled parental behavior in such a complex way. According to her, regulating the brain using a hormone forged in the adrenal gland is less direct than the simple development of a new neuronal circuit.
“That’s what happened with voles, where you find monogamous and non-monogamous voles,” McCarthy said, referring to these small rodents that are sometimes confused with mice. “Evolution always surprises. There are a million ways to solve a problem. »
According to scientists, the results obtained in mice could help to better understand parental behavior in humans.
In mice, the parental hormone is often converted to a compound that closely resembles allopregnanolone, which was approved by the Food and Drug Administration in 2019 to treat depression postpartum. The drug is known as brexanolone and sold under the brand name Zulresso.
Tali Kimchi, associate professor at the Weizmann Institute of Science in Israel, noted via email that the paper opens the way for further research into depression postpartum“one of the most devastating and incurable psychopathologies known to us, with lasting and sometimes even fatal effects on both parents and offspring.”