Most mammals grow up in single parent homes. It’s estimated that over 95 percent of the planet’s nearly 6,000 known mammalian species rely almost exclusively on mothers to nurture and raise their offspring. But even when dads stick around, it’s not always smooth sailing. Fatherhood can range from attentive and caring to downright violent behaviors—but why this spectrum exists remains largely a mystery to evolutionary biologists.
Take the African striped mouse (Rhabdomys pumilio), for instance. Males can exhibit diverse responses to their young after becoming fathers. Particularly caring mice dads will groom their pups and even insulate them with their bellies against inclement weather. Meanwhile, other fathers may ignore or hurt a litter’s weaker siblings.
Although a simplified example, the striped mouse can serve as a proxy for other mammals including humans. Knowing this, researchers at Princeton University recently investigated the neurological underpinnings of rodent fatherhood. Their new study published today in the journal Nature indicates that a specific molecular group inside the brain may largely determine how dads react to their progeny.
To learn more, the team recorded the neural activity of male striped mice when placed in various situations both with and without pups. They soon noticed that neural activity in the brain’s medial preoptic area (MPOA) increased whenever the males encountered a young mouse. These MPOA spikes weren’t uniform, however. Higher activity in the region corresponded with helpfulness, while lower recordings aligned with hostility. Although this isn’t the first time that biologists noted MPOA’s relation to parenting, past research largely linked it to rodent females after becoming mothers.
“But in the case of these males, it’s not pregnancy or even parenthood that transforms their brains,” Forrest Rogers, a neuroscientist and study co-author, said in a statement. “Bachelors can be just as capable of caring as experienced dads.”
Rogers and his colleagues noticed MPOA was not the only area that tied to parenting. Surprisingly, the more caring mice dads also displayed lower levels of a gene called Agouti. This gene is typically known for its influence on metabolism and skin pigmentation, not fatherhood.
“Discovering this previously unknown role in the brain for parenting behavior was exciting,” said Rogers.
After finding this new link, researchers wanted to know what conditions influenced Agouti gene expression in the MPOA. Contrary to what one may initially assume, they found that solitary males possessed low levels of Agouti compared to males who lived in groups. Especially high levels also sometimes muted neural activity in the MPOA.
Artificially boosting Agouti through gene therapy reinforced these observations, too. Male mice who were previously nurturing became less interested or even volatile towards pups if they produced more Agouti. As a remedy, the team later relocated some of these males from communal to solitary living conditions. This naturally lowered their Agouti levels, making them more interested in the mice pups again.
“Our findings point to Agouti as a potential evolutionary mechanism that allows animals to integrate environmental information, such as social competition or population density, and adjust the balance between self-preservation and investment in offspring,” added study co-author and neuroscientist Catherine Peña.
Forrest, Peña, and their collaborators are still interested in examining which specific environmental factors may influence Agouti levels in mammals like striped mice and humans. At the same time, they warned against viewing their findings as a one-size-fits-all solution for parenting behaviors. Instead, they hope to help other researchers identify factors that may contribute to higher risks of issues like father figure neglect or abuse.
“Parenting is a complex trait. We’re not suggesting that you can take a pill to become a better parent, or that struggles with parenting reflect some molecular deficiency,” said Peña.
