Human evolution has rewired male biology to be caregivers to their children, neuroscientist James Rilling says. Photo: Unsplash
It turns out dad bods are real thing, and there's a biological reason for it.
Human evolution has rewired male biology to be caregivers to their children, neuroscientist James Rilling says.
And the more men are involved in parenting early on, the more physiology kicks in to support a nurturing role, Rilling tells RNZ's Afternoons.
He explores the topic in his new book; Father Nature: The Science of Paternal Potential.
Men experience hormonal change when they become fathers, he said.
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"We're used to thinking about biological changes that women experience when they become mothers, very profound hormonal changes. But what's less well appreciated is that men also experience hormonal changes when they become fathers.
"When men become involved fathers, their testosterone levels decrease, and there's now emerging evidence that oxytocin levels are increasing."
Most mammalian species have little to do with infant rearing, he said, but among those that do, similar hormonal changes can be observed.
"Testosterone typically biases males towards investing more of their energy in mating and the pursuit of mates and the pursuit of social status, and it takes energy away from parenting.
"The thought is that when males become parents, this decrease in testosterone is an adaptive shift in their energy allocation, so they can use less energy for mating and more for parenting."
This drop in testosterone contributes to the so-called 'dad bod'.
"It is the case that testosterone basically burns fat, and so when your testosterone levels go down, that contributes to the accumulation of abdominal fat, and so that's quite well established now."
His research also showed men with smaller testicles were more nurturing dads.
"If you look across species, in species where males have large testes, basically they're investing a lot in mating effort, usually for sperm competition, and in those species, you tend to see less investment in parenting.
"And so, we had the thought that it might be interesting to look among our sample of human fathers to see if that variable was correlated with anything."
Men who had smaller testes were reported by their partner as being more involved in the day-to-day, instrumental care of their children, he said.
"We found some differences in terms of brain function, in terms of how the father's brain responded to looking at pictures of their toddler."
Men paid a price for that reduced testosterone, he said.
"Low testosterone is a risk factor for depression. Another thing people don't realise we often talk about postpartum depression in new mothers, and that is a is a big problem, but it's also the case that rates of depression are about twice as high in new fathers compared to men who are not new fathers."
While other factors contribute to that, he believed a decline in testosterone was a factor in these elevated rates of depression among new fathers.