Fathers of newborn babies can be forgiven for feeling a little hormonal this Father’s Day.
A new study has shown they are being flooded by the same chemical as a breastfeeding mother.
In the mother, the hormone activates lactation. In the father, it makes him a better dad.
New Dunedin dad and ODT reporter Jacob McSweeny (31) said he had not thought about his own emotions after his daughter Lorea was born on August 18, but had noticed some changes.
It was a feeling of awe and love, rather than tear-jerking emotions, he said.
"It is a really nice warm feeling."
He said he had not thought about his emotions, but was focused on thinking practically.
"Change a nappy, get baby dressed, or get food for mum, or deal with your other child who needs stuff."
But it was perhaps the prolactin that had influenced that practical thinking, he said.
"Perhaps me acting practically is an example of that prolactin increase, where I am being a good dad or doing the things I need to get done for the family to be functional."
But it certainly had not kicked in when his wife Brooke was in the early stages of pregnancy.
"The joke at the start was that I kept forgetting that she was pregnant ... that shows my head was not in it until ... she was quite close to the time of the birth."
The University of Otago study, with international partners, shed new light on the role of prolactin.
The research, which is published in the journal Cell, demonstrated that prolactin directly influences male behaviour by unlocking the paternal instinct.
Conversely, if the action of the hormone is blocked, male parents will ignore their duties.
Raised levels of prolactin in the blood of new fathers have previously been observed in different species, and in male human parents it rises both pre-birth and during parenting activities such as holding their child.
But its presence was not fully understood, until now.
Dr Kristina Smiley, from the Centre for Neuroendocrinology’s department of anatomy, said the two-year study looked at prolactin function in rats and mice.
Male rats ignore their offspring, whereas male mice are actively involved in the care of their pups.
When researchers elevated prolactin levels in male rats, they engaged in paternal care such as grooming their pups.
In contrast, when male mice had their prolactin blocked, they stopped taking care of their pups.
“It's a really dramatic change," Dr Smiley said.
But paternal behaviour was not induced when prolactin levels were raised in virgin males.
The research team would now look at how surges of prolactin might change the male’s brain after mating to prepare them for fatherhood.
Mr McSweeny said there was one element to the study that had not surprised him.
"I never trusted rats, and it shows they don’t have prolactin, and they are bad fathers.
"Mice just look friendlier, so, no surprises there."
It is Father’s Day tomorrow.
