Studies of the prefrontal cortex structure of first-time and experienced marmoset parents show that "Fatherhood produces changes in very high-cognitive-level areas" said Yvesgenia Kozorovitskiy of Princeton University.
It's important to remember, though, that this information is coming from people who live and work with small monkeys.
Fatherhood boosts male brains [newscientist.com via dailyindia, thanks to dt reader buck]
That monkey page is honestly one of the saddest/funniest things I have ever seen. I can't look away from it.
[neither can I, but only because all the glitter animation freezes my browser window every damn time I visit. -ed.]