Anecdotal I know, but when my first was born and I was outside with them for the first time, I got a weird sense of extreme overprotectiveness - exaggerating somewhat to make the point, but it almost felt like I would punch anyone if they looked vaguely threateningly at my new child (a particularly memorable experience for me because I'm usually such a placid person). Not an expert, but that felt like it could be something hormonal.

And at risk of sounding a bit handwavy, I think I've adapted to being a parent much better than I thought I would, which I guess could be thanks to some biological help.

Also used to look a lot younger than I actually was, and now I look my age or even older, but that could simply be lack of sleep.