If you didn't know the way, you weren't a man. (1.68)
This has both literal and figurative meanings. Literally: the way from the Boys' Island to the main island. Figuratively: finding the proverbial path to manhood. Becoming an adult is about finding direction. And everyone knows that men don't ask for directions!
It would be better to drown than [to fail]. Everyone would know you'd failed, and you'd probably never get a wife, and if you did get a wife, she'd be a woman none of the real men wanted, with bad teeth and smelly breath. (1.72)
Mau is worried about failing his culture's coming-of-age ritual, and the punishment for failing is seriously dire. Many things are the same across cultures. Like in this case: men want hot wives.
MEN HELP OTHER MEN (1.75)
This is a carved onto the tree on the Boys' Island. You can't become a man by yourself, you have to have help. And a real man helps others find themselves. Isn't that nice? We sometimes think that men are supposed to be all loners—but Mau's culture sees men as being part of a community.
Animals flee. His father had told him so. Boys flee. A man does not flee. (1.93)
Mau considers not trying to seek shelter from the incoming wave. However, it's the fleeing that saves his life. So, does this mean that being a boy is like being an animal and being a man is about finding humanity but paying attention the animal instinct within? It's not quite civilization = manliness, but it's something close.
Only the best men, the greatest hunters and warriors, became Grandfathers when they died. (2.59)
So, the best men are hunters and warriors? Why does killing things make them the best? We take it none of the Grandfathers were vegetarian.
Now [Mau] could kill things. And wasn't that part of being a man? (2.87)
Goodness gracious, it's kill kill kill to Mau and his tribe. Is it possible to be a great man without killing something? (Well, maybe—but probably not if you live in a tribal community that depends on hunting for survival.)
"Even great heroes of history [...] wouldn't get down in the dirt because a baby was dying of hungry and crawl up to a pig and [...] YUCK, but... in a good way." (5.31)
Daphne respects Mau while also being revolted when she hears he suckled a pig—with his own mouth—to get milk for the baby. Doing what it takes is the mark of a true man. We can't picture Arnold Schwarzenegger suckling a pig. He's not enough of a man for that. Mau is.
Because what? [...] Because I must act like a man, or they will think less of me? (5.35)
Mau's worried about keeping up appearances so that his people will respect him. It seems like they don't really get behind a strong female leader either... Anyway, it seems that a lot of being a man is simply acting like one, whether you are or not. Fake it 'til you make it!
This warrior is fighting Death. (5.77)
The Grandfathers don't think much of Mau for rooting around in the mud and suckling a pig. But Mau did it to save a baby: he was fighting Death. You can't fight Death by killing. That's just fighting for Death. Mau's trying to save lives.
"[Isaac Newton] was not a warrior, he had no spear, but the sun and the moon spun inside his head and he stood on the shoulders of giants." (15.167)
A-ha! So it is possible to be a great man by using brains instead of brawn. Mau triumphs at the end with a combination of both. (Fun fact: Newton actually said in a letter that he was "standing on the shoulders of Giants."