Kanga and Roo never explained this.
In this episode of “Animals are Better Than Us At Literally Everything” here comes the Kangaroo. Not only could it probably beat Floyd Mayweather in a boxing match, but the Duggars have nothing on these procreators. Female kangaroos, or “does”, can stay pregnant continually, making sure there are plenty of roos to box.
Dolphins are pretty smart.
In addition to playing games, dolphins are all smart enough to be curious about what their naughty bits look like. Though to be fair, maybe not every animal has had equal enough access to mirrors for us to know if dolphins are truly unique in this regard. But considering what dolphin penises look like, it’s not that weird to think they’d wanna have a gander.