Gentoo penguins propose to their mates by presenting them with the smoothest pebble they can find. If the female accepts, she will gift him one back. They will do this back-and-forth for weeks until they have the perfect amount of pretty pebbles to build their nest together.
Source
Potter, A. (n.d.). The gift to win a penguin’s heart. BBC Earth. Retrieved from https://www.bbcearth.com/news/the-gift-to-win-a-penguins-heart
