Adelie and Gentoo penguins have a thing for pebbles and small stones. Males fight over them as though they were valuable gems, and couples prize them for their role in nest building. They are, in fact, a symbol of penguin love. When a male offers a pebble to a female, it is a token of his desire to settle down and have a family. If she accepts and places it in her nest, they are officially a couple.
More practically, these penguins need pebbles and other small rocks to properly construct their nests. Because they live in harsh, barren environments such as Antarctica, pebbles can be difficult to find. Males are notorious for stealing pebbles from each other and fighting ferociously over these valuable little rocks.
More on the mating habits of penguins: