The gift

For gentoo penguins, it's all about who's got the best rocks. Like a human offering jewelry, this gentoo male at the Edinburgh Zoo presents a large pebble on Tuesday in hopes of winning over a female.

But sometimes finding the flashiest gifts isn't enough. During the mating season—which begins in March and lasts up to six weeks—males suffering from "pebble envy" will steal the best-looking rocks and pawn them off as their own.

The pebbles will eventually become parts of the penguins' nests—large circular piles of stone that can reach 8 inches (20 centimeters) in height and 10 inches (25 centimeters) in diameter.