This order contains the species penguins and rockhoppers, which inhabit the southern hemisphere. They can be recognised by their upright posture and rigid wings that cannot be folded along the body and can swim in a way that allows them to move efficiently underwater in pursuit of their prey. Adults have a thick layer of subcutaneous fat that ensures they can survive in areas with extremely low temperatures. On ice, they often move by sliding on their stomach. They are known to breed several times with the same partner in the same nest from year to year. Males and females both care for the nest and the young. Most species live in colonies of several hundred to thousands of individuals and are socially organised animals. Evolutionarily, their closest relatives belong to the order Procelariiformes.