http://www.newsweek.com/newsweek-com-how-crocodile-ruthless-efficient-apex-predator-1773709

The Nile crocodile is one of the most dangerous animals in the world. They are very efficient hunters, relying on ambush tactics to indiscriminately capture any prey in their path. National Geographic estimates that up to 200 people are killed by these deadly reptiles every year, but what is the secret to their success?

It has long been known that crocodilians—crocodiles and alligators—are able to hold their breath underwater for an extraordinarily long time. This adaptation allows them to kill large mammal prey by dragging them into the water and drowning them. Their remarkable breath-holding capacity is believed to have evolved due to adaptive alterations in the protein that carries oxygen around their blood: hemoglobin.