Crocodiles of the Nile often attack and kill large animals (such as wildebeest) by dragging them under the water and drowning them! Crocodiles can do this because they have the amazing ability to stay under the water for an hour or more. Part of the explanation lies in the structure of crocodile hemoglobin. Bicarbonate ions can bind to crocodile hemoglobin and displace oxygen. Why should the increased affinity of crocodile hemoglobin for bicarbonate ion allow the crocodile to hold its breath for long periods of time?