The crocodile, which can remain under water without breathing for up to 1 h, drowns its air-breathing prey and then dines at its leisure. An adaptation that aids the crocodile in doing so is that it can utilize virtually 100% of the O2 in its blood whereas humans, for example, can extract only ~65% of the O2 in their blood. Crocodile Hb does not bind BPG. However, crocodile deoxyHb preferentially binds HCO3-. How does this help the crocodile obtain its dinner?