The mammalian diving reflex optimizes respiration to allow mammals to stay underwater for a long time. It is exhibited strongly in aquatic mammals (seals, otters, dolphins, etc.), but exists in a weaker version in other mammals, including humans. Diving birds, such as penguins, have a similar reflex. Every animal’s diving reflex is triggered specifically by cold water contacting the face – water that is warmer than 21 °C (70 °F) does not cause the reflex, and neither does submersion of body parts other than the face.
Also, the reflex is always exhibited more dramatically, and thus can grant longer survival, in young individuals. Beyond the effect of the reflex when submerged in water, the reflex is used consciously in ‘splashing cold water on one’s face,’ either to relax when upset or wake up when drowsy. Additionally, the reflex is also elicited through water boarding torture, in order to create the sense of drowning upon the individual.
The Daily Omnivore 