What triggers the mammalian dive reflex?

The mammalian dive reflex is a physiological response that occurs when a mammal's face is submerged in water, or when it experiences certain environmental conditions related to temperature. This reflex includes a series of changes in heart rate, blood flow redistribution, and oxygen conservation to help the organism survive while submerged.

When considering the trigger for this reflex, environmental temperature plays a key role. Specifically, the reflex is more pronounced and effectively activated at lower temperatures, particularly those below 21 degrees Celsius. This is because colder water can stimulate the receptors in the face, particularly when the nose and mouth are submerged, signaling the body to engage in the dive reflex to conserve oxygen and reduce metabolic demands.

While icy water can certainly trigger the dive reflex, the more accurate choice highlights the critical role of the environmental temperature at or below the threshold of 21 degrees Celsius for this reflex to be effectively activated in a broader context.