A secondary feature of the mammalian diving reflex is what?

• A. Diverts blood centrally after going in cold water
• B. Lowers heart rate
• C. Increases respiratory rate
• D. Stimulates sweating

Answer: (B)

The diving reflex is an oxygen-conserving response that kicks in when the face contacts cold water, triggering a set of autonomic changes to prioritize brain and heart function during submersion. The best example of a secondary feature is the heart rate slowing down—bradycardia. This reduction in pulse lowers overall oxygen consumption, helping to extend the time before hypoxia worsens. While blood flow is also redirected toward vital organs through vasoconstriction, the question highlights the cardiac adjustment, so the slower heart rate is the most representative secondary feature. The other options don’t fit: increasing respiratory rate isn’t part of this reflex (apnea is more typical), sweating is unrelated, and while central blood diversion occurs, it describes a vascular change rather than the cardiac response being tested.