All mammals — whether on land or in the sea — have adaptations to help them survive in cold water. One adaptation is known as the mammalian diving reflex (or MDR). When exposed to cold water, all mammals' bodies respond in a similar way. The heart rate slows, and the blood vessels in the arms, hands, legs and feet constrict, or tighten. These changes help the body conserve oxygen, and make sure that the brain, heart and other vital organs get the most blood flow. The MDR happens in all mammals, from sea lions to your dog to YOU. Marine mammals stay underwater for long periods of time and have additional adaptations to help them do this. For example, animals such as whales and seals can handle more carbon dioxide in their blood than land mammals can. Overview: In this activity, you and an adult will explore how cold water affects your circulation. By measuring the pulse on your neck and your wrist before and after you place your face in icy water, you'll see for yourself how the mammalian diving reflex works. Scientists have found that the mammalian diving response is much stronger in children than it is in adults. This is one reason why rescue workers have been able to revive children who were underwater for more than an hour.