Saturday, April 3, 2010

Mammalian Diving Reflex

Mammalian diving reflex optimizes respiration to allow mammals to stay underwater for extended periods of time. When an animal's face comes in contact with cold water, a reflex is triggered that may lower heart rate, induce vasoconstriction, or shift blood around the body. Humans, being mammals, have may exercise this reflex to some extent, but to one not nearly as adept as that of seals, which may lower their heart rate from 125 to as low as 10 beats per minute. From our circulation unit, how is it possible that the heart muscle may still supply the body with oxygen? Does this make reptiles (such as alligators) similar to mammals? And, how is it possible that two species that diverged much earlier than the aquatic animals which returned to water both display traits that are beneficial towards survival in cold water submersion via the face? Why is it that humans may only exercise such a reflex to the limited extent? What may have caused this limitation?


  1. The mammalian diving reflex is made possible by a few adaptations in aquatic mammals (although humans do experience the reflex to a lesser extent). The main adaptation of diving mammals is the prevalence of myoglobin, an oxygen-storing protein, in their muscles. The presence of myoglobin allows animals such as the Weddell seal to store 25% of its oxygen in the muscles so that breathing is not as important for the time being. In addition, the heart rate slows and so does oxygen consumption; this slower metabolism means that the heart doesn't have to provide as much oxygen to the body cells. The oxygen that it does pump via the blood is directed toward the vital organs such as the brain and spinal cord. (Campbell 926) These adaptations allow less oxygen to be used during prolonged dives, and thus diving mammals can last a long time underwater without replenishing their air.

    As for the question of the relationship of diving mammals to aquatic reptiles such as alligators, they are not very similar at all. Jerry Coyne includes several diagrams in his book which show that the alligator first appeared, along with many other reptiles, during the Permian period, while aquatic mammals first began their move back to the water around 50 million years ago (pg. 27). This difference in when the species diverged has led to some major differences. For example, alligators have one-way breathing instead of the dead-end breathing of mammals (Wikipedia), in which air passes into and out of the alveoli through the same pathway. In addition, alligators don't contain as much myoglobin as aquatic mammals, and they live in a different climate. Alligators prefer fresh water, while most aquatic mammals live in salt water. Finally, the mammals are thermoregulators, so they must worry about keeping their body temperatures at acceptable levels while diving; on the contrary, alligators are thermoconformers and thus can't dive very deep for fear of losing any source of heat.

    As for the reason that humans developed a mammalian diving reflex, I look at the fact that humans diverged later in the evolutionary pathway than whales, dolphins, and seals. Whereas those aquatic mammals first appeared about 50 million years ago, the first humans and humanoid primates began to appear about eight million years ago (197). This means that the genes which regulate the mammalian diving reflex must have existed in all mammals by the time that aquatic mammals were selected to express them strongly. Genes don't disappear over evolutionary history; they can simply change levels of expression or perhaps become "dead". Therefore, the genes which regulate the mammalian diving reflex in aquatic animals also appear in humans, although to a lesser extent since natural selection has selected for the expression of genes that suit us to life on land. For example, humans can store 13% of their oxygen in muscles via myoglobin (Campbell 926). This is a smaller number than that of the Weddell seal, but it represents a good deal of oxygen nonetheless. It shows the relationship between humans and other mammals; this is a common trait.

  2. When submerged underwater, the supply of oxygen is cut off from the circulatory system. The heart's ability to then supply the body with O2 is limited to the remaining amount of O2 that the alveoli can transfer into the blood stream. The Mammalian Diving Reflex simply maximizes the intake of the remainder of O2 by using the minimum amount of O2. The reflex is only available for a limited time because there is a minimum amount of O2 required by the cells to survive and there is a limited amount of O2 remaining in the lungs.

    As for the part of the question that pertains to evolution: Most reptiles have a 3 chambered heart. This means they cannot sustain vigorous activity for long because the hearts are not capable of supplying the amount of O2 required. To a certain degree, the Mammalian Diving Reflex does have the same results as the reptilian circulatory system because both utilize a decreased rate of blood flow and decreased activity to survive. These are two separate adaptations which would allow increased survival underwater. I believe the one question which you strangely worded is trying to ask how these adaptations accomplish the same thing if the two species diverged much earlier. Divergent evolution occurs when adaptations in different environments causes one species to split into multiple species. In this example of reptiles and mammals, survival underwater was not a major distinction between the two groups. Also, even in two separate species, it is still possible to obtain similar adaptations if they require similar talents. Therefore, although reptiles and mammals are distinctly different, they can both have the ability to maximize O2 underwater if it is required for their survival.