Looking back through our blogs, I realized that we were missing some of the really cool aspects of marine mammals. So our intern, Lucie Drozd, is going to be doing a series on the adaptions allowing mammal to live in the water. As the whale watch season comes to an end, I’ve had a great chance to reflect and look back on some of the amazing things I have seen on the boats this fall. It’s always fun to see the dynamic and entertaining humpback behaviors at the surface, but for me what is even more amazing is what’s going on inside their bodies that allow these mammals to live in the ocean. The definition of being a mammal is that you are warm-blooded and air breathing.
You give birth to live young and nurse them with milk produced by mammary glands…..and you have hair. All of these mammalian characteristics aren’t well suited for living in the water, so marine mammals have evolved several physiological adaptations.
In this post I’ve listed some of these adaptations that allow these air-breathing mammals to go on dives for up to 45 minutes at a time (humpbacks); some even 2 hours long (sperm whales) – holding their breath the entire time. It’s easy to assume that since whales are so large, they must have big lungs in which they take down enough air for these long dives. But in fact, relative to your body size, your lungs are actually larger than that of a whale. Your lungs take up 7% of your internal body cavity while a whale’s lungs only take up 3% – meaning that lung size is not the secret.
The most important way to get oxygen for these animals is not through taking large breaths of air, but how they store oxygen molecules within their bodies. Oxygen is stored and transported around your body in your red blood cells in a protein called hemoglobin. Whales have twice the amount of hemoglobin in their blood then we do. So while your blood is 30% hemoglobin, a whale’s blood is 60% hemoglobin – allowing them to store twice as much oxygen for long dives. In addition to more hemoglobin, whales’ bodies have a higher percentage of blood – allowing even more oxygen storage. Blood takes up 10-20% of a whale’s body volume, while our blood volume to body ratio is only around 7%.We also store oxygen directly in our muscle tissue in a protein called myoglobin. Whales’ myoglobin concentrations in their muscle are up to 30% higher than their terrestrial relatives. This molecule is distributed throughout the muscles in the body and holds up to 35% of whales’ oxygen stores. This is crucial as it’s important for oxygen to not only last as long as possible, but also be constantly supplied to the brain while they are under water.
In order to conserve this precious oxygen, whales can do several things – one being that they have conscious control over their heart rate and can greatly reduce their cardiac output by slowing down their heart rate by over half. This reduces blood flow to non-essential organs (like skin and organs related to digestion) and some muscles because they have their own blood supply in the myoglobin. This is called ischemia. Additionally, blood pathways are shunted (or blocked) to certain tissues that are less important to the animal during dives—such as the stomach, while others sustain a steady blood flow- such as the brain. This process can be harmful to animals if prolonged which emphasizes the importance of rest periods.
Unlike us, whales are voluntary breathers so they have to think about every breath they take. They also are much more efficient at gas exchange than we are. When they come to the surface
they exhale first – getting rid of all the stale air in their lungs before taking in a fresh, clean breath. Humans, on the other hand, breathe in first and then exhale, leaving a lot of stale air in our lungs. We exchange only about 10-15% of the air in our lungs with every breath, while whales exchange about 80 to 90%. They also have a very quick gas exchange. This short window of time at the surface also requires the whales to have extremely efficient breaths. This is possible because their lungs have a large surface area to rapidly exchange gases. This surface area comes from all of the small alveolar sacs in the lungs at the end of the larger airways that come in contact with the fresh oxygen.Next time you are out on a whale watch, listen when the whales are at the surface – it is easy to hear the forceful exhale but takes some practice to hear the fainter inhale.