Adaptations of Hunters Who Return to the Sea

As we continue our collaboration with the Hawaiʻi Wildlife Center to develop educational resources focused on Hawaiian seabirds, I thought I would share some thoughts and information on the structures of marine mammals and seabirds that make them supremely adapted to life at sea. 

As everyone who knows me is probably aware, I have spent most of my life fascinated with the complexities and intricacies of the diving physiology of marine mammals, but for the next few blogs, I am going to turn my focus to illuminating and explaining the equally impressive adaptations found in seabirds. Not only do I find this a fascinating topic, but seabird adaptations and conservation are the focus for the current Kula Naiʻa Foundation project in collaboration with the Hawaiʻi Wildlife Center. As many of you know, we have recently finished two children's books (“The Seabird Egg Book,” available in the US and the UK, and “Out on a Limb,” also available in the US and in the UK). We are currently working on 2 or 3 more titles in the Seabirds series.

Let me start by stating the simple fact that both marine mammals and seabirds evolved on land and returned to the ocean in the pursuit of food. But, because they are distantly related (birds evolved from dinosaurs, after all), their adaptations to the same constraints are remarkably different. Both birds and mammals are tetrapods, 4-limbed vertebrates that started with a central backbone supported by four limbs for walking on the ground. Their ancestral lines separated some 320 million years ago. Mammals evolved from small, warm-blooded creatures living in burrows underground or climbing among the trees, while birds evolved from warm-blooded feathered dinosaurs. Members of both groups, birds and mammals, eventually found themselves increasingly looking for food in the ocean. Over many millions of years, they evolved adaptations that helped them become some of the most effective hunters in the ocean.

From BBC Bitesize: (Evolution of the five-fingered limb)

Their ability to maintain a warm body temperature (mammals - 37℃, seabirds - 38-42℃) even in cold conditions may have been the starting point for these evolutionary adaptations to feeding in the water. In addition, their air-filled lungs provided a higher concentration of oxygen to fuel their muscles and brains. The result was predators who had more effective muscles and a larger, more sophisticated brain. In addition, some of the more deep diving species among both seabirds and marine mammals, including some of the penguins, larger dolphins, toothed whales and seals, can get down to the oxygen minimum layer, a few hundred meters below the surface. This is a part of the ocean where there is such a density of marine life hiding in the dark during the day that they use up most of the oxygen in the water. As a result, a deep-diving bird (such as an emperor penguin) or marine mammal (such as a pilot whale) can swim around, using their stored oxygen, and relatively easily pick up prey, unable to move very much since they have little access to oxygen.

As they continued to pursue prey through the water, their Tetrapod inheritance provided further opportunities for evolutionary adaptations to their method of locomotion. Penguins, Puffins, Sea Lions and Fur Seals all move through the water by flapping their forelimbs. This is a very efficient way of moving, allowing all these groups to attain speeds of 10-30 km/hr or more. However, birds and mammals all need to return to the surface to breathe air. At these speeds, swimming just below the surface is not very efficient, as some of your energy is wasted by pushing up a wave of water at the surface. The more efficient way of moving and breathing like this includes swimming a few body widths below the surface, where the fast-moving body no longer kicks up a wave at the surface. As a result, the best way to breathe is to get past the water's surface as fast as possible, creating small leaps into the air when it is time to breathe and then, upon reentering the water, quickly passing through the water's surface back down to a few body widths below it again. This mode of moving is known as porpoising (Penguins, Sea Lions)

The forelimbs developed into pectoral flippers for whales and dolphins, mainly used for steering and maneuvering. However, just like among the seabirds, the skeleton of the forelimbs still have the same bones as we do, with a few exceptions. Some whales and dolphins have more finger bones (phalanges) than the normal 3. The extremes are the pilot whales, which may have up to 17 finger bones in their flippers. In birds, including seabirds, the bones in the hand (metatarsals) are elongated, while the finger bones are reduced in size and number. The humpback whale is a species with extremely long flippers. This adaptation, however, is to be able to stay cool, see this blog post.

There are numerous adaptations to the marine environment that we could explore in both marine mammals and sea birds. So, over the next few weeks, I will write more in-depth explorations of the unique adaptations and diversity of some of the common seabirds found in Hawaiian waters.  The next blog will focus on the Sulidae family of seabirds - these are the plunge-diving Boobies and Gannets.  Three species are found in Hawaiian waters (Red-footed Booby Sula sula, Brown Booby Sula leucogaster and Masked Booby Sula dactylatra), there are a total of 10 species found worldwide.