Last weekend, I took my Marine Ecology students on a trip to Cobscook Bay. One of our activities was a whale watch off Head Harbour on Campobello Island. Birds were abundant with lots of Northern Gannets, Sooty Shearwaters, a few Manx Shearwaters, Black-legged Kittiwakes, Common Loons, many Bonaparte’s Gulls and even a few Atlantic Puffins. And yes, we saw marine mammals. A couple of Minke Whales and groups of Harbor Porpoises tantalized us by breaking the surface of the water briefly.
Perhaps you feel a bit frustrated like me watching these marine vertebrates because a good bit of their lives is lived under the water out of our vision. Thanks to underwater cameras, small submersibles and SCUBA equipment, we do have some understanding of life under the water surface for marine birds and mammals.
It is not surprising that these animals have particular adaptations that permit them to make a living by diving underwater. Some of these adaptations are morphological and others are physiological. Let’s explore some of these adaptations using the Common Loon.
Common Loons are excellent divers, capable or reaching depths of about 200 feet. A loon can stay under water for as long as 15 minutes.
Loons swim underwater using only their feet. The wings of loons are relatively short and are held tightly against the body during a dive.
The legs of a loon are set far back on the body. The legs are splayed out laterally when the bird swims. When a loon is first diving from the surface, it breaks the surface by alternating strokes with the left and right leg. Once underwater, the legs beat synchronously.
The lateral placement of the legs makes for hydrodynamic efficiency. If the legs were close together, the turbulent eddies created by one leg would interfere with smooth movement through the water of the other leg. The lateral arrangement allows a loon to generate maximum thrust while minimizing hydrodynamic drag.
The feet of loons are large and webbed. The real power in swimming is generated by the rearward movement of those webbed feet against the water. When the loon moves its feet forward during the recovery stroke, the toes are brought together causing the web to collapse and minimizing the effort needed to get the foot ready for the next power stroke.
Loons have a peculiar yet elegantly adapted leg. Unlike most birds, the major lower bone (the tibiotarsus or the large drumstick bone on a chicken) of a loon has a prominent extension, called the cnemial crest, that extends well beyond the joint where the upper leg bone (the femur) joins. This cnemial crest provides a broad attachment for the large muscles of the upper leg. The massive thigh muscles generate the huge force that allows loons to dive so deeply and so quickly.
The long bones of birds are not hollow as seen in most species of birds. These heavier bones make it easier for a loon to dive. Just before a dive, a loon compresses its body, driving out the air trapped within its feathers. Air trapped in the feathers would increase the buoyancy of the loon and make it harder to dive.
On to a physiological adaptation. Loons, like other diving birds and marine mammals, have the ability to store large quantities of oxygen in their blood. Even so, staying underwater for 15 minutes is no easy task. During diving, a loon undergoes a physiological change called the diving reflex. Oxygen flow to most body parts is greatly reduced except to the heart and nervous system.
This reflex causes the heart rate to slow, decreasing the amount of oxygen used by the heart. Muscles and other parts of the body have to perform anaerobic (oxygen-free) metabolism until the bird surfaces.
[First published on October 14, 2012]