Archive for February 2011
Wing-propelled diving is one of the most interesting modes of vertebrate locomotion, in part because it evolved when a structure formerly used for flight (the wing) was modified for underwater propulsion. Many different vertebrate groups have returned to the sea, and each of these has inherited different phylogenetic baggage from their terrestrial ancestors. Cetaceans (whales and dolphins) cannot bend much side to side due to their mammalian rib cage and thus produce thrust primarily with up and down movements of their powerful fluked tails. Sea turtles share the same boxed-in body plan of their land-based ancestors and rely on flippered limbs to paddle through the oceans. Sea snakes evolved from limbless terrestrial snakes, and thus must undulate their entire bodies to swim. Just like these other groups, birds returning to the water inherited a suite of features that originally evolved for other functions, such as feathers, a foldable wing, and an air-filled skeleton. Some features are advantageous (an up and down wingstroke works well underwater too), others are hindrances (needing to lay eggs), and some are a little bit of both (feathers insulate, but require molting on land).
Wing-propelled diving has evolved many times in birds. Penguins are one of the most widely recognized examples, but several other living groups use wing-propelled diving for feeding trips while retaining the ability to fly. Auks, murres, puffins and dovekies all belong to the clade Alcidae and inherited the ability to dive from a common ancestor. These birds are very accomplished divers and some can reach depths over 100m below the surface. Alcidae also includes several extinct species. The most famous of these is the Great Auk, wiped out by humans in the 19th century. A much older fossil group, the Lucas Auks, also lost the ability to fly during the Miocene, over 10 million years ago. Lucas Auks are classified in the clade Mancallinae. We’ll meet these unusual birds in a later post. Another extinct group, distantly related to auks or penguins, is the Plotopteridae (featured three posts back).
Aside from Alcidae, wing-propelled diving is seen in some members of the petrel and albatross clade Procellariformes. Most adept at the underwater lifestyle is the tiny diving petrel, which looks like a veritable giant bumblebee when it flys over the water. An even smaller songbird, the dipper, is the only landbird that uses wing propelled diving (the dipper is up next for coverage here). Each of these groups shares some evolutionary novelities with the others due to the constraints of underwater flight, and each shows some unique features of their own. Figuring out what characteristics are general to all wing-propelled divers, what characteristics only appear after the loss of flight, and what order these features evolve in can tell us so much about this incredible evolutionary transition. Our team is working hard on the problem right now, and there will be many updates in the next few months. First, though, I’d like introduce the non-penguin members of the wing-propelled diving club. We’ve already mentioned plotopterids, so the next few posts will deal with dippers, alcids, and petrels.
Race to the End of the Earth is a special traveling exhibit organized by the American Museum of Natural History, Musée des Confluences, and Royal BC Museum. I was lucky enough to catch the show while it was still in New York, and it was one of the finest special exhibits I have seen. Walking through the exhibit, visitors learn about the Norwegian team led by Roald Amundsen and the British team led by Robert Falcon Scott. A large illustrated timeline tracks the adventures and misfortunes of the two teams as they raced towards the last great geographic landmark during the austral summer of 1911-1912. Displays include artifacts from the expeditions, full size reconstructions of some of the innovative shelters built by the teams to weather the winter before they set out for the pole, even a wax model of the daily ration for each man on Scott’s team (they got a soup can sized hunk of pemmican, about a quarter stick of butter, a few large biscuits, 3-4 sugar cubes, and some tea). And of course, there are penguins! Several displays focus on living penguins encountered by the teams, including a spectacular scene of Emperors gathered under the polar lights during the dark, winter part of their breeding cycle. Scott’s team marched for days, incurring severe frostbite, to obtain some eggs from an Emperor colony (this “side trip” happened before the team set off on the main journey to the pole). At the time, the Emperor Penguin was considered to be the most primitive living bird, and some ornithologists believed this species and other penguins had evolved directly from reptiles and never had the ability to fly. Thus, eggs would be very valuable to science. It is interesting to note that the journey to the colony was apparently much more arduous for the seasoned human explorers than for the three-foot tall, stumpy-legged birds, which carry their food in their blubber and can convert themselves into avian toboggans by lying on their stomachs and sliding across the ice.
Fossil penguins are featured as well. While Antarctica is portrayed as a penguin paradise today, only a few species actually live on the continent. In the Eocene, however, Antarctica was incredibly rich in penguin diversity. At least a dozen species occupied Seymour Island, at the tip of the Antarctic Peninsula. Their weathered bones are found in roughly 40 million year old rocks there today. More species may have dwelt on the main body of the continent, but we know almost nothing about ancient life there because ice cover prevents paleontologists from accessing the rocks. The special exhibit has two life-size steel outline models of Anthropornis nordenskjoeldi and Delphinornis larseni designed by Lindsay Foehrenbach of the AMNH. These species were respectively one of the largest and one of the smallest species that lived in Antarctica during the Eocene. Inside the frames are cast fossil bones of these species and to the right are the bones of a living Emperor penguin for scale. I helped a bit with the specs for these models and I love the sleek, elegant layout of the final exhibit – the AMNH always does these displays right.
Check out the exhibit’s website, which contains a walk-through video, maps, photos and info about the accompanying book by exhibit curator Dr. Ross MacPhee. The exhibit will be touring for the next few years, and if you are lucky, one of the next stops may be in your neighborhood.