Wing Propelled Diving
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.