|Mute swan, Cygnus olor|
A lovely example of a structure evolved for one purpose being used for another, unrelated purpose was published very recently in a short communication in The Wilson Journal of Ornithology.
The author, Olle Terenius, of the Department of Ecology, Swedish University of Agricultural Sciences, Uppsala, Sweden, described seeing mute swans, Cygnus olor, using their raised wings to windsurf for some considerable distance. Regrettably, the full text sits behind a paywall.
Mute Swans (Cygnus olor) were observed using tailwind as a support for high-speed water transportation on three different occasions in three different locations in Sweden. With the wings arched over the back, they traveled ∼100 m in an inlet of the Baltic Sea in Stockholm, several hundred meters in Lake Hjälstaviken in Enköping, and ∼350 m in Lake Krankesjön in Lund. The speed of the movement was estimated to be much higher than normally seen for swimming swans. The first observation included two Mute Swans traveling one after another in the same direction, the second observation was of one single individual traveling towards a group of conspecifics, and the third observation was of a single individual traveling by itself. This behavior may serve as a means of medium-distance water transportation in this heavy bird species.
The mute swan is the world's heaviest flying bird and it takes some considerable effort for them to get airborne. When they do, however, they are powerful fliers, reaching speeds of 50-55 mph. So, using windsurfing as a means of locomotion on water not only saves considerable energy but gives them a 'running' start when taking off since the inertia in a relatively heavy body will already have been partly overcome. To paddle through water at the speeds recorded by Olle Terenius would have required about the same energy as a person playing squash.
What we have here is an example of nature using a structure for a purpose other than the main one selection pressure clearly selected it for. Interestingly, there are several examples of this principle from the evolutionary history of birds which involve feathers and wings. Firstly, feathers probably evolved as insulation in dinosaurs, then wings which were feathered modifications of the increasingly redundant forelimbs in the bipedal theropod dinosaurs, were probably used for gliding and maybe mating displays in addition to keeping the owner and its eggs warm. Then flight became the main function of the wings. Now swans are using these wings for windsurfing sails. We also have the example of wings being used for swimming, in penguins and some diving birds.
This principle is also behind what appear superficially to be irreducibly complex structures, for example, Michael Behe's notorious E. coli flagella, where structures evolved for one purpose were used for another because they were available, so evolution had something to work on.
Nature is ever invented simply because nature doesn't read the rule book. If it works it will be used. Utilitarianism rules because there is no plan or pre-designated purpose for anything.
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