Meeting Abstract
It has long been debated that morphological trait appears only in breeding season (i.e. ornament) is evolved through sexual selection. On the other hand, there are ornament-like traits whose evolutionary reasons are difficult to be explained solely by the theory of sexual selection. One such example is the horn of rhinoceros auklet (Cerorhinca monocerata, a seabird species), which appears on the top of beak in both male and female only in breeding season. It is reported that there are no sexual differences in size of the horn. In addition, a protruding object like the horn on a flying animal would increase aerodynamic or hydrodynamic drag (retarding force) and hence the energetic cost of locomotion. To date, no one has yet suggested a plausible function of the horn of rhinoceros auklet. In fact, there are some cases where a protruding object can reduce aerodynamic drag. For example, a vortex generator, a small plate attached on the surface of an aircraft, affects the airflow downstream of it and reduces drag. Moreover, in the breeding season of rhinoceros auklets, both male and female fly with a bunch of small fish for feeding chicks, which would cause substantially large drag. Taken together, we hypothesized that the horn of the rhinoceros auklet may have the similar role of vortex generator, particularly when the bird flies with fish in its beak. In order to test our hypothesis, we used computational fluid dynamics (CFD) technique to simulate the airflow around the bird, and compared the flow-field and drag force among the presence/absence of horn and fish. We will report the aerodynamic phenomenon and the drag reduction effect due to the horn.
