ABSTRACT

Unlike most other animal Classes, Aves (birds) is a taxon in which males of some species possess an intromittent organ (IO), whereas males of other species do not (King 1981a). Indeed, birds are virtually unique among internal fertilizers that most species lack an IO. Thus, in birds at least, the IO is not necessary for internal fertilization. This raises the question, then, whether the avian IO has evolved as a primary sexual trait simply for the delivery of sperm, as is sometimes assumed for other taxa, or as a secondary sexual trait (Eberhard 1990; Briskie and Montgomerie 1997). Most of the scant literature on avian IOs has focused on the seemingly odd presence of IOs in a few orders, but it is in fact their absence in so many species that is the evolutionary puzzle.