Submitted to: Proceedings of the Royal Society of London: Biological Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/2007
Publication Date: 5/8/2008
Citation: Rowe, M., Bakst, M.R., and Pruett-Jones, S. 2008. Good vibrations? Structure and function of the cloacal tip of male Australian Maluridae. Journal of Avian Biology. 39(3):348-354.
Interpretive Summary: When females mate with several males, sperm from one male out competes the remaining sperm. This ‘sperm competition’ provides the males producing the best sperm with a fertilization advantage. Some of the Australian passerine birds known as wrens experience sperm competition and we propose that the competition is influenced by a unique morphological elaboration of the cloacal region called the cloacal tip. It is a prominent, rounded, cone-like tip projecting toward the birds head. It appears to be an extension of the cloacal protuberance (a bulging of the sides of the cloacal created by the highly coiled ductus deferens) in several of the wrens studied. The cloacal tip was a muscular hydrostatic appendage (structurally like that of an elephant’s trunk but on the magnitude of 1 cm or less), comprised of longitudinal striated muscle, and a honey-comb matrix of connective tissue. We suggest that the tip wiggles or vibrates, possibly stimulating the female at copulation. Interestingly, the mean length, width and area of the tip differed significantly across wren species and mean tip length was positively related to testes weight and cloacal protuberance size. Thus species known to experience more intense sperm competition exhibited a longer cloacal tip. This work will interest biologists studying breeding practices and sperm competition in feral birds but is also of particular interest to biologists interested in oviductal sperm transport and selection in domestic birds.
Technical Abstract: When females mate multiply, sperm competition can generate strong selection for traits that provide males with a fertilization advantage. This study examined the cloacal tip, a unique morphological structure of males that appears to be associated with sperm competition in Australian Maluridae. Species in this group experience a range of sperm competition intensities and a diversity of associated reproductive behaviours and adaptations. A prominent tip was present in the striated grasswren, and superb, splendid, red-backed and white-winged fairy-wrens, while variegated and blue-breasted fairy-wrens possessed a rudimentary tip, and the southern emu-wren lacked a tip. The tip was a quasi-muscular hydrostatic appendage of the cloacal protuberance comprised of longitudinal striated muscle, a two-dimensional matrix of connective tissue and a keratinised epithelium. The mean length, width and area of the tip differed significantly across species and, after controlling for body size and phylogenetic relationships, mean tip length was positively related to testes mass and cloacal protuberance size. Thus species known or inferred to experience more intense sperm competition exhibited a longer cloacal tip. We discuss the potential function of this structure and suggest the cloacal tip may increase a male’s likelihood of paternity success and represents a novel adaptation to sperm competition.