Good vibrations? Structure and function of the cloacal tip of male Australian Maluridae

Abstract

When females mate multiply, sperm competition can generate strong selection for traits that provide males with a fertilisation 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 appear to experience a range of sperm competition intensities and exhibit a diversity of associated reproductive adaptations and behaviours. A prominent cloacal tip was present in the striated grasswren Amytornis striatus striatus, and superb Malurus cyaneus cyanochlamys, splendid M. splendens melanotus, red-backed M. melanocephalus and white-winged fairy-wrens M. leucopterus leuconotus, while variegated M. lamberti assimilis and blue-breasted M. pulcherrimus fairy-wrens possessed a rudimentary tip, and the southern emu-wren Stipiturus malachurus malachurus lacked a tip. The tip was a muscular hydrostatic appendage of the cloacal protuberance comprised of longitudinal striated muscle, a two-dimensional matrix of connective tissue and a keratinised epithelium. After controlling for phylogenetic relationships, cloacal tip length was positively related to combined testes mass but not body mass. Therefore, species inferred to experience more intense sperm competition exhibited a longer cloacal tip. We discuss the potential function of the cloacal tip and suggest that it may increase a male's likelihood of paternity success and represents a novel adaptation to sperm competition. © 2008 The Authors.