Individual and temporal variation in cloacal protuberance size of male Bearded Tits (Panurus biarmicus)

The intensity of sperm competition is one of the factors known to determine interspecific variation in the size of male reproductive organs. However, individual variation in the size of reproductive organs and its relation to male quality have not been considered appropriately. We investigated the a nnual pattern of variation in the size of the cloacal protuberance (CP) in male Bearded Tits (Panurus biarmicus) and correlated CT volume during the f ertile period of females with reproductive and morphological features of ma les. Contrary to other passerines, the cloacal protuberance does not consti tute a sperm reservoir but functions as a copulatory organ in Bearded Tits. CP volume differed significantly between unmated and mated males and provi ded a good indicator of their reproductive status. Within individual mated Bearded Tits, CP size changed in conjunction with their mate's fertile cycl e, peaking around the day of clutch initiation. During the fertile period, mated Bearded Tits possess the largest CP (relative to body size) known for a passerine (average volume index = 59.14 mm(3) per g; n = 16). The adapti ve value of this feature could be that it improves ejaculate transfer owing to better and longer cloacal contact. Relative to unmated males, mated mal es have more total sperm stored in their seminal glomera and larger testes, suggesting the occurrence of large disadvantages for unmated males to gain reproductive success. An isolation experiment showed that female presence positively influences the development of CP size in virgin male Bearded Tit s.