Sperm competition and the reproductive organs of the male and female Dunnock Prunella modularis

The morphology of the external and internal reproductive organs of male and female Dunnocks Prunella modularis are described in relation to the intense sperm competition characteristic of this species' mating system. Males have a relatively large cloacal protuberance, seminal glomera (sperm store) and testes. The seminal glomera contain about 1060 times 10 6 spermatozoa, probably sufficient for numerous copulations. The female's uterovaginal junction comprised 16 primary mucosal folds, each containing an average of 87 sperm storage tubules (SSTs): 1398 in total. SSTs were 370 μm in length and SSTs filled with sperm had an internal diameter of 12.2 μm. The morphology, number, size and storage capacity of SSTs were similar to that for other passerines, despite the female Dunnock's high copulation rate. Sperm ejected during the Dunnock's precopulatory display is thought to originate from the vagina rather than from the SSTs.