Female cloacal protuberance of the polygynandrous Alpine Accentor Prunella collaris: histological features and possible functional significance

This paper describes the histological features of the prominent cloacal protuberance (CP) of sexually mature females of the polygynandrous Alpine Accentor Prunella collaris . In the copulation period, the CP of the prelaying mature female developed remarkably and the lips of the vent turned brilliant scarlet. Macroscopic and microscopic examination of the cloacal region from females of different ovarian maturity showed that the formation of the CP was mainly due to the hypertrophy of the cloacal musculature, i.e. the sphincter cloacae muscle and levator cloacae muscle, and to the development of the associated connective tissue in the sexually mature female. The scarlet coloration of the cloacal lips was ascribed exclusively to the development of the dermal vascular plexus accompanied by hyperaemia, as characterized by the dilated blood vessels filled with blood. There was no evidence that skin pigmentation was involved in the scarlet coloration of the cloacal lips. We speculate that the morphological changes in the cloacal lips of the female Alpine Accentor are mediated hormonally by the brain–pituitary–gonadal axis.