但多数鸟类仅以泄殖腔互相接触,藉肌肉收缩而把雄鸟的精子送给雌鸟。有胎盘哺乳动物及多数硬骨鱼无泄殖腔。

• Most Birds mate by joining their cloacas in a "cloacal kiss"; muscular contractions transfer the sperm from the male to the female.