<P> The mare's reproductive system is responsible for controlling gestation, birth, and lactation, as well as her estrous cycle and mating behavior . The stallion's reproductive system is responsible for his sexual behavior and secondary sex characteristics (such as a large crest). </P> <P> Male and female birds have a cloaca, an opening through which eggs, sperm, and wastes pass . Intercourse is performed by pressing the lips of the cloacae together, which is sometimes known as intromittent organ which is known as a phallus that is analogous to the mammals' penis . The female lays amniotic eggs in which the young fetus continues to develop after it leaves the female's body . Unlike most vertebrates female birds typically have only one functional ovary and oviduct . As a group, birds, like mammals, are noted for their high level of parental care . </P> <P> Reptiles are almost all sexually dimorphic, and exhibit internal fertilization through the cloaca . Some reptiles lay eggs while others are viviparous (animals that deliver live young). Reproductive organs are found within the cloaca of reptiles . Most male reptiles have copulatory organs, which are usually retracted or inverted and stored inside the body . In turtles and crocodilians, the male has a single median penis - like organ, while male snakes and lizards each possess a pair of penis - like organs . </P> <P> Most amphibians exhibit external fertilization of eggs, typically within the water, though some amphibians such as caecilians have internal fertilization . All have paired, internal gonads, connected by ducts to the cloaca . </P>

Compare the reproductive system of the fish and frog