such as frogs and toads. Anurans are commonly used as a model organism for amphibians, because of the large, easy to manipulate eggs, fast developmental rate, high fecundity rate, no parental involvement, and external fertilization. Males will congregate near a lake or pond and establish calling stations. Females approach the area and listen to all of the different male calls, and then continue to move towards the mate she chooses. This is the anuran's sexual selection. It has been concluded that females prefer a male with a more attractive call, which is also the larger male. Copulation occurs when a male anuran hops onto the back of a female. They then move to a spot near water to simultaneously release their sperm and eggs. Other males in the area can also release sperm onto the eggs to also attempt to fertilize the eggs. If the female does not want to reproduce with the male that jumps onto her back, she will wait until the male leaves or move to a new location. Sperm released into the water must be in close proximity, get to the egg first, and then enter the gel layer of the egg to have the best chance of fertilizing. When the anurans are not close to eggs, they sometimes release their sperm into oocyte containing foam nests, or terrestrial breeders go right to the gel coat of the oocyte to release their sperm. Over the course of a breeding season, males can copulate numerous times by releasing sperm anywhere he finds unfertilized eggs or encounters a female who is/wants to spawn. Females, however, can only release eggs once per breeding season. Releasing sperm directly into the water increases sperm competition through agonistic behavior and spawning in groups. This has been tested, with good evidence associated with a larger sperm number and testes size. Smaller testes size and a slower sperm velocity were seen with the anurans that released the sperm into the foam nests instead of into open water. To further increase sperm competition, there is a larger thickness to an anuran oocyte gel, specifically the green tree frog. Anuran sperm also have high longevity and osmotic tolerance compared to fresh water fish. The Caudata order contains all of the salamanders and newts, amphibians that have tails. Within this, the only subgroups that externally fertilize are Cryptobranchidae (giant salamanders) Sirenidae, and Hynobiidae. The females release egg