uctive activity and inactivity. These phases produce regular changes in organisms' gonads, and are often attuned to certain environmental changes. For example, in the higher latitudes, vertebrate reproductive phases are generally coupled with the changing of the seasons (1:23-24).
Vertebrate species mate in different ways. The two primary types of mating include polygamy and monogamy. With polygamy, one individual mates with more than one partner. Polygyny occurs when the male mates with several females; polyandry involves a female mating with several males. In monogamous species, individuals mate with only one sexual partner. Monogamous relationships may be characterized by strong pair bonding between individuals. This bonding may endure throughout the species' lifespan (1:29).
Different vertebrate species may also be classified as being oviparous, ovoviviparous, and viviparous. It has been hypothesized that ancestral vertebrate reproduction consisted of oviparity, or egg-laying, with external fertilization. For such organisms, the period of gestation must have been very short. The eggs may have only remained in the oviducts for only a few hours or days (5:R854).
With the evolutionary development of internal fertilization, however, lengths of gestation could increase (5:R854). In ovoviviparous organisms, the embryos develop inside their egg membranes within some part of the female urogenital system. Although such embryos are not supplied with maternal nutrients, gases and water may diffuse through the egg membranes. The mother's body also gives the embryos protection. Ovoviviparity is known to occur in the following vertebrates: Elasmobranchii, Teleostei, Amphibia, and Reptilia (1:237).
In contrast, viviparous species provide for their embryos' needs up until the time of birth. Mechanisms have evolved which not only furnish embryos with nutrients, but facilitate gas exchange and remove wastes as well....