Vertebrate Sexual Behavior
Vertebrates use behavior to support and facili tate both the production of eggs and sperm and their conjoinment at fertilization. Sexual behav ior is defined by these functional criteria. The organization of sexual behavior is generally similar among vertebrates, but there is also great diversity. In many species, behavioral pat terns and underlying mechanisms that are used in gamete production and copulation overlap those used in courtship or parental care. (See Ehrman Sc Kim [courtship] and Rosenblatt [parenting], this volume, to get a more complete account of sexual behavior.)
The general pattern for vertebrates is to reproduce sexually, that is, by merging two ga metes, each of which provides one member of each chromosome pair to the newly formed zygote. The two gametes typically differ mor phologically; thus, with few exceptions, verte brate species have two sexes, which are defined by differences in the gametes (gonochorism). Males produce sperm, the relatively smaller gamete with little cytoplasm and few organelles other than those specialized for motility. Fe males produce eggs, which are relatively large and contain more cytoplasm. The degree of dif ference varies among vertebrate classes; it is particularly marked in birds, for which the yolk deposits in eggs can support extensive embry onic growth. An ostrich egg is an exaggerated case in point.