PREFACE

Mammals are hairy endothermic vertebrates that suckle their offspring (lactation distinguishes mammals from other animals). The mammalian class first evolved from therapsid reptiles more than 200 million years ago. Approximately 4500 terrestrial, arboreal, flying, and aquatic species of mammals exist today. About two-thirds of all mammals belong to orders consisting of rodents or bats. Most, the eutherian mammals, form a placenta and give birth to a sexually-differentiated fetus. The metatherian marsupials are born early in embryonic life and develop within a maternal pouch; these viviparous animals produce a live young without an external egg stage. There are only two known species of prototherian (egg-laying, oviparous) mammals.

Our base of data concerning mammals has been generated mainly from controlled experiments with laboratory animals, the domesticated farm species, and primates. Rudimentary knowledge of the reproductive biology of many wild mammals remains unknown. There is an exception to virtually every rule when dealing with reproductive processes across species - unique strategies have presumably served evolution in response to widely divergent selection pressures. In no other field of comparative physiological function are species distinctions so prevalent. Whenever possible, I have emphasized elemental mechanisms.

The reproductive system of both males and females is arranged onto four elementary levels - the hypothalamus, hypophysis (pituitary gland), gonads (testes or ovaries), and reproductive tract. The male tract consists of ducts (epididymides, vas deferens, and urethra) and accessory glands (prostate, seminal vesicles, and bulbourethral). Female ducts include the oviducts, uterus, cervix, and vagina. The gonads release exocrine products, gametes (sperm or ova), into their respective duct systems. Contact among the reproductive organs is achieved by complex interactions involving the two great communication systems of the body - nervous and endocrine.

The courier of information for the endocrine system is the hormone. A hormone is a chemical that is synthesized by an endocrine (ductless) gland. Hormones are secreted directly into the vascular system and act upon a genetically-responsive tissue at a distant site to cause a biological response. Endocrine glands generally originate as embryonic outgrowths of tubular epithelia. However, hormones are also synthesized by nervous tissues. Hormones secreted by neurons are called neurohormones. Products of nerves once thought to work strictly within a localized environment (neurotransmitters) also can utilize the blood stream as a medium for transport, and consequently, assume the role of hormone. Target tissues contain discriminating recognition molecules (receptors) for their companion hormone. Tropic hormones exert their action on another endocrine gland.