While it is true that members of most sexually reproducing species can be defined as either male or female, those who belong to the rest of the biological world are not so simply understood. Hermaphroditic creatures reproduce both as male and as female individuals, providing a fascinating glimpse into alternative sexual practices in nature and their ecological and evolutionary successes and failures.

Eloquently written by an award-winning biologist and pioneer in molecular ecology, this primer on hermaphroditism traces the phenomenon throughout Earth's myriad species, accounting for the adaptive significance of alternative sexual systems. Accessible and richly illustrated, the text maps the evolutionary origins of hermaphroditism, as well as its historical instances and fictional representations, underscoring the relevance of dual sexuality to our biological, intellectual, and cultural making. John C. Avise describes the genetics, ecology, phylogeny, and natural history of hermaphroditic plants, fish, and invertebrate animals and details organisms that either reproduce simultaneously as male and female or switch routinely between one sex and the other. Filled with surprising creatures and compelling revelations, this textbook stands alone in its clear yet comprehensive treatment of hermaphroditism and its unique challenge to the supremacy of separate sexes.

Covering both the internal and external incubation of offspring, this book provides a biology-rich survey of the natural history, ecology, genetics, and evolution of pregnancy-like phenomena. From mammals and other live-bearing organisms to viviparous reptiles, male-pregnant fishes, larval-brooding worms, crabs, sea cucumbers, and corals, the world's various species display pregnancy and other forms of parental devotion in surprisingly multifaceted ways. An adult female (or male) can incubate its offspring in a womb, stomach, mouth, vocal sac, gill chamber, epithelial pouch, backpack, leg pocket, nest, or an encasing of embryos, and by studying these diverse examples from a comparative vantage point, the ecological and evolutionary-genetic outcomes of different reproductive models become fascinatingly clear.

John C. Avise discusses each mode of pregnancy and the decipherable genetic signatures it has left on the reproductive structures, physiologies, and innate sexual behaviors of extant species. By considering the many biological aspects of gestation from different evolutionary angles, Avise offers captivating new insights into the significance of "heavy" parental investment in progeny.