Chapter 21: Movement and Muscle: Plasticity in Response to Use and Disuse

Movement and Muscle: Plasticity in Response to Use and Disuse also introduces asexual reproductive strategies such as budding, fission, and parthenogenesis, and compares the ecological advantages of sexual versus asexual modes of reproduction. The chapter delves into gonadal development and function, outlining the differentiation of testes and ovaries and the hormonal cascades that govern reproductive cycles. In males, the hypothalamic-pituitary-gonadal axis regulates the production of sperm through the action of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which stimulate testosterone synthesis and spermatogenesis. In females, cyclical changes in hormone levels coordinate follicular development, ovulation, and preparation of the uterus for implantation. Estrous and menstrual cycles are compared across species, with examples highlighting how environmental factors and seasonal cues influence reproductive timing. Fertilization is covered in both internal and external contexts, with attention to species-specific adaptations in sperm morphology, egg coatings, and mating behaviors. The chapter also discusses reproductive anatomy across taxa, including cloacas in birds and reptiles, accessory glands in mammals, and the diversity of reproductive ducts and structures used for gamete transfer. The physiology of pregnancy, parturition, and lactation is presented with a focus on mammals. Placental function is described as a complex exchange interface for gases, nutrients, and waste between mother and embryo. Hormonal control of pregnancy—including the roles of progesterone, estrogen, oxytocin, and prolactin—is examined in relation to maintaining gestation and initiating birth and postnatal care. Energetic costs of gestation and lactation are quantified, emphasizing the trade-offs animals face between current and future reproductive investment. Finally, the chapter discusses reproductive strategies from an evolutionary and ecological perspective, including r/K selection theory, semelparity versus iteroparity, and parental investment patterns. The diverse mechanisms that animals have evolved to maximize reproductive success underscore the complexity and adaptability of reproductive physiology across environments.