Chapter 4: Physiological Development and Epigenetics

Physiological Development and Epigenetics begins with the case study of hooded seals, illustrating how oxygen storage capacity—crucial for diving—develops postnatally. Young seals possess much lower myoglobin concentrations in muscle tissue than adults, drastically limiting their dive time and depth. The chapter generalizes this principle, showing that juvenile animals almost always differ in key physiological functions from adults. In humans, for instance, the metabolic demands of a large brain dominate early development, accounting for about 50% of a 4- to 5-year-old’s energy use. Other examples include thermoregulation in white-footed mice, which improves as insulation and metabolic heat production mature. At the cellular level, changes in enzyme expression—such as liver enzymes in rats—are shown to follow developmentally programmed sequences, while chloride cells in killifish shift location from skin and yolk sac to gills as the fish matures. The chapter then transitions into phenotypic plasticity, describing how a single genotype can yield different phenotypes based on environmental cues. Seasonal polyphenism in butterflies and predator-induced changes in mollusk shell thickness exemplify adaptive plasticity. Polyphenic development, like that in locusts and Daphnia, demonstrates how dramatic differences in morphology and behavior can emerge from environmental influence. Finally, the chapter examines epigenetics: the stable transmission of gene expression changes without altering DNA sequences. Mechanisms such as DNA methylation and histone modification are explored, including their roles in tissue differentiation, caste determination in insects, and long-term physiological effects of early-life stress, like the Dutch Hunger Winter. Epigenetic inheritance—whether mitotic within an individual or meiotic across generations—provides a powerful framework for understanding how environmental conditions can influence physiology for life and even across generations.