Critical Windows in Animal Development: Interactions Between Environment, Phenotype, and Time

Abstract

Observable phenotypic traits of an animal are a result of the interaction between the genome and environment. Differences in phenotypic traits between individuals induced by the environment, an indicator of phenotypic plasticity, may have immediate and long-term consequences for individuals, populations, and species. During development, animals are often most responsive or susceptible to changes in their environment, and phenotypic plasticity can be particularly prevalent. It is increasingly apparent that the way in which the environment influences an animal’s physiology may differ not just across a species’ lifetime but also within a species’ ontogeny. Periods of development during which an animal may show greater likelihood of phenotypic changes are termed “critical windows” or “sensitive periods.” Across animal taxa, experiments utilize exposures to particular environmental, chemical, or pharmacological stressors at certain time points of development to detect and understand critical windows during development. This chapter examines the emergence of critical windows as an important physiological concept using examples from the literature that span model and non-model invertebrates and vertebrates exposed to a range of environmental conditions. This chapter also outlines considerations for the continued search for critical windows. Critical window experimental designs can range in complexity, and variables such as the timing of exposures, if a single or multiple doses of a stressor are used, and when endpoints are assessed should be considered. A continued focus on critical windows will no doubt contribute to our growing knowledge of the interaction between the environment and physiology during animal development.