379 Chapter 21 Physiological adaptation to hot and cold environments Michael J. Tipton Kent B. Pandolf Michael N. Sawka Jürgen Werner Nigel A.S. Taylor CHAPTER CONTENTS 1 Introduction 379 2 Principles of adaptation 380 2.1 Phenotypic and genotypic adaptation 380 2.2 Functional and morphological adaptation 380 2.3 Adaptation theory 381 2.4 Physiological forcing functions 382 3 Adaptation to cold 383 3.1 An overview of cold-adaptive response patterns 383 3.2 Cold adaptation: evidence from ethnic groups 385 3.3 The nature of human adaptation to cold 386 4 Adaptation to heat 387 4.1 Heat adaptation: evidence from ethnic groups 387 4.2 Heat acclimation: its induction, retention and decay 389 4.3 Impact of aerobic fitness 391 4.4 Adaptive actions and physiological mechanisms 392 4.5 Dry-heat versus humid-heat comparisons and cross-acclimation 395 5 The relationship between adaptation to cold and heat 396 References 396 1 INTRODUCTION The 3–4 million-year evolution of humans has been influ- enced and determined by the environment. The colonisa- tion of the planet from man’s origins as a tropical animal of the African or Asian plains was largely complete by 30,000– 15,000 bc. These migrations were dependent on the ability to adapt behaviourally and physiologically to the environ- mental extremes experienced by humans as they moved further from their equatorial origins. The term adaptation can be defined as changes or adjust- ments, often hereditary, in the structure or habits of a species or individual that improve its condition in relationship to its environment. In its broadest sense, adaptation can influ- ence all the systems and states of a living organism and, in so doing, is fundamental to survival and prosperity. In this chapter, we focus on human adaptation to hot and cold environments and the integrated physiological responses that accompany these adaptations, resulting in a more effec- tive defence of body temperature in the presence of thermal challenges. As described in Chapters 18 and 19, humans possess regulatory mechanisms that ensure the stability of the internal environment (homeostasis). The thermoregulatory system contains sensors, signal integrators, effector organs and a communication network designed to regulate body temperature within a narrow range. Significant perturba- tions in body temperature evoke corrective responses in this system to restore normality. Repeated perturbations evoke adaptations. These minimise the impact of an acute stimu- lus, enabling the body to cope more effectively with thermal stresses. Humans have a large capacity to adapt anatomi- cally, biochemically, physiologically and psychologically to a broad range of thermal environments. They also possess the intellect to employ technology to modify the thermal environment, returning it to an acceptable (stress-free) state. For example, the oldest building made by homonids (Tanzania) was a windbreak constructed by Australopithecus ∼3.25 million years ago. Over a million years ago, Homo erectus was building huts from stones, branches and furs. The earliest evidence for the use of fire, a source of light and