Book excerpt: Survival in extreme conditions is not about running for cover, or coming up for air, but rather in many instances working within the confines of the environment and instead suppressing bodily function. Yogis do it, seals do it, even sleeping bears do itthat is, alter their physiology in order to survive. This physiology of survival is explored here, including its evolution and varied manifestations across the animal kingdom. In the course of exploration over the years, researchers in comparative physiology have discovered fascinating and unanticipated commonalities. One might not expect to find a common theme relating the physiological reactions of seals, and yogis, and the comparisons extend even further afield, to hibernating animals, infants during birth, near-drowning victims, and clams at low tide. The common threads linking this unlikely mix of animals and situations are shared reactions to unfavorable environments, reactions that include lowering energetic requirements and retreating into states of depressed metabolism. Scrutiny of these diverse examples reveals some suggestive insights into the biology of survival and well-being. Animals in these withdrawn states are less dependent upon their customary levels of oxygen consumption, temporarily lessening their need for that life-sustaining resource. Instead they rely upon temporary strategic retreats of reduced metabolism, later resuming normal activity when conditions become more favorable. These states, and also the regulatory functions, including the neural and endocrine, that integrate to maintain equilibrium in altered environments or in temporarily challenging situations are examined. Breath-hold diving and its inevitable progressive asphyxia, often with cold exposure and swimming exercise that may accompany underwater submergence, comprises an assault on the ordinary homeostatic condition of the animal. These encounters, for which seals and other marine mammals are well adapted (but humans less so) alter resting equilibrium, and entail remarkable physiological orchestration."