Both technological and conceptual advances continue to enhance our ability to evaluate physiological mechanisms in free‐living animals. Although complex and uncontrolled natural environments may challenge our ability to define causal mechanistic relationships, they provide opportunities not available in more conventional laboratory settings. Among these opportunities are the ability to observe the interplay between physiology and behavior, the potential inspiration to physiological studies from novel observations in the field, and the ability to evaluate the extent to which particular physiological systems are challenged under natural conditions. As we accumulate information about physiological function in the field, we are often forced to reconsider established paradigms: hibernating bears may contract their muscles to maintain strength and tone, testosterone levels in male stonechats maintaining territories in winter are exceptionally low, wintering emperor penguins may risk overheating, and large desert mammals may eschew brain‐cooling mechanisms. Measuring and quantifying the organismal response to a changing environment provides a link between mechanistic physiology and behavior, ecology, and evolution and gives us new tools to understand population, community, and ecosystem‐level processes.