Beyond canonical homeostasis: Dynamic, plastic, and intermittently unregulated physiology as the norm.

Homeostasis is often taught as steady-state regulation keeping internal variables within narrow limits-reflecting a mammalian bias. Yet, canonical homeostasis is far from universal. In many animals, including humans, normal physiological states allow some variables to fluctuate cyclically or stochastically while others remain tightly controlled. Indeed, across non-mammalian species, broad physiological variability is frequently normal rather than exceptional. Here we present case studies showing that plastic, variable regulation-sometimes approaching randomness-is both common and adaptive. Examples include intermittent breathing during diving, episodic feeding and digestion, pulsatile hormone release, variable sleep cycles, irregular gastric emptying, fluctuating renal blood flow, and stochastic immune responses. These complex regulatory patterns represent evolved strategies-not failures of homeostasis-allowing organisms to function efficiently in dynamic environments. Such flexible regulation, including unregulated intervals, may minimize energetic costs compared with continuous tight control, enhancing evolutionary fitness. We propose that variable degrees of physiological regulation have evolved as energy-efficient solutions shaped by natural selection. Expanding the concept of homeostasis to include intermittent and stochastic forms will improve our understanding of physiological adaptation, with significant implications for ecology and evolutionary biology. We further advocate new experimental approaches to distinguish adaptive variability from true regulatory failure.