Homeostatic (allostatic, teleonomic) efficiency and vitality

This Opinion Article provides a brief overview of the environment – animal relationship as a foundation for describing vitality as a holistic outcome of agency and environmental mastery. The account draws on the ecological view of animals as dynamic adaptive networks that convert energy and information into order, complexity and predictions within an animal’s structures and functions. In accord with the Extended Evolutionary Synthesis, actions of the animal extend beyond self-maintenance to provide gifts to conspecific and ecological communities, as well as to offspring. These self- and other-focused outcomes are facilitated by the efficiency with which the capture and use of energy and information are attuned to environmental dynamics. A challenge for animal and health sciences is to describe how well individuals are mastering their environment: are they surviving or thriving? In the early 1900s, before control theory proposed feedback pathways drive physiological variables towards fixed set points, many physiologists including Walter Cannon recognised the importance of dynamic changes, adaptation, and complexity in enabling relative stability of the internal environment: the outcome Cannon described as homeostasis. Cannon and Hoskins suggested that the efficiency with which animals maintain homeostasis in the face of environmental challenges could provide an index of vitality. As a holistic outcome of coherent internal functions and external harmony, it is suggested that Hoskins’ and Cannon’s concept of homeostatic efficiency has current-day utility as a measure of the animal’s mastery of its environment: its vitality. The outcome is agnostic of underlying pathways and can equally be described as allostatic or teleonomic efficiency. Hence, the term homeostatic efficiency does not imply rigidity or an underlying set-point model of regulation. The characteristics of day-to-day variations in products such as milk yield, body weight, egg output, and feed intake are currently interpreted as indicators of resilience. From the ecological view of the animal – environment dynamic, it is suggested that homeostatic efficiency provides a holistic indicator of vitality that encompasses the tribulations of daily life and resilience in adversity. This shifts studies of day-to-day fluctuations from a focus on external causes towards descriptions of the animal’s capacity to maintain coherence and harmony. Homeostatic efficiency complements other holistic indicators of vitality including behavioural indicators of affective states, qualitative behavioural assessment, fluctuating asymmetry, and functional and structural entropy (including the microbiome).