HomoTherm: An Open-Source Approach to Modelling Heat Exchange in Humans and Other Hominins in Diverse Environments

Climate change is increasing human exposure to novel environments and generating serious practical challenges in human health, while the role of past climates in shaping hominin evolution remains a fascination. Models of human heat exchange vary from simple environmental indices to extremely detailed physiological calculations. Physiologically explicit models vary in their physical explicitness, and significant trade-offs exist between model realism and computational speed. We presently lack agile models that are not so over-parameterised that they are slow, that maintain generality across diverse and complex physical environments, and that have sufficient biological realism to capture human (and, more generally, hominin) diversity in physiology, body size and proportions, activity level, clothing, hair, and skin. Here we present a model (HomoTherm) to fill this gap, built upon the general endotherm model from the NicheMapR package for biophysical modelling in R. This steady-state heat budget model is unique among existing open-source models in that it solves for the metabolic rate, evaporation rate, skin temperature, and clothing temperature of a multi-part (head, torso, and limbs) human of variable shape, size, or colour and at variable levels of physical activity. The model can handle complex microclimates, including environments that differ in downwelling and upwelling radiation, and connects directly with the microclimate modelling capabilities of NicheMapR. Because of its general nature, it can easily be adapted to model other hominins by changing proportions, fur properties, and any other known physiological trait. We illustrate the model's capabilities through comparisons with actual human responses in published empirical studies of indoor and outdoor exposures to heat and cold, and compare HomoTherm's performance with that of other models. Vignettes and an open-source Shiny app facilitate the use of HomoTherm in addressing fundamental and applied problems in human thermal biology.