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Thermoregulation of Bird Body Temperature by Reflecting Near-infrared Radiation

Paper Reviewed
Medina, I., Newton, E., Kearney, M.R., Mulder, R.A., Porter, W.P. and Stuart-Fox, D. 2018. Reflection of near-infrared light confers thermal protection in birds. Nature Communications 9: 3610, DOI: 10.1038/s41467-018-05898-8.

Writing as background for their intriguing paper that was published in the journal Nature Communications, Medina et al. (2018) note that most (~55%) of the radiant energy from sunlight falls within the unseen near-infrared wavelengths. Consequently, it was their hypothesis that animals, such as birds, might be capable of reflecting solar radiation at near-infrared (NIR) wavelengths so as "to control heat gain and remain within their critical thermal limits." If correct, this thermoregulation could offer animals protection against predictions of widespread decline, death and even extinction in consequence of model-projected future increases in global temperature by the end of this century.

Hoping to provide some insight into this possibility of relief, the team of six researchers "employed phylogenetic comparative methods and biophysical modelling to explore the adaptive significance of NIR reflectivity." More specifically, while controlling for ultraviolet and visible reflectance, they tested whether NIR reflectivity varies "in relation to thermal environment in a continent-wide analysis of 90 species (12%) of Australian birds (spanning 35% of the families and 66% of the orders in Australia; n = 616 individuals) from all major habitat types, including sea and shore birds, waterbirds, forest or arid specialists and habitat generalists." And what did their work reveal?

Results of the analysis indicated, in the words of the authors, that "species occupying hot, arid environments reflect more radiant energy in NIR wavelengths than species in thermally benign environments, even when controlling for variation in visible color." What is more, this response was stronger in smaller, as opposed to larger, bird species; for as noted by Medina et al., "biophysical models confirm that smaller species gain a greater advantage from high NIR reflectivity in hot, arid environments, reducing water loss from compensatory evaporative cooling by up to 2% body mass per hour."

In commenting on their important findings, the authors say that they "highlight the importance of NIR reflectivity for thermal protection," adding that "empirical measures of NIR reflectivity must be incorporated into mechanistic models predicting the effects of climate change, particularly extreme heat events, on individual fitness and species distribution." Without the incorporation of this thermoregulatory ability, model predictions will overestimate the impacts of future warming on birds, and quite possibly many other animal species as well.

Posted 12 April 2019