Martina S. Müller, Alexei L. Vyssotski, Maki Yamamoto, and Ken Yoda

Animals in the same population consistently differ in their physiology and behaviour, but the underlying mechanisms remain poorly understood. As the autonomic nervous system regulates wide-ranging physiological functions, many of these phenotypic differences may be generated by autonomic activity. We investigated for the first time in a free-living animal population (the long-lived seabird Streaked Shearwater, Calonectris leucomelas), whether individuals consistently differ in autonomic activity, over time and across contexts. We repeatedly recorded electrocardiograms from individual shearwaters, and from heart rate and heart rate variability quantified sympathetic activity, which drives the ‘fight-or-flight’ response, and parasympathetic activity, which promotes ‘rest-and-digest’ processes. We found a broad range of autonomic phenotypes that persisted even across years: heart rate consistently differed among individuals during stress and non-stress and these differences were driven by parasympathetic activity, thus identifying the parasympathetic ‘rest-and-digest’ system as a central mechanism that can drive broad phenotypic variation in natural animal populations.

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