Heart rate variability reveals that a decrease in parasympathetic ('rest-and-digest') activity dominates autonomic...

Area of Science:

• Physiology
• Animal Behavior
• Ecology

Background:

• The autonomic nervous system, comprising the parasympathetic (PNS) and sympathetic (SNS) divisions, regulates physiological responses to stress.
• The 'fight-or-flight' (SNS) and 'rest-and-digest' (PNS) responses are crucial for survival but are often studied independently.
• Previous research on wild animals primarily focused on SNS activity during stress, neglecting the PNS role.

Purpose of the Study:

• To investigate the autonomic stress response in a free-living wild animal, the Streaked Shearwater (Calonectris leucomelas).
• To differentiate the roles of PNS and SNS activity during stress using heart rate variability (HRV) metrics.
• To assess the impact of different stressor intensities (handling, restraint, recovery) on autonomic regulation.

Main Methods:

• Utilized external electrocardiogram loggers to measure heart rate and HRV in Streaked Shearwaters.
• Quantified PNS and SNS activity through specific HRV indexes.
• Compared autonomic responses during handling, restraint in a cloth bag, and recovery in a nest burrow.

Main Results:

• Handling stress induced a significant and prolonged decrease in PNS activity, with recovery taking up to two hours.
• A smaller and shorter-lived increase in SNS activity was observed during handling stress.
• Confinement in a cloth bag was less stressful than handling but more stressful than burrow recovery, indicating varying stressor impacts.

Conclusions:

• The autonomic stress response in Streaked Shearwaters is primarily mediated by a reduction in PNS activity.
• HRV analysis provides an effective non-invasive method for studying stress physiology in free-living wild animals.
• This study highlights the dominant role of PNS in the acute stress response of vertebrates.