High heart rates in hunting porpoises

The impressive breath-hold capabilities of marine mammals are facilitated by both enhanced O2 stores and reductions in the rate of O2 consumption via peripheral vasoconstriction and bradycardia, coined the dive response. Many studies have focused on the extreme role of the dive response in maximizing dive duration in marine mammals, but few have addressed how these adjustments may compromise the capability to hunt, digest and thermoregulate during routine dives. Here we use DTAGs which record heart rate together with foraging and movement behaviour to investigate how O2 management is balanced between the need to dive and forage in five wild harbour porpoises that hunt thousands of small prey daily during continuous shallow diving. Dive heart rate was moderate (median minimum 47-69 bpm) and relatively stable across dive types, dive duration (0.5-3.3 min), and activity. A moderate dive response, allowing for some perfusion of peripheral tissues, may be essential for fuelling the high field metabolic rates required to maintain body temperature and support digestion during diving in these small, continuously-feeding cetaceans. Thus, despite having the capacity to prolong dives via a strong dive response, for these shallow-diving cetaceans, it appears to be more efficient to maintain circulation while diving: extreme heart-rate gymnastics are for deep dives and emergencies, not everyday use.

Methods
Data was collected using ECG-DTAGS DTAGs which record heart rate together with acoustic, depth, and movement data.

Data processing was performed using in MATLAB (MathWorks, Inc.). Code used to process data are available at www.animaltags.org.

The acceleration data has been processed to calculate Minimum Specific Acceleration (m s-2) at 25 Hz. This is an activity index. (files ending in MSA).

The heart rate data files identify the time of each heart beat and the associated instantaneous heart rate. Heart beats were identified by marking R peaks in the ECG QRS complex in 250 Hz ECG data. 

The Dive Summary file provides information on each dive analyzed in the paper including time of dive, duration, heart rate metrics, and activity metrics.

The Interval Summary file provided data for each interval analyzed in the paper including time of interval, heart rate metrics and activity metrics.  

Scripts used to process data are available at: www.animaltags.org

More details are provided in the Readme spreadsheet.