Towards non-invasive heart rate monitoring in free-ranging cetaceans: a unipolar suction cup tag measured the heart rate of trained Risso's dolphins

Abstract

Heart rate monitoring in free-ranging cetaceans to understand their behavioural ecology and diving physiology is challenging. Here, we developed a simple, non-invasive method to monitor the heart rate of cetaceans in the field using an electrocardiogram-measuring device and a single suction cup equipped with an electrode. The unipolar suction cup was placed on the left lateral body surface behind the pectoral fin of Risso's dolphins (Grampus griseus) and a false killer whale (Pseudorca crassidens) in captivity; their heart rate was successfully monitored. We observed large heart rate oscillations corresponding to respiration in the motionless whales during surfacing (a false killer whale, mean 47 bpm, range 20-75 bpm; Risso's dolphins, mean ± s.d. 61 ± 15 bpm, range 28-120 bpm, n = 4 individuals), which was consistent with the sinus arrhythmia pattern (eupneic tachycardia and apneic bradycardia) observed in other cetaceans. Immediately after respiration, the heart rate rapidly increased to approximately twice that observed prior to the breath. Heart rate then gradually decreased at around 20-50 s and remained relatively constant until the next breath. Furthermore, we successfully monitored the heart rate of a free-swimming Risso's dolphin. The all-in-one suction cup device is feasible for field use without restraining animals and is helpful in further understanding the diving physiology of free-ranging cetaceans. This article is part of the theme issue 'Measuring physiology in free-living animals (Part II)'.

Keywords: biologging; cetaceans; diving physiology; electrocardiogram; heart rate; marine mammal.

Figure 1.

(a) A suction cup tag attached to a Risso's dolphin. (b) A suction cup tag equipped with an ECG logger. (c) The region at which the heartbeats were measured: from just behind the posterior edge of the scapula under the thoracic vertebrae. (Online version in colour.)

Figure 2.

Examples of ECG data and filtered ECG data of a false killer whale and a Risso's dolphin that were motionless at the surface. Grey circles indicate each heartbeat.

Figure 3.

Depth, longitudinal acceleration, instantaneous heart rate and filtered ECG data of a Risso's dolphin (ID: gg_mf) during a swimming trial. The thick orange line shows the median instantaneous heart rate per minute. The plus and minus of longitudinal acceleration are downward and upward direction of the logger, respectively. (b) An enlarged section of time series data from a dashed square of figure part (a). The arrow in (a) indicates when the dolphin started swimming. (Online version in colour.)

Figure 4.

Examples of variations in the instantaneous heart rates of two delphinid species with respiration (a) and (b). The red dashed lines and circles show pronounced sinus arrhythmia patterns associated with respiration. The grey dashed lines and crosses show less synchronized sinus arrhythmia patterns with respiration. The panels on the right show the changes in heart rate from the end of each respiration to the next respiration when pronounced sinus arrhythmia patterns were observed. The black lines and the error bars show the median and the quantile deviance at intervals of 5 s. We did not calculate the median and the quantile deviance after 70 s for ID pc_km, 60 s for ID gg_mf and 45 s for ID gg_rm because of few data points. (Online version in colour.)