High heart rates in hunting harbour porpoises

Abstract

The impressive breath-hold capabilities of marine mammals are facilitated by both enhanced O 2 stores and reductions in the rate of O 2 consumption via peripheral vasoconstriction and bradycardia, called 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 O 2 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 rates were 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.

Key questions in marine mammal bioenergetics

“… Biuw, 2003 ; Biuw et al, 2007 ) and acceleration, breath rate and heart rate (used to infer FMRs; e.g. Isojunno et al, 2018 ; Wilson et al, 2020 ; McDonald et al, 2021 ). They also provide information on the physical and biological environment that can characterise the prey landscape ( Arranz et al, 2011 ; Goulet et al, 2019 ; McMahon et al, 2019 , 2021 ).…”

Morphology and physiology in some small pelagic cetaceans: Is Dall's porpoise a deep diver and a thoroughbred of the sea?

“…The one exception is the Mb concentration in Dall's porpoise. We assume a Mb concentration of 6 g 100 g −1 muscle in Dall's porpoise as a conservative estimate because (1) stomach content analyses suggest deep‐diving capacity and deep divers typically have elevated Mb concentration (Kooyman & Ponganis, 1998; Noren et al, 2014); (2) Mb concentrations are high in cetaceans with large blood volumes and elevated Hb concentrations such as in the beluga and sperm whale (Table 4); (3) striped dolphins and Fraser's dolphins, both suspected deep divers with large muscle masses, have epaxial muscle Mb concentrations of 6.6–8.2 and 7.1 g 100 g −1 muscle, respectively, (Arregui et al, 2021; Dolar et al, 1999; Kroeger et al, 2020; Polasek & Davis, 2001); and (4) Mb concentration of Dall's porpoise is expected to be much higher than 2.5 g 100 g −1 muscle Mb content in harbor porpoises (Noren et al, 2014) because harbor porpoises are predominantly shallow divers with mean dive durations less than two min and mean dive depths less than 50 m (McDonald et al, 2021; Westgate et al, 1995).…”

Current status of tagging harbour porpoises - aplication to the Dutch North Sea : knowledge base report

“…External tag attachments include devices that do not penetrate the skin, such as harnesses and suction cups (Figure 3.2, Figure 3.3d). They have been successfully applied to Yangtse finless porpoises (Neophocaena asiaeorientalis asiaeorientalis), Dall's porpoise (Phocoenoides dalli) and harbour porpoises (Akamatsu et al, 2005;Hanson et al, 1998;McDonald et al, 2021;Sørensen et al, 2018;Wisniewska et al, 2016Wisniewska et al, , 2018b. Attachments can last for hours up to days (McDonald et al, 2021;Wisniewska et al, 2016).…”

“…In a different study three trained harbour porpoises were equipped with a multi-sensor data logger to record heart rate while they were swimming and capturing prey (McDonald et al, 2018). In the wild these kinds of measurements have been done for blue whales (Balaenoptera musculus), narwhals (Monodon monoceros) and harbour porpoises (Goldbogen et al, 2019;McDonald et al, 2021;Williams et al, 2017b).…”

“…It was first applied on porpoises in human care providing high-resolution information on porpoise acoustic behaviour (Deruiter et al, 2009). Wild porpoises have been tagged with the DTAG3 which also provide data on swimming and hunting behaviour, allowing the assessment of both vocalisations, social interactions, heart rate and movement changes in relation to anthropogenic sounds (Wisniewska et al, 2016;Sørensen et al, 2018;Wisniewska et al, 2018b, McDonald et al 2021.…”