High heart rates in hunting harbour porpoises

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.

Is V̇O2 supressed during nonapnoeic facial submersion?

The mammalian dive response (DR) is described as oxygen-conserving based on measures of bradycardia, peripheral vasoconstriction, and decreased ventilation (V̇E). Using a model of simulated diving, this study examined the effect of nonapnoeic facial submersions (NAFS) on oxygen consumption (V̇O2). 19 participants performed four 2-min NAFS with 8 min of rest between each. Two submersions were performed in 5 °C water, 2 in 25 °C water. Heart rate (HR) was collected using chest strap monitors. A tube connected to the inspired port of a non-rebreathing valve allowed participants to breathe during facial submersion. Expired air was directed to a metabolic cart to determine V̇O2 and V̇E. Baseline (BL) HR, V̇O2, and V̇E values were determined by the average during the 2 min prior to facial submersion; cold shock response (CSR) values were the maximum during the first 30 s of facial submersion; and NAFS values were the minimum during the last 90 s of facial submersion. A 2-way repeated-measures ANOVA indicated that both HR and V̇E were greater during the CSR (92.5 ± 3.6 beats/min, 16.3 ± 0.8 L/min) compared with BL (78.9 ± 3.2 beats/min, 8.7 ± 0.4 L/min), while both were decreased from BL during the NAFS (60.0 ± 4.0 beats/min, 6.0 ± 0.4 L/min) (all, p < 0.05). HRCSR was higher and HRNAFS lower in 5 °C versus 25 °C water (p < 0.05), while V̇E was greater in 5 °C conditions (p < 0.05). V̇O2 exceeded BL during the CSR and decreased below BL during the NAFS (BL: 5.3 ± 0.1, CSR: 9.8 ± 0.4, NAFS: 3.1 ± 0.2 mL·kg−1·min−1, p < 0.05). The data illustrate that NAFS alone contributes to the oxygen conservation associated with the human DR.