Metabolic trade-offs in tropical and subtropical marine mammals: unique maintenance and locomotion costs in West Indian manatees and Hawaiian monk seals

Unlike the majority of marine mammal species, Hawaiian monk seals (Neomonachus schauinslandi) and West Indian manatees (Trichechus manatus latirostris) reside exclusively in tropical or subtropical waters. Although potentially providing an energetic benefit through reduced maintenance and thermal costs, little is known about the cascading effects that may alter energy expenditure during activity, dive responses and overall energy budgets for these warm-water species. To examine this, we used open-flow respirometry to measure the energy expended during resting and swimming in both species. We found that the average resting metabolic rates (RMRs) for both the adult monk seal (753.8±26.1 kJ h-1, mean±s.e.m.) and manatees (887.7±19.5 kJ h-1) were lower than predicted for cold-water marine mammal species of similar body mass. Despite these relatively low RMRs, both total cost per stroke and total cost of transport (COTTOT) during submerged swimming were similar to predictions for comparably sized marine mammals (adult monk seal: cost per stroke=5.0±0.2 J kg-1 stroke-1, COTTOT=1.7±0.1 J kg-1 m-1; manatees: cost per stroke=2.0±0.4 J kg-1 stroke-1, COTTOT=0.87±0.17 J kg-1 m-1). These lower maintenance costs result in less variability in adjustable metabolic costs that occur during submergence for warm-water species. However, these reduced maintenance costs do not appear to confer an advantage in overall energetic costs during activity, potentially limiting the capacity of warm-water species to respond to anthropogenic or environmental threats that require increased energy expenditure.

Postnatal development of diving physiology: implications of anthropogenic disturbance for immature marine mammals

Marine mammals endure extended breath-holds while performing active behaviors, which has fascinated scientists for over a century. It is now known that these animals have large onboard oxygen stores and utilize oxygen-conserving mechanisms to prolong aerobically supported dives to great depths, while typically avoiding (or tolerating) hypoxia, hypercarbia, acidosis and decompression sickness (DCS). Over the last few decades, research has revealed that diving physiology is underdeveloped at birth. Here, I review the postnatal development of the body's oxygen stores, cardiorespiratory system and other attributes of diving physiology for pinnipeds and cetaceans to assess how physiological immaturity makes young marine mammals vulnerable to disturbance. Generally, the duration required for body oxygen stores to mature varies across species in accordance with the maternal dependency period, which can be over 2 years long in some species. However, some Arctic and deep-diving species achieve mature oxygen stores comparatively early in life (prior to weaning). Accelerated development in these species supports survival during prolonged hypoxic periods when calves accompany their mothers under sea ice and to the bathypelagic zone, respectively. Studies on oxygen utilization patterns and heart rates while diving are limited, but the data indicate that immature marine mammals have a limited capacity to regulate heart rate (and hence oxygen utilization) during breath-hold. Underdeveloped diving physiology, in combination with small body size, limits diving and swimming performance. This makes immature marine mammals particularly vulnerable to mortality during periods of food limitation, habitat alterations associated with global climate change, fishery interactions and other anthropogenic disturbances, such as exposure to sonar.

Cardiorespiratory coupling in cetaceans; a physiological strategy to improve gas exchange?

In the current study we used transthoracic echocardiography to measure stroke volume (SV), heart rate (fH) and cardiac output (CO) in adult bottlenose dolphins (Tursiops truncatus), a male beluga whale calf [Delphinapterus leucas, body mass (Mb) range: 151-175 kg] and an adult female false killer whale (Pseudorca crassidens, estimated Mb: 500-550 kg) housed in managed care. We also recorded continuous electrocardiogram (ECG) in the beluga whale, bottlenose dolphin, false killer whale, killer whale (Orcinus orca) and pilot whale (Globicephala macrorhynchus) to evaluate cardiorespiratory coupling while breathing spontaneously under voluntary control. The results show that cetaceans have a strong respiratory sinus arrythmia (RSA), during which both fH and SV vary within the interbreath interval, making average values dependent on the breathing frequency (fR). The RSA-corrected fH was lower for all cetaceans compared with that of similarly sized terrestrial mammals breathing continuously. As compared with terrestrial mammals, the RSA-corrected SV and CO were either lower or the same for the dolphin and false killer whale, while both were elevated in the beluga whale. When plotting fR against fH for an inactive mammal, cetaceans had a greater cardiac response to changes in fR as compared with terrestrial mammals. We propose that these data indicate an important coupling between respiration and cardiac function that enhances gas exchange, and that this RSA is important to maximize gas exchange during surface intervals, similar to that reported in the elephant seal.

Stroke effort and relative lung volume influence heart rate in diving sea lions

The dive response, bradycardia (decreased heart rate) and peripheral vasoconstriction, is the key mechanism allowing breath-hold divers to perform long-duration dives while actively swimming and hunting prey. This response is variable and modulated by factors such as dive duration, depth, exercise and cognitive control. This study assessed the potential role of exercise and relative lung volume in the regulation of heart rate (fH) during dives of adult female California sea lions instrumented with electrocardiogram (ECG), depth and tri-axial acceleration data loggers. A positive relationship between activity (minimum specific acceleration) and fH throughout dives suggested increased muscle perfusion associated with exercise. However, apart from late ascent, fH during dives was still less than or equal to resting fH (on land). In addition, the activity-fH relationship was weaker in long, deep dives consistent with prioritization of blood oxygen conservation over blood oxygen delivery to muscle in those dives. Pulmonary stretch receptor reflexes may also contribute to fH regulation as fH profiles generally paralleled changes in relative lung volume, especially in shallower dives and during early descent and late ascent of deeper dives. Overall, these findings support the concept that both exercise and pulmonary stretch receptor reflexes may influence the dive response in sea lions.

Dive heart rate in harbour porpoises is influenced by exercise and expectations

The dive response, a decrease in heart rate (f_H) and peripheral vasoconstriction, is the key mechanism allowing breath-hold divers to perform long-duration dives. This pronounced cardiovascular response to diving has been investigated intensely in pinnipeds, but comparatively little is known for cetaceans, in particular in ecologically relevant settings. Here, we studied the dive f_H response in one of the smallest cetaceans, the harbour porpoise (Phocoena phocoena). We used a novel multi-sensor data logger to record dive behaviour, f_H, ventilations and feeding events in three trained porpoises, providing the first evaluation of cetacean f_H regulation while performing a variety of natural behaviours, including prey capture. We predicted that tagged harbour porpoises would exhibit a decrease in f_H in all dives, but the degree of bradycardia would be influenced by dive duration and activity, i.e. the dive f_H response would be exercise modulated. In all dives, f_H decreased compared with surface rates by at least 50% (mean maximum surface f_H=173 beats min^-1, mean minimum dive f_H=50 beats min^-1); however, dive f_H was approximately 10 beats min^-1 higher in active dives as a result of a slower decrease in f_H and more variable f_H during pursuit of prey. We show that porpoises exhibit the typical breath-hold diver bradycardia during aerobic dives and that the f_H response is modulated by exercise and dive duration; however, other variables such as expectations and individual differences are equally important in determining diving f_H.

Deep-diving sea lions exhibit extreme bradycardia in long-duration dives

Heart rate and peripheral blood flow distribution are the primary determinants of the rate and pattern of oxygen store utilisation and ultimately breath-hold duration in marine endotherms. Despite this, little is known about how otariids (sea lions and fur seals) regulate heart rate (fH) while diving. We investigated dive fH in five adult female California sea lions (Zalophus californianus) during foraging trips by instrumenting them with digital electrocardiogram (ECG) loggers and time depth recorders. In all dives, dive fH (number of beats/duration; 50±9 beats min(-1)) decreased compared with surface rates (113±5 beats min(-1)), with all dives exhibiting an instantaneous fH below resting (<54 beats min(-1)) at some point during the dive. Both dive fH and minimum instantaneous fH significantly decreased with increasing dive duration. Typical instantaneous fH profiles of deep dives (>100 m) consisted of: (1) an initial rapid decline in fH resulting in the lowest instantaneous fH of the dive at the end of descent, often below 10 beats min(-1) in dives longer than 6 min in duration; (2) a slight increase in fH to ~10-40 beats min(-1) during the bottom portion of the dive; and (3) a gradual increase in fH during ascent with a rapid increase prior to surfacing. Thus, fH regulation in deep-diving sea lions is not simply a progressive bradycardia. Extreme bradycardia and the presumed associated reductions in pulmonary and peripheral blood flow during late descent of deep dives should (a) contribute to preservation of the lung oxygen store, (b) increase dependence of muscle on the myoglobin-bound oxygen store, (c) conserve the blood oxygen store and (d) help limit the absorption of nitrogen at depth. This fH profile during deep dives of sea lions may be characteristic of deep-diving marine endotherms that dive on inspiration as similar fH profiles have been recently documented in the emperor penguin, another deep diver that dives on inspiration.