Living in the fast lane: rapid development of the locomotor muscle in immature harbor porpoises (Phocoena phocoena)

Building Cetacean Locomotor Muscles throughout Ontogeny to Support High-Performance Swimming into Adulthood

The demands on the locomotor muscles at birth are different for cetaceans than terrestrial mammals. Cetacean muscles do not need to support postural costs as the neonate transitions from the womb because water's buoyant force supports body weight. Rather, neonatal cetacean muscles must sustain locomotion under hypoxic conditions as the neonate accompanies its mother swimming underwater. Despite disparate demands at birth, cetaceans like terrestrial mammals require postnatal development to attain mature musculature. Neonatal cetaceans have a low proportion of muscle mass, and their locomotor muscles have lower mitochondrial density, myoglobin content (Mb), and buffering capacity than those found in the adult locomotor muscle. For example, the locomotor muscle of the neonatal bottlenose dolphin has only 10 and 65% of the Mb and buffering capacity, respectively, found in the adult locomotor muscle. The maturation period required to achieve mature Mb and buffering capacity in the locomotor muscle varies across cetacean species from 0.75 to 4 and 1.17 to 3.4 years, respectively. The truncated nursing interval of harbor porpoises and sub-ice travel of beluga whales may be drivers for faster muscle maturation in these species. Despite these postnatal changes in the locomotor muscle, ontogenetic changes in locomotor muscle fiber type seem to be rare in cetaceans. Regardless, the underdeveloped aerobic and anaerobic capacities of the locomotor muscle of immature dolphins result in diminished thrusting capability and swim performance. Size-specific stroke amplitudes (23-26% of body length) of 0-3-month-old dolphins are significantly smaller than those of >10-month-olds (29-30% of body length), and 0-1-month-olds only achieve 37 and 52% of the mean and maximum swim speed of adults, respectively. Until swim performance improves with muscle maturation, young cetaceans are precluded from achieving their pod's swim speeds, which could have demographic consequences when fleeing anthropogenic disturbances.

High heart rates in hunting harbour 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, 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 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 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.

Epizootiology of a Cryptococcus gattii outbreak in porpoises and dolphins from the Salish Sea

Cryptococcus gattii is a fungal pathogen that primarily affects the respiratory and nervous systems of humans and other animals. C. gattii emerged in temperate North America in 1999 as a multispecies outbreak of cryptococcosis in British Columbia (Canada) and Washington State and Oregon (USA), affecting humans, domestic animals, and wildlife. Here we describe the C. gattii epizootic in odontocetes. Cases of C. gattii were identified in 42 odontocetes in Washington and British Columbia between 1997 and 2016. Species affected included harbor porpoises Phocoena phocoena (n = 26), Dall's porpoises Phocoenoides dalli (n = 14), and Pacific white-sided dolphins Lagenorhynchus obliquidens (n = 2). The probable index case was identified in an adult male Dall's porpoise in 1997, 2 yr prior to the initial terrestrial outbreak. The spatiotemporal extent of the C. gattii epizootic was defined, and cases in odontocetes were found to be clustered around terrestrial C. gattii hotspots. Case-control analyses with stranded, uninfected odontocetes revealed that risk factors for infection were species (Dall's porpoises), age class (adult animals), and season (winter). This study suggests that mycoses are an emerging source of mortality for odontocetes, and that outbreaks may be associated with anthropogenic environmental disturbance.

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.

Extremely Elevated Myoglobin Contents in the Pelagic Melon-Headed Whale (Peponocephala electra) after Prolonged Postnatal Maturation

Muscle biochemistry of aquatic birds and mammals varies in accordance with swimming and diving performance, as well as with ontogeny. Similar to other odontocetes, the locomotor muscles (longissimus dorsi) of neonatal melon-headed whales (Peponocephala electra) have low myoglobin content (Mb; 1.06±0.20 g Mb/100 g wet muscle mass; mean ± SE; n=2] and low muscle nonbicarbonate buffering capacity (37.78±3.75 slykes; n=2), representing only 16% of adult Mb (6.64±0.33 g Mb/100 g wet muscle mass; n=5) and 56% of adult muscle nonbicarbonate buffering capacities (66.90±4.80 slykes; n=5). By the juvenile stage, Mb (2.75±0.80⁢ g Mb/100 g wet muscle mass; n=3) is still only 41% of adult levels, but nonbicarbonate buffering capacity (65.61±2.62 slykes; n=3) has matured. Despite the observation that Hawaiian melon-headed whales are not deep divers or long-duration divers, their Mb rivals that found in ziphiids that forage in the bathypelagic zone and monodontids that forage under sea ice. The pelagic lifestyle of melon-headed whales likely requires sustained swimming, such that endurance training could elevate Mb in the locomotor muscle. Indeed, elevated Mb in the locomotor muscles of other pelagic odontocetes has been observed. Unlike deep-diving and Arctic-dwelling odontocetes, melon-headed whales do not achieve mature muscle characteristics before nursing. It is likely that early in life, the hydrodynamic benefits of swimming in echelon position with their mothers minimizes the endurance training of the calves that would otherwise promote rapid elevations in Mb.

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.

Comparative physiology of vocal musculature in two odontocetes, the bottlenose dolphin (Tursiops truncatus) and the harbor porpoise (Phocoena phocoena)

The mechanism by which odontocetes produce sound is unique among mammals. To gain insight into the physiological properties that support sound production in toothed whales, we examined myoglobin content ([Mb]), non-bicarbonate buffering capacity (β), fiber-type profiles, and myosin heavy chain expression of vocal musculature in two odontocetes: the bottlenose dolphin (Tursiops truncatus; n = 4) and the harbor porpoise (Phocoena phocoena; n = 5). Both species use the same anatomical structures to produce sound, but differ markedly in their vocal repertoires. Tursiops produce both broadband clicks and tonal whistles, while Phocoena only produce higher frequency clicks. Specific muscles examined in this study included: (1) the nasal musculature around the phonic lips on the right (RNM) and left (LNM) sides of the head, (2) the palatopharyngeal sphincter (PPS), which surrounds the larynx and aids in pressurizing cranial air spaces, and (3) the genioglossus complex (GGC), a group of muscles positioned ventrally within the head. Overall, vocal muscles had significantly lower [Mb] and β than locomotor muscles from the same species. The PPS was predominately composed of small diameter slow-twitch fibers. Fiber-type and myosin heavy chain analyses revealed that the GGC was comprised largely of fast-twitch fibers (Tursiops: 88.6%, Phocoena: 79.7%) and had the highest β of all vocal muscles. Notably, there was a significant difference in [Mb] between the RNM and LNM in Tursiops, but not Phocoena. Our results reveal shared physiological characteristics of individual vocal muscles across species that enhance our understanding of key functional roles, as well as species-specific differences which appear to reflect differences in vocal capacities.

Muscle biochemistry of a pelagic delphinid (Stenella longirostris longirostris): insight into fishery-induced separation of mothers and calves

The length of time required for postnatal maturation of the locomotor muscle (longissimus dorsi) biochemistry [myoglobin (Mb) content and buffering capacity] in marine mammals typically varies with nursing duration, but it can be accelerated by species-specific behavioral demands, such as deep-diving and sub-ice transit. We examined how the swimming demands of a pelagic lifestyle influence postnatal maturation of Mb and buffering capacity in spinner dolphins (Stenella longirostris longirostris). Mb content of newborn (1.16±0.07 g Mb per 100 g wet muscle mass, n=6) and juvenile (2.77±0.22 g per 100 g, n=4) spinner dolphins were only 19% and 46% of adult levels (6.00±0.74 g per 100 g, n=6), respectively. At birth, buffering capacity was 52.70±4.48 slykes (n=6) and increased to 78.53±1.91 slykes (n=6) once a body length of 141 cm was achieved, representing 1.6- to 2.0-year-old dolphins. Based on the age of weaning (1.3–1.6 years post-partum), muscle maturation occurred just after weaning as described for coastal bottlenose dolphins (Tursiops truncatus). Thus, a pelagic lifestyle does not promote rapid maturation of muscle biochemistry. Rather, it promotes enhanced muscle biochemistry: newborn and adult spinner dolphins had four- and two-times greater Mb contents than newborn and adult bottlenose dolphins, respectively. Indeed, adult levels rivaled those of deep-diving cetaceans. Nonetheless, the relatively underdeveloped muscle biochemistry of calves likely contributes to documented mother–calf separations for spinner dolphins chased by the tuna purse-seine fishery.

Navigating under sea ice promotes rapid maturation of diving physiology and performance in beluga whales

Little is known about the postnatal development of the physiological characteristics that support breath-hold in cetaceans, despite their need to swim and dive at birth. Arctic species have the additional demand of avoiding entrapment while navigating under sea ice, where breathing holes are patchily distributed and ephemeral. This is the first investigation of the ontogeny of the biochemistry of the locomotor muscle in a year-round Arctic-dwelling cetacean (beluga whale, Delphinapterus leucas). Compared with what we know about other cetaceans, belugas are born with high myoglobin content (1.56±0.02 g 100 g−1 wet muscle mass, N=2) that matures rapidly. Myoglobin increased by 452% during the first year after birth and achieved adult levels (6.91±0.35 g 100 g−1 wet muscle mass, N=9) by 14 months postpartum. Buffering capacity was 48.88±0.69 slykes (N=2) at birth; adult levels (84.31±1.38 slykes, N=9) were also achieved by 14 months postpartum. As the oxygen stores matured, calculated aerobic dive limit more than doubled over the first year of life, undoubtedly facilitating the movements of calves under sea ice. Nonetheless, small body size theoretically continues to constrain the diving ability of newly weaned 2 year olds, as they only had 74% and 69% of the aerobic breath-hold capacity of larger adult female and male counterparts. These assessments enhance our knowledge of the biology of cetaceans and provide insight into age-specific flexibility to alter underwater behaviors, as may be required with the ongoing alterations in the Arctic marine ecosystem associated with climate change and increased anthropogenic activities.

Tracking the Development of Muscular Myoglobin Stores in Mysticete Calves

For marine mammals, the ability to tolerate apnea and make extended dives is a defining adaptive trait, facilitating the exploitation of marine food resources. Elevated levels of myoglobin within the muscles are a consistent hallmark of this trait, allowing oxygen collected at the surface to be stored in the muscles and subsequently used to support extended dives. In mysticetes, the largest of marine predators, details on muscular myoglobin levels are limited. The developmental trajectory of muscular myoglobin stores has yet to be documented and any physiological links between early behavior and the development of muscular myoglobin stores remain unknown. In this study, we used muscle tissue samples from stranded mysticetes to investigate these issues. Samples from three different age cohorts and three species of mysticetes were included (total sample size = 18). Results indicate that in mysticete calves, muscle myoglobin stores comprise only a small percentage (17–23%) of conspecific adult myoglobin complements. Development of elevated myoglobin levels is protracted over the course of extended maturation in mysticetes. Additionally, comparisons of myoglobin levels between and within muscles, along with details of interspecific differences in rates of accumulation of myoglobin in very young mysticetes, suggest that levels of exercise may influence the rate of development of myoglobin stores in young mysticetes. This new information infers a close interplay between the physiology, ontogeny and early life history of young mysticetes and provides new insight into the pressures that may shape adaptive strategies in migratory mysticetes. Furthermore, the study highlights the vulnerability of specific age cohorts to impending changes in the availability of foraging habitat and marine resources.

Rapid maturation of the muscle biochemistry that supports diving in pacific walruses (Odobenus rosmarus divergens)

Physiological constraints dictate animals' abilities to exploit habitats. For marine mammals, it is important to quantify physiological limits that influence diving and their ability to alter foraging behaviors. We characterized age-specific dive limits of walruses by measuring anaerobic (acid buffering capacity) and aerobic (myoglobin content) capacities of the muscles that power hind (longissimus dorsi) and fore (supraspinatus) flipper propulsion. Mean buffering capacities were similar across muscles and age classes (a fetus, 5 neonatal calves, a 3-month old, and 20 adults), ranging from 41.31 – 54.14 slykes and 42.00 – 46.93 slykes in the longissimus and supraspinatus, respectively. Mean myoglobin in the fetus and neonatal calves fell within a narrow range (longissimus: 0.92 – 1.68 g 100 g wet muscle mass−1; supraspinatus: 0.88 – 1.64 g wet muscle mass−1). By 3 months postpartum, myoglobin in the longissimus increased by 79%, but levels in the supraspinatus remained unaltered. From 3-months postpartum to adulthood, myoglobin increased by an additional 26% in the longissimus and increased by 126% in the supraspinatus; myoglobin remained greater in the longissimus compared to the supraspinatus. Walruses are unique among marine mammals because they are born with mature muscle acid buffering capacity and attain mature myoglobin content early in life. Despite rapid physiological development, small body size limits the diving capacity of immature walruses and extreme sexual dimorphism reduces the diving capacity of adult females compared to adult males. Thus, free-ranging immature walruses likely exhibit the shortest foraging dives while adult males are capable of the longest foraging dives.