D. Ann Pabst

The glymphatic system, an analog of the peripheral lymphatic system in the brain, and the meningeal lymphatic system are critical to central nervous system health. The glymphatic system functions to distribute cerebrospinal fluid and important compounds throughout the brain and to remove metabolic waste. The flow of cerebrospinal fluid through this system is affected by changes in cerebral blood flow, intracranial pressure, and vascular tone. Cetaceans experience profound cardiorespiratory alterations while diving that can directly affect cerebrospinal fluid and blood flow and, thus, glymphatic function. Our goal was to investigate glymphatic and lymphatic system structures, including perivascular spaces, aquaporin-4 water channels, meningeal lymphatic, and dural venous sinus vessels in the common dolphin (Delphinus delphis), using immunofluorescent labeling, histochemical staining, and postmortem computed tomography (CT) angiography. We highlight perivascular spaces and aquaporin-4 water channels surrounding blood vessels in the parenchyma and demonstrate evidence of meningeal lymphatic vessels and associated dural venous sinuses. These results demonstrate that common dolphins possess the key anatomical structures required for functional glymphatic and meningeal lymphatic systems. Future studies can build upon these anatomical discoveries to study the function and role of these systems in brain health in this species.

Compared with terrestrial mammals, marine mammals possess increased muscle myoglobin concentrations (Mb concentration, g Mb · 100g⁻¹ muscle), enhancing their onboard oxygen (O₂) stores and their aerobic dive limit. Although myoglobin is not homogeneously distributed, cetacean muscle O₂ stores have been often determined by measuring Mb concentration from a single muscle sample (longissimus dorsi) and multiplying that value by the animal’s locomotor muscle or total muscle mass. This study serves to determine the accuracy of previous cetacean muscle O₂ stores calculations. For that, body muscles from three delphinid species: Delphinus delphis, Stenella coeruleoalba, and Stenella frontalis, were dissected and weighed. Mb concentration was calculated from six muscles/muscle groups (epaxial, hypaxial and rectus abdominis; mastohumeralis; sternohyoideus; and dorsal scalenus), each representative of different functional groups (locomotion powering swimming, pectoral fin movement, feeding and respiration, respectively). Results demonstrated that the Mb concentration was heterogeneously distributed, being significantly higher in locomotor muscles. Locomotor muscles were the major contributors to total muscle O₂ stores (mean 92.8%) due to their high Mb concentration and large muscle masses. Compared to this method, previous studies assuming homogenous Mb concentration distribution likely underestimated total muscle O₂ stores by 10% when only considering locomotor muscles and overestimated them by 13% when total muscle mass was considered.

Mesoplodont beaked whales are extreme divers, diving for over 45 mins and to depths of over 800 m. These dives are of similar depth and duration to those of the giant sperm whale (Physeter macrocephalus) whose body mass can be 50 times larger. Velten et al. (2013) provided anatomical data that demonstrated that on-board oxygen stores were sufficient to aerobically support the extreme dives of mesoplodonts if their diving metabolic rates are low. Because no physiological data yet exist, we utilized an anatomical approach—the body composition technique—to examine the relative metabolic rates of mesoplodonts. We utilized a systematic mass dissection protocol to compare the body composition of mesoplodonts with those of two short duration, shallow divers—the harbor porpoise (Phocoena phocoena) and bottlenose dolphin (Tursiops truncatus). We then investigated the body composition of two other extreme divers, the southern elephant seal (Mirounga leonina) and P. macrocephalus using data from the literature. Our results demonstrate that extreme divers invest a smaller percentage of their total body mass (TBM) in metabolically expensive brain and viscera, and a larger percent of their TBM in inexpensive integument, bone, and muscle, than do the shallow divers. Deep divers also share features of their locomotor muscle that contribute to relatively low tissue metabolic rates and high oxygen storage capacity, including large muscle fiber diameters, low mitochondrial volume densities, and high myoglobin concentrations. One feature of the locomotor muscle of mesoplodonts, though, is unique among deep divers investigated to date. Rather than having an endurance athlete’s muscle fiber profile, dominated by slow oxidative fibers, mesoplodonts possess a sprinter’s profile, dominated by fast glycolytic fibers. Velten et al. (2013) hypothesized that these fibers are likely inactive during routine swimming and provide a large, metabolically inexpensive oxygen store for the slow oxidative fibers to aerobically power swimming. We suggest that future anatomical analyses, coupled with performance data transduced through tagging studies, will enhance our understanding of the extreme diving capabilities of marine mammals.

Most mammals possess stamina because their locomotor and respiratory (i.e., ventilatory) systems are mechanically coupled. These systems are decoupled, however, in bottlenose dolphins (Tursiops truncatus) as they swim on a breath hold. Locomotion and ventilation are coupled only during their brief surfacing event, when they respire explosively (up to 90% of total lung volume in approximately 0.3 s) (Ridgway et al. 1969 Science 166:1651–1654). The predominantly slow‐twitch fiber profile of their diaphragm (Dearolf 2003 J Morphol 256:79–88) suggests that this muscle does not likely power their rapid ventilatory event. Based on Bramble's (1989 Amer Zool 29:171–186) biomechanical model of locomotor‐respiratory coupling in galloping mammals, it was hypothesized that locomotor muscles function to power ventilation in bottlenose dolphins. It was further hypothesized that these muscles would be composed predominantly of fast‐twitch fibers to facilitate the bottlenose dolphin's rapid ventilation. The gross morphology of craniocervical (scalenus, sternocephalicus, sternohyoid), thoracic (intercostals, transverse thoracis), and lumbopelvic (hypaxialis, rectus abdominis, abdominal obliques) muscles (n = 7) and the fiber‐type profiles (n = 6) of selected muscles (scalenus, sternocephalicus, sternohyoid, rectus abdominis) of bottlenose dolphins were investigated. Physical manipulations of excised thoracic units were carried out to investigate potential actions of these muscles. Results suggest that the craniocervical muscles act to draw the sternum and associated ribs craniodorsally, which flares the ribs laterally, and increases the thoracic cavity volume required for inspiration. The lumbopelvic muscles act to draw the sternum and caudal ribs caudally, which decreases the volumes of the thoracic and abdominal cavities required for expiration. All muscles investigated were composed predominantly of fast‐twitch fibers (range 61–88% by area) and appear histochemically poised for rapid contraction. These combined results suggest that dolphins utilize muscles, similar to those used by galloping mammals, to power their explosive ventilation. J. Morphol., 2008.

The melon is a lipid‐rich structure located in the forehead of odontocetes that functions to propagate echolocation sounds into the surrounding aquatic environment. To date, the melon's ability to guide and impedance match biosonar sounds to seawater has been attributed to its unique fatty acid composition. However, the melon is also acted upon by complex facial muscles derived from the m. maxillonasolabialis. The goal of this study was to investigate the gross morphology of the melon in bottlenose dolphins (Tursiops truncatus) and to describe how it is tendinously connected to these facial muscles. Standard gross dissection (N = 8 specimens) and serial sectioning (N = 3 specimens) techniques were used to describe the melon and to identify its connections to the surrounding muscles and blubber in three orthogonal body planes. The dolphin forehead was also thin‐sectioned in three body planes (N = 3 specimens), and polarized light was used to reveal the birefringent collagen fibers within and surrounding the melon. This study identified distinct regions of the melon that vary in shape and display locally specific muscle‐tendon morphologies. These regions include the bilaterally symmetric main body and cone and the asymmetric right and left caudal melon. This study is the first to identify that each caudal melon terminates in a lipid cup that envelopes the echolocation sound generators. Facial muscles of the melon have highly organized tendon populations that traverse the melon and insert into either the surrounding blubber, the connective tissue matrix of the nasal plug, or the connective tissue sheath surrounding the sound generators. The facial muscles and tendons also lie within multiple orthogonal body planes, which suggest that the melon is capable of complex shape change. The results of this study suggest that these muscles could function to change the frequency, beam width, and directionality of the emitted sound beam in bottlenose dolphins. The echolocation sound propagation pathway within the dolphin forehead appears to be a tunable system. J. Morphol., 2008.

This study investigated the functional morphology of the blubber that forms the caudal keels of the harbor porpoise (Phocoena phocoena). Blubber is a pliant biocomposite formed by adipocytes and structural fibers composed of collagen and elastic fibers. Caudal keels are dorsally and ventrally placed triangular wedges of blubber that define the hydrodynamic profile of the porpoise tailstock. Mechanical tests on carcasses demonstrate that when keels are bent, they strain nonuniformly along their lengths, with highest strains just caudal to the dorsal fin and lowest at the insertion of the flukes. Therefore, caudal keels undergo nonuniform longitudinal deformation while maintaining a stable, triangular cross‐sectional shape. Polarizing and transmitted light microscopy techniques were used to investigate blubber's 3D fiber architecture along the length of the dorsal keel. The triangular cross‐sectional shape of the keel appears to be maintained by structural fibers oriented to act as tensile stays. The construction of the blubber composite is regionally specific :structural fiber densities and diameters are higher in the relatively stiff caudal region of the keel than in the more deformable cranial keel region. The orientations of structural fibers also change along the length of the keel. Cranially, no fibers are oriented along the long axis, whereas a novel population of longitudinally oriented fibers reinforces the keel at the insertion of the flukes. Thus, differences in the distribution and orientation of structural fibers contribute to the regionally specific mechanical properties of the dorsal keel. J. Morphol. 261:105–117, 2004.

Blubber, the lipid‐rich hypodermis of cetaceans, functions in thermoregulation, buoyancy control, streamlining, metabolic energy storage, and locomotion. This study investigated the development of this specialized hypodermis in bottlenose dolphins (Tursiops truncatus) across an ontogenetic series, including fetuses, neonates, juveniles, subadults, and adults. Blubber samples were collected at the level of the mid‐thorax, from robust specimens (n = 25) that stranded along the coasts of North Carolina and Virginia. Blubber was dissected from the carcass and its mass, and the depth and lipid content at the sample site, were measured. Samples were prepared using standard histological methods, viewed by light microscopy, and digital images of blubber captured. Images were analyzed through the depth of the blubber for morphological and structural features including adipocyte size, shape, and numbers, and extracellular, structural fiber densities. From fetus to adult, blubber mass and depth increased proportionally with body mass and length. Blubber lipid content increased dramatically with increasing fetal length. Adult and juvenile blubber had significantly higher blubber lipid content than fetuses, and this increase was reflected in mean adipocyte size, which increased significantly across all robust life history categories. In juvenile, subadult, and adult dolphins, this increase in cell size was not uniform across the depth of the blubber, with the largest increases observed in the middle and deep blubber regions. Through‐depth counts of adipocytes were similar in all life history categories. These results suggest that blubber depth is increased during postnatal growth by increasing cell size rather than cell number. In emaciated adults (n = 2), lipid mobilization, as evidenced by a decrease in adipocyte size, was localized to the middle and deep blubber region. Thus, in terms of both lipid accumulation and depletion, the middle and deep blubber appear to be the most metabolically dynamic. The superficial blubber likely serves a structural role important in streamlining the animal. This study demonstrates that blubber is not a homogeneous tissue through its depth, and that it displays life history‐dependent changes in its morphology and lipid content. J. Morphol. 259:7–20, 2004.