Open questions in marine mammal sensory research.

Although much research has focused on marine mammal sensory systems over the last several decades, we still lack basic knowledge for many of the species within this diverse group of animals. Our conference workshop allowed all participants to present recent developments in the field and culminated in discussions on current knowledge gaps. This report summarizes open questions regarding marine mammal sensory ecology and will hopefully serve as a platform for future research.

Essential anatomy for core clerkships: A clinical perspective.

Clerkships are defining experiences for medical students in which students integrate basic science knowledge with clinical information as they gain experience in diagnosing and treating patients in a variety of clinical settings. Among the basic sciences, there is broad agreement that anatomy is foundational for medical practice. Unfortunately, there are longstanding concerns that student knowledge of anatomy is below the expectations of clerkship directors and clinical faculty. Most allopathic medical schools require eight "core" clerkships: internal medicine (IM), pediatrics (PD), general surgery (GS), obstetrics and gynecology (OB), psychiatry (PS), family medicine (FM), neurology (NU), and emergency medicine (EM). A targeted needs assessment was conducted to determine the anatomy considered important for each core clerkship based on the perspective of clinicians teaching in those clerkships. A total of 525 clinical faculty were surveyed at 24 United States allopathic medical schools. Participants rated 97 anatomical structure groups across all body regions on a 1-4 Likert-type scale (1 = not important, 4 = essential). Non-parametric ANOVAs determined if differences existed between clerkships. Combining all responses, 91% of anatomical structure groups were classified as essential or more important. Clinicians in FM, EM, and GS rated anatomical structures in most body regions significantly higher than at least one other clerkship (p = 0.006). This study provides an evidence-base of anatomy content that should be considered important for each core clerkship and may assist in the development and/or revision of preclinical curricula to support the clinical training of medical students.

Elephant trunks use an adaptable prehensile grip.

Elephants have long been observed to grip objects with their trunk, but little is known about how they adjust their strategy for different weights. In this study, we challenge a female African elephant at Zoo Atlanta to lift 20-60 kg barbell weights with only its trunk. We measure the trunk's shape and wrinkle geometry from a frozen elephant trunk at the Smithsonian. We observe several strategies employed to accommodate heavier weights, including accelerating less, orienting the trunk vertically, and wrapping the barbell with a greater trunk length. Mathematical models show that increasing barbell weights are associated with constant trunk tensile force and an increasing barbell-wrapping surface area due to the trunk's wrinkles. Our findings may inspire the design of more adaptable soft robotic grippers that can improve grip using surface morphology such as wrinkles.

Skin wrinkles and folds enable asymmetric stretch in the elephant trunk.

The elephant's trunk is multifunctional: It must be flexible to wrap around vegetation, but tough to knock down trees and resist attack. How can one appendage satisfy both constraints? In this combined experimental and theoretical study, we challenged African elephants to reach far-away objects with only horizontal extensions of their trunk. Surprisingly, the trunk does not extend uniformly, but instead exhibits a dorsal "joint" that stretches 15% more than the corresponding ventral section. Using material testing with the skin of a deceased elephant, we show that the asymmetry is due in part to patterns of the skin. The dorsal skin is folded and 15% more pliable than the wrinkled ventral skin. Skin folds protect the dorsal section and stretch to facilitate downward wrapping, the most common gripping style when picking up items. The elephant's skin is also sufficiently stiff to influence its mechanics: At the joint, the skin requires 13 times more energy to stretch than the corresponding length of muscle. The use of wrinkles and folds to modulate stiffness may provide a valuable concept for both biology and soft robotics.

Evidence of traumatic brain injury in headbutting bovids.

Traumatic brain injury (TBI) is a leading cause of neurologic impairment and death that remains poorly understood. Rodent models have yet to produce clinical therapies, and the exploration of larger and more diverse models remains relatively scarce. We investigated the potential for brain injury after headbutting in two combative bovid species by assessing neuromorphology and neuropathology through immunohistochemistry and stereological quantification. Postmortem brains of muskoxen (Ovibos moschatus, n = 3) and bighorn sheep (Ovis canadensis, n = 4) were analyzed by high-resolution MRI and processed histologically for evidence of TBI. Exploratory histological protocols investigated potential abnormalities in neurons, microglia, and astrocytes in the prefrontal and parietal cortex. Phosphorylated tau protein, a TBI biomarker found in the cerebrospinal fluid and in neurodegenerative lesions, was used to detect possible cellular consequences of chronic or acute TBI. MRI revealed no abnormal neuropathological changes; however, high amounts of tau-immunoreactive neuritic thread clusters, neurites, and neurons were concentrated in the superficial layers of the neocortex, preferentially at the bottom of the sulci in the muskoxen and occasionally around blood vessels. Tau-immunoreactive lesions were rare in the bighorn sheep. Additionally, microglia and astrocytes showed no grouping around tau-immunoreactive cells in either species. Our preliminary findings indicate that muskoxen and possibly other headbutting bovids suffer from chronic or acute brain trauma and that the males' thicker skulls may protect them to a certain extent.

Characterizing the suckling behavior by video and 3D-accelerometry in humpback whale calves on a breeding ground.

Getting maternal milk through nursing is vital for all newborn mammals. Despite its importance, nursing has been poorly documented in humpback whales (Megaptera novaeangliae). Nursing is difficult to observe underwater without disturbing the whales and is usually impossible to observe from a ship. We attempted to observe nursing from the calf's perspective by placing CATS cam tags on three humpback whale calves in the Sainte Marie channel, Madagascar, Indian Ocean, during the breeding seasons. CATS cam tags are animal-borne multi-sensor tags equipped with a video camera, a hydrophone, and several auxiliary sensors (including a 3-axis accelerometer, a 3-axis magnetometer, and a depth sensor). The use of multi-sensor tags minimized potential disturbance from human presence. A total of 10.52 h of video recordings were collected with the corresponding auxiliary data. Video recordings were manually analyzed and correlated with the auxiliary data, allowing us to extract different kinematic features including the depth rate, speed, Fluke Stroke Rate (FSR), Overall Body Dynamic Acceleration (ODBA), pitch, roll, and roll rate. We found that suckling events lasted 18.8 ± 8.8 s on average (N = 34) and were performed mostly during dives. Suckling events represented 1.7% of the total observation time. During suckling, the calves were visually estimated to be at a 30-45° pitch angle relative to the midline of their mother's body and were always observed rolling either to the right or to the left. In our auxiliary dataset, we confirmed that suckling behavior was primarily characterized by a high average absolute roll and additionally we also found that it was likely characterized by a high average FSR and a low average speed. Kinematic features were used for supervised machine learning in order to subsequently detect suckling behavior automatically. Our study is a proof of method on which future investigations can build upon. It opens new opportunities for further investigation of suckling behavior in humpback whales and the baleen whale species.

Specializations of somatosensory innervation in the skin of humpback whales (Megaptera novaeangliae).

Cetacean behavior and life history imply a role for somatosensory detection of critical signals unique to their marine environment. As the sensory anatomy of cetacean glabrous skin has not been fully explored, skin biopsy samples of the flank skin of humpback whales were prepared for general histological and immunohistochemical (IHC) analyses of innervation in this study. Histology revealed an exceptionally thick epidermis interdigitated by numerous, closely spaced long, thin diameter penicillate dermal papillae (PDP). The dermis had a stratified organization including a deep neural plexus (DNP) stratum intermingled with small arteries that was the source of intermingled nerves and arterioles forming a more superficial subepidermal neural plexus (SNP) stratum. The patterns of nerves branching through the DNP and SNP that distribute extensive innervation to arteries and arterioles and to the upper dermis and PDP provide a dense innervation associated through the whole epidermis. Some NF-H+ fibers terminated at the base of the epidermis and as encapsulated endings in dermal papillae similar to Merkel innervation and encapsulated endings seen in terrestrial mammals. However, unlike in all mammalian species assessed to date, an unusual acellular gap was present between the perineural sheaths and the central core of axons in all the cutaneous nerves perhaps as mechanism to prevent high hydrostatic pressure from compressing and interfering with axonal conductance. Altogether the whale skin has an exceptionally dense low-threshold mechanosensory system innervation most likely adapted for sensing hydrodynamic stimuli, as well as nerves that can likely withstand high pressure experienced during deep dives.

The unique rectus extraocular muscles of cetaceans: Homologies and possible functions.

Each rectus extraocular muscle in cetaceans divides into two portions: a massive palpebral belly that inserts into the deep surface of the eyelids and a smaller scleral belly that inserts onto the eyeball. While the cetacean palpebral insertions have long been recognized, their homologies and functions remain unclear. To compare cetacean rectus EOM insertions with the global and orbital rectus EOM insertions of other mammals we dissected orbital contents of 20 odontocete species, 2 mysticete species and 18 non-cetacean species, both aquatic and terrestrial. Four cetacean species were also examined with magnetic resonance imaging (MRI). All four rectus muscles in cetaceans had well-developed palpebral bellies and insertions. Adjacent palpebral bellies showed varying degrees of fusion, from near independence to near complete fusion. Fusion was most complete towards palpebral insertions and less towards origins. A medial moiety of the superior rectus palpebral belly is likely the levator palpebrae superioris. Smaller but still robust scleral insertions were present on all recti, with the medial rectus (MR) being significantly more muscular than the others. All non-cetacean species examined had recti with distinct global and orbital insertions, the latter generally onto Tenon's capsule. Orbital insertions in pygmy hippopotamus and Florida manatee extended into the deep surfaces of the eyelids, hence qualifying as palpebral insertions. Our results suggest that rectus EOMs of mammals generally have both global and orbital insertions, and that palpebral bellies of cetaceans and other species are modified homologs of the orbital insertions. The presence of palpebral insertions in pygmy hippopotamus and absence in other cetartiodactyls suggests an intermediate condition between terrestrial cetartiodactyls and cetaceans. Palpebral insertions in Florida manatee and reports of their presence in some pinnipeds suggest parallel evolution in multiple aquatic lineages. Various functions of cetacean palpebral recti have been proposed, including eyelid dilators, protection during diving and thermogenesis for warming eye and brain. For further insight into their possible functions, we observed eye movements of captive bottlenose dolphins (Tursiops truncatus) at the U.S. National Aquarium. Our observations showed that in addition to rotation of the eyeball the entire surrounding palpebral region also moves during gaze changes. For example during upward gaze the globe not only rotates in supraduction but translates dorsally as well. It appears the rectus palpebral bellies are responsible for flexing the palpebral structures and thus also translating the globe, while the scleral insertions act directly for ocular rotation. Along with frequent non-conjugate eye movements, the oculomotor mechanics and repertoire of cetaceans are thus quite distinctive. Summarily, axial displacement within the orbit is a major 'eye movement' in cetaceans, with protrusion and retraction mediated by well-developed circular muscles and retractor bulbi respectively. Torsional eye movements driven by elaborate oblique EOMs are likewise significant. The roles of rectus EOMs for ocular rotation via their scleral insertions, especially the highly muscular MR, are for typical supra/infraductions and nasal/temporal ductions. The palpebral bellies accentuate these ductions by translating the globe and surrounding structures in the same direction.

Marine mammal sensory systems: Special issue overview.

Marine mammals are a unique group of organisms that are secondarily adapted to the aquatic environment. Their specific lifestyle requires numerous adaptations of anatomy and physiology in general, and sensory physiology in particular. During the course of evolution, marine mammal senses changed to fit with the specific requirements of underwater sensing, while at the same time retaining aerial sensing to various degrees. In this special issue, state of the art science in the field of marine mammal sensory research is reported for representatives of all marine mammal groups, unfortunately with the exclusion of the polar bear. The articles focus on somatosensation of the glabrous skin of cetaceans and mechanoreception, including haptics, hydrodynamics, and acoustics, to chemoreception. Articles even deal with electroreception, highlighting that the bottlenose dolphin can perceive weak electric stimuli, and vision, indicating that harbor seals are able to derive temporal information from an optical stimulus. Altogether this special issue illustrates the diversity of research in the field regarding sensory systems, species, or experimental approaches. The strength of this special issue lies in the combination of carefully conducted anatomical studies paired with observations and behavioral studies attempting to relate "form" and "function" as well as in the many impulses and future avenues mentioned by numerous contributions.

Comparative examination of pinniped craniofacial musculature and its role in aquatic feeding.

Secondarily aquatic tetrapods have many unique morphologic adaptations for life underwater compared with their terrestrial counterparts. A key innovation during the land-to-water transition was feeding. Pinnipeds, a clade of air-breathing marine carnivorans that include seals, sea lions, and walruses, have evolved multiple strategies for aquatic feeding (e.g., biting, suction feeding). Numerous studies have examined the pinniped skull and dental specializations for underwater feeding. However, data on the pinniped craniofacial musculoskeletal system and its role in aquatic feeding are rare. Therefore, the objectives of this study were to conduct a comparative analysis of pinniped craniofacial musculature and examine the function of the craniofacial musculature in facilitating different aquatic feeding strategies. We performed anatomic dissections of 35 specimens across six pinniped species. We describe 32 pinniped craniofacial muscles-including facial expression, mastication, tongue, hyoid, and soft palate muscles. Pinnipeds broadly conform to mammalian patterns of craniofacial muscle morphology. Pinnipeds also exhibit unique musculoskeletal morphologies-in muscle position, attachments, and size-that likely represent adaptations for different aquatic feeding strategies. Suction feeding specialists (bearded and northern elephant seals) have a significantly larger masseter than biters. Further, northern elephant seals have large and unique tongue and hyoid muscle morphologies compared with other pinniped species. These morphologic changes likely help generate and withstand suction pressures necessary for drawing water and prey into the mouth. In contrast, biting taxa (California sea lions, harbor, ringed, and Weddell seals) do not exhibit consistent craniofacial musculoskeletal adaptations that differentiate them from suction feeders. Generally, we discover that all pinnipeds have well-developed and robust craniofacial musculature. Pinniped head musculature plays an important role in facilitating different aquatic feeding strategies. Together with behavioral and kinematic studies, our data suggest that pinnipeds' robust facial morphology allows animals to switch feeding strategies depending on the environmental context-a critical skill in a heterogeneous and rapidly changing underwater habitat.

Unconventional animal models for traumatic brain injury and chronic traumatic encephalopathy.

Traumatic brain injury (TBI) is one of the main causes of death worldwide. It is a complex injury that influences cellular physiology, causes neuronal cell death, and affects molecular pathways in the brain. This in turn can result in sensory, motor, and behavioral alterations that deeply impact the quality of life. Repetitive mild TBI can progress into chronic traumatic encephalopathy (CTE), a neurodegenerative condition linked to severe behavioral changes. While current animal models of TBI and CTE such as rodents, are useful to explore affected pathways, clinical findings therein have rarely translated into clinical applications, possibly because of the many morphofunctional differences between the model animals and humans. It is therefore important to complement these studies with alternative animal models that may better replicate the individuality of human TBI. Comparative studies in animals with naturally evolved brain protection such as bighorn sheep, woodpeckers, and whales, may provide preventive applications in humans. The advantages of an in-depth study of these unconventional animals are threefold. First, to increase knowledge of the often-understudied species in question; second, to improve common animal models based on the study of their extreme counterparts; and finally, to tap into a source of biological inspiration for comparative studies and translational applications in humans.

A first radiotherapy application of functional bulboclitoris anatomy, a novel female sexual organ-at-risk, and organ-sparing feasibility study.

OBJECTIVE: The bulboclitoris (clitoris and vestibular bulbs) is the primary organ responsible for female sexual arousal and orgasm. Effects of radiotherapy on the bulboclitoris are unknown, as its structure/function has yet to be described in radiotherapy, and it overlaps only partially with the external genitalia structure. Our aim was to: describe bulboclitoris structure, function and delineation; compare volume of and dose delivered to the bulboclitoris vs external genitalia; and, compare bulboclitoris-sparing IMRT (BCS-IMRT) to standard IMRT (S-IMRT) to determine reoptimization feasibility. METHODS: Our expert team (anatomist, pelvic radiologist, radiation oncologist) reviewed bulboclitoris anatomy and developed contouring guidance for radiotherapy. 20 female patients with anal cancer treated with chemoradiation were analyzed. Sexual organs at risk (OARs) included the external genitalia and the bulboclitoris. Volumes, dice similarity coefficients (DSCs) and dose received using S-IMRT were compared. Plans were reoptimized using BCS-IMRT. Dose-volume histograms (DVHs) for PTVs and all OARs were compared for BCS-IMRT vs S-IMRT. RESULTS: Bulboclitoris structure, function and delineation are described herein. The bulboclitoris occupies 20cc (IQR:12-24), largely distinct from the external genitalia (DSC <0.05). BCS-IMRT was superior to S-IMRT in reducing the dose to the bulboclitoris, with the greatest reductions in V30 and V40, with no significant changes in dose to other OARs or PTV 1/V95. CONCLUSION: The bulboclitoris can be contoured on planning imaging, largely distinct from the external genitalia. Compared with S-IMRT, BCS-IMRT dramatically reduced dose to the bulboclitoris in anal cancer planning. BCS-IMRT might safely reduce sexual toxicity compared with standard approaches. ADVANCES IN KNOWLEDGE: The structure and function of the bulboclitoris, the critical primary organ responsible for female sexual arousal and orgasm, has yet to be described in the radiotherapy literature. Structure, function and delineation of the bulboclitoris are detailed, delineation and bulboclitoris-sparing IMRT were feasible, and sparing reduces the dose to the bulboclitoris nearly in half in female patients receiving IMRT for anal cancer, warranting further clinical study.

Suction feeding by elephants.

Despite having a trunk that weighs over 100 kg, elephants mainly feed on lightweight vegetation. How do elephants manipulate such small items? In this experimental and theoretical investigation, we filmed elephants at Zoo Atlanta showing that they can use suction to grab food, performing a behaviour that was previously thought to be restricted to fishes. We use a mathematical model to show that an elephant's nostril size and lung capacity enables them to grab items using comparable pressures as the human lung. Ultrasonographic imaging of the elephant sucking viscous fluids show that the elephant's nostrils dilate up to [Formula: see text] in radius, which increases the nasal volume by [Formula: see text]. Based on the pressures applied, we estimate that the elephants can inhale at speeds of over 150 m s-1, nearly 30 times the speed of a human sneeze. These high air speeds enable the elephant to vacuum up piles of rutabaga cubes as well as fragile tortilla chips. We hope these findings inspire further work in suction-based manipulation in both animals and robots.

Cetacean Orbital Muscles: Anatomy and Function of the Circular Layers.

Dissections of cetacean orbits identified two distinct circular muscle layers that are uniquely more elaborate than the orbitalis muscles described in numerous mammals. The circular orbital muscles in cetaceans form layers that lie both external and internal to the rectus extra ocular muscles (EOMs). A cone-shaped external circular muscle (ECM) that invests the external surface of the rectus EOMs was found in all cetacean specimens examined. The cetacean ECM corresponds generally to descriptions of the musculus orbitalis in various mammals but is more strongly developed and has more layers than in noncetaceans. A newly identified internal circular muscle (ICM) is located internal to the rectus EOMs and external to the retractor bulbi (RB). The RB is massive in cetaceans and is encased in a connective tissue layer containing convoluted bundles of blood vessels. The most robust ECM and ICM layers were in sperm whale (Physeter macrocephalus) where they form complete rings. Surprisingly, histological analysis showed the sperm whale ECM to contain both smooth and striated (skeletal) muscle layers while the ICM appeared to contain solely skeletal muscle fibers. The extreme development of the ECM (orbitalis) and RB suggest a co-evolved system mediating high degrees of protrusion and retraction in cetaceans. We know of no homolog of the ICM but its function seems likely related to the complex vascular structures surrounding and deep to the retractor muscle. Skeletal muscle components in orbital circular muscles appear to be highly derived specializations unknown outside of cetaceans. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:1792-1811, 2020. © 2019 American Association for Anatomy.

Mysticetes to MiniConference to Manuscripts: Introduction to Thematic Issue on Mysticete Anatomy.

This issue of the Anatomical Record is focused on the theme of Mysticete Anatomy. There are six included articles that explore the anatomy of the nasal region (Marquez et al., 2018; Maust-Mohl et al., 2018), larynx (Damien et al., 2018), lungs (Fetherston et al., 2018), sublingual fascia (Werth et al., 2018), and brain (Raghanti et al., 2018). These papers document anatomical features exhibited by mysticetes (baleen whales) and their related cousins (including other whales, and the semiaquatic moose and hippopotamus). This theme stems from a 2-day MiniConference on Mysticete Anatomy, hosted at the Icahn School of Medicine at Mount Sinai in New York City on May 2016. Anatomy is explored in the contexts of function and evolution of aquatic adaptations. Anat Rec, 2019. © 2019 Wiley Periodicals, Inc. Anat Rec, 302:663-666, 2019. © 2019 Wiley Periodicals, Inc.

A Comparison of Common Hippopotamus (Artiodactyla) and Mysticete (Cetacea) Nostrils: An Open and Shut Case.

Hippos are considered the closest living relatives to cetaceans and they have some similar adaptations for aquatic living, such as a modified respiratory tract. Behavioral observations of male and female common hippos (Hippopotamus amphibius) at Disney's Animal Kingdom® and the Adventure Aquarium were conducted to describe and examine movements of the nostrils during respiration (inspiration, expiration, and inter-breath interval). The hippo nostril is a crescent shaped opening with lateral and medial aspects that are mobile and can be adducted and abducted to regulate the nostril opening. Notably, the default (resting) position of the nostrils is closed during the inter-breath interval, even when hippos are resting in water and their heads are not submerged. Similar to cetaceans, this aquatic adaptation protects the respiratory tract from an accidental incursion of water that can occur even when the nostrils are above water. Dissection of a deceased captive common hippo suggests there are separate muscles that pull the medial and lateral aspects for abduction. The internal nasal passage has a nasal plug that is similar in shape but less pronounced than the nasal plugs of two baleen whale species studied (minke whale Balaenoptera acutorostrata, fin whale Balaenoptera physalus). Examination of the musculature suggests fibers attach from the premaxillae and extend caudally to retract the plug to open the nasal passage. We discuss similarities and differences of the nostrils/blowholes of fully aquatic, semi-aquatic, and terrestrial species to assess adaptations related to environmental conditions that may be convergent or derived from a common ancestor. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc. Anat Rec, 302:693-702, 2019. © 2018 Wiley Periodicals, Inc.

A Comparison of the Cortical Structure of the Bowhead Whale (Balaena mysticetus), a Basal Mysticete, with Other Cetaceans.

Few studies exist of the bowhead whale brain and virtually nothing is known about its cortical cytoarchitecture or how it compares to other cetaceans. Bowhead whales are one of the least encephalized cetaceans and occupy a basal phylogenetic position among mysticetes. Therefore, the bowhead whale is an important specimen for understanding the evolutionary specializations of cetacean brains. Here, we present an overview of the structure and cytoarchitecture of the bowhead whale cerebral cortex gleaned from Nissl-stained sections and magnetic resonance imaging (MRI) in comparison with other mysticetes and odontocetes. In general, the cytoarchitecture of cetacean cortex is consistent in displaying a thin cortex, a thick, prominent layer I, and absence of a granular layer IV. Cell density, composition, and width of layers III, V, and VI vary among cortical regions, and cetacean cortex is cell-sparse relative to that of terrestrial mammals. Notably, all regions of the bowhead cortex possess high numbers of von Economo neurons and fork neurons, with the highest numbers observed at the apex of gyri. The bowhead whale is also distinctive in having a significantly reduced hippocampus that occupies a space below the corpus callosum within the lateral ventricle. Consistent with other balaenids, bowhead whales possess what appears to be a blunted temporal lobe, which is in contrast to the expansive temporal lobes that characterize most odontocetes. The present report demonstrates that many morphological and cytoarchitectural characteristics are conserved among cetaceans, while other features, such as a reduced temporal lobe, may characterize balaenids among mysticetes. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc. Anat Rec, 302:745-760, 2019. © 2018 Wiley Periodicals, Inc.

Anatomy and Functional Morphology of the Mysticete Rorqual Whale Larynx: Phonation Positions of the U-Fold.

Many Mysticetes (baleen whales) are acoustically active marine mammals. This is epitomized by rorquals, and specifically male humpback whales (Megaptera novaeangliae) whose complex songs comprise a wide range of vocalizations. The sound production mechanism of odontocetes (toothed whales, including dolphins and porpoises) is well described, in contrast to that of mysticetes whose vocalization mechanism remains a subject of active scientific investigation. Anatomical observations and acoustic signal processing have led to divergent hypotheses under the framework of a production-based approach. We attempt to unify these hypotheses by broadening existing data with our new anatomical investigation, interpreted in light of known acoustical properties of mysticete vocalizations. We examined 15 specimens of four rorqual species: sei whale (Baleanoptera borealis), fin whale (Baleanoptera physalus), minke whale (Baleanoptera acutorostrata), and humpback whale (Megaptera novaeangliae). Based on these data and on previous literature, we propose a description of three functional positions (rest, breathing, and recirculation), unidirectional egressive airflow for sound production (from lungs to laryngeal sac), and new nomenclature for different parts of the U-fold (distal section, midsection, and corniculate flaps). Each of these sections has specific morphological and acoustical properties that support the concept of "mode variation" in baleen whale vocalizations. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc. Anat Rec, 302:703-717, 2019. © 2018 Wiley Periodicals, Inc.