Hearing and Smell in cetacean History

Immunohistochemistry of the nasal cavity-associated lymphoid tissue in the dolphin (Stenella coeruleoalba, Meyen 1833)

The striped dolphin (Stenella coeruleoalba) is a medium-sized pelagic dolphin with a single external nasal opening (blowhole) located in the rostral and dorsal regions of the skull. The nasal cavity is divided into three sections: the olfactory, respiratory, and vestibular areas. The surface epithelium lining the regio vestibularis is the first tissue in the nose to be directly affected by environmental antigens. Cetaceans have a significant amount of mucosa-associated lymphoid tissue (MALT) located throughout their bodies. The lymphoid tissue found in the nasal mucosa is known as nose- or nasopharynx-associated lymphoid tissue (NALT). NALT has not yet been studied in dolphins, but it has been identified and documented in humans and laboratory rodents. This study utilized toll-like receptor 2 (TLR2), CD4, Langerin/CD207, and inducible nitric oxide synthase to characterize, for the first time, immune cells in the mucosal regio vestibularis of the S. coeruleoalba nasal cavity using confocal microscopy immunofluorescence techniques. The findings revealed scattered immune cells immunoreactive to the tested antibodies, present in both the epithelial tissue lining the nasal cavity vestibulum and the underlying connective tissue. This study enhances our comprehension of the immune system of cetaceans. RESEARCH HIGHLIGHTS: This study provides new insights into NALT in S. coeruleoalba. This research deepens the knowledge of the skin of cetaceans.

Evolution of whale sensory ecology: Frontiers in nondestructive anatomical investigations

Studies surrounding the evolution of sensory system anatomy in cetaceans over the last ~100 years have shed light on aspects of the early evolution of hearing sensitivities, the small relative size of the organ of balance (semicircular canals and vestibule), brain (endocast) shape and relative volume changes, and ontogenetic development of sensory-related structures. Here, I review advances in our knowledge of sensory system anatomy as informed by the use of nondestructive imaging techniques, with a focus on applied methods in computed tomography (CT and μCT), and identify the key questions that remain to be addressed. Of these, the most important are: Is lower frequency hearing sensitivity the ancestral condition for whales? Did echolocation evolve more than once in odontocetes; and if so, when and why? How has the structure of the cetacean brain changed, through the evolution of whales, and does this correspond to changes in hearing sensitivities? Finally, what are the general pathways of ontogenetic development of sensory systems in odontocetes and mysticetes? Answering these questions will allow us to understand important macroevolutionary patterns in a fully aquatic mammalian group and provides baseline data on species for which we have limited biological information because of logistical limitations.

Co-evolution of cerebral and cerebellar expansion in cetaceans

Cetaceans possess brains that rank among the largest to have ever evolved, either in terms of absolute mass or relative to body size. Cetaceans have evolved these huge brains under relatively unique environmental conditions, making them a fascinating case study to investigate the constraints and selection pressures that shape how brains evolve. Indeed, cetaceans have some unusual neuroanatomical features, including a thin but highly folded cerebrum with low cortical neuron density, as well as many structural adaptations associated with acoustic communication. Previous reports also suggest that at least some cetaceans have an expanded cerebellum, a brain structure with wide‐ranging functions in adaptive filtering of sensory information, the control of motor actions, and cognition. Here, we report that, relative to the size of the rest of the brain, both the cerebrum and cerebellum are dramatically enlarged in cetaceans and show evidence of co‐evolution, a pattern of brain evolution that is convergent with primates. However, we also highlight several branches where cortico‐cerebellar co‐evolution may be partially decoupled, suggesting these structures can respond to independent selection pressures. Across cetaceans, we find no evidence of a simple linear relationship between either cerebrum and cerebellum size and the complexity of social ecology or acoustic communication, but do find evidence that their expansion may be associated with dietary breadth. In addition, our results suggest that major increases in both cerebrum and cerebellum size occurred early in cetacean evolution, prior to the origin of the major extant clades, and predate the evolution of echolocation.

Behavioural responses of humpback whales to food-related chemical stimuli

Baleen whales face the challenge of finding patchily distributed food in the open ocean. Their relatively well-developed olfactory structures suggest that they could identify the specific odours given off by planktonic prey such as krill aggregations. Like other marine predators, they may also detect dimethyl sulfide (DMS), a chemical released in areas of high marine productivity. However, dedicated behavioural studies still have to be conducted in baleen whales in order to confirm the involvement of chemoreception in their feeding ecology. We implemented 56 behavioural response experiments in humpback whales using two food-related chemical stimuli, krill extract and DMS, as well as their respective controls (orange clay and vegetable oil) in their breeding (Madagascar) and feeding grounds (Iceland and Antarctic Peninsula). The whales approached the stimulus area and stayed longer in the trial zone during krill extract trials compared to control trials, suggesting that they were attracted to the chemical source and spent time exploring its surroundings, probably in search of prey. This response was observed in Iceland, and to a lesser extend in Madagascar, but not in Antarctica. Surface behaviours indicative of sensory exploration, such as diving under the stimulus area and stopping navigation, were also observed more often during krill extract trials than during control trials. Exposure to DMS did not elicit such exploration behaviours in any of the study areas. However, acoustic analyses suggest that DMS and krill extract both modified the whales' acoustic activity in Madagascar. Altogether, these results provide the first behavioural evidence that baleen whales actually perceive prey-derived chemical cues over distances of several hundred metres. Chemoreception, especially olfaction, could thus be used for locating prey aggregations and for navigation at sea, as it has been shown in other marine predators including seabirds.

New records of the archaic dolphin Agorophius (Mammalia: Cetacea) from the upper Oligocene Chandler Bridge Formation of South Carolina, USA

The stem odontocete Agorophius pygmaeus (Ashley Formation, lower Oligocene, South Carolina; 29.0–26.57 Ma) has been a critical point of comparison for studies of early neocete evolution owing to its early discovery as well as its transitional anatomy relative to archaeocete whales and modern odontocetes. Some time during the late nineteenth century the holotype skull went missing and has never been relocated; supplementary reference specimens have since been recently referred to the species from the Ashley Formation and the overlying Chandler Bridge Formation (upper Oligocene; 24.7–23.5). New crania referable to Agorophius sp. are identifiable to the genus based on several features of the intertemporal region. Furthermore, all published specimens from the Chandler Bridge Formation consistently share larger absolute size and a proportionally shorter exposure of the parietal in the skull roof than specimens from the Ashley Formation (including the holotype). Furthermore, these specimens include well-preserved ethmoid labyrinths and cribriform plates, indicating that Agorophius primitively retained a strong olfactory sense. These new crania suggest that at least two species of Agorophius are present in the Oligocene of South Carolina, revealing a somewhat more complicated taxonomic perspective.

Neuroanatomy and inner ear labyrinths of the narwhal, Monodon monoceros, and beluga, Delphinapterus leucas (Cetacea: Monodontidae)

Narwhals (Monodon monoceros) and belugas (Delphinapterus leucas) are the only extant members of the Monodontidae, and are charismatic Arctic-endemic cetaceans that are at risk from global change. Investigating the anatomy and sensory apparatuses of these animals is essential to understanding their ecology and evolution, and informs efforts for their conservation. Here, we use X-ray CT scans to compare aspects of the endocranial and inner ear labyrinth anatomy of extant monodontids and use the overall morphology to draw larger inferences about the relationship between morphology and ecology. We show that differences in the shape of the brain, vasculature, and neural canals of both species may relate to differences in diving and other behaviors. The cochleae are similar in morphology in the two species, signifying similar hearing ranges and a close evolutionary relationship. Lastly, we compare two different methods for calculating 90var - a calculation independent of body size that is increasingly being used as a proxy for habitat preference. We show that a 'direct' angular measurement method shows significant differences between Arctic and other habitat preferences, but angle measurements based on planes through the semicircular canals do not, emphasizing the need for more detailed study and standardization of this measurement. This work represents the first comparative internal anatomical study of the endocranium and inner ear labyrinths of this small clade of toothed whales.

Diversity in olfactory bulb size in birds reflects allometry, ecology, and phylogeny

The relative size of olfactory bulbs (OBs) is correlated with olfactory capabilities across vertebrates and is widely used to assess the relative importance of olfaction to a species’ ecology. In birds, variations in the relative size of OBs are correlated with some behaviors; however, the factors that have led to the high level of diversity seen in OB sizes across birds are still not well understood. In this study, we use the relative size of OBs as a neuroanatomical proxy for olfactory capabilities in 135 species of birds, representing 21 orders. We examine the scaling of OBs with brain size across avian orders, determine likely ancestral states and test for correlations between OB sizes and habitat, ecology, and behavior. The size of avian OBs varied with the size of the brain and this allometric relationship was for the most part isometric, although species did deviate from this trend. Large OBs were characteristic of more basal species and in more recently derived species the OBs were small. Living and foraging in a semi-aquatic environment was the strongest variable driving the evolution of large OBs in birds; olfaction may provide cues for navigation and foraging in this otherwise featureless environment. Some of the diversity in OB sizes was also undoubtedly due to differences in migratory behavior, foraging strategies and social structure. In summary, relative OB size in birds reflect allometry, phylogeny and behavior in ways that parallel that of other vertebrate classes. This provides comparative evidence that supports recent experimental studies into avian olfaction and suggests that olfaction is an important sensory modality for all avian species.

Anatomical evidence for low frequency sensitivity in an archaeocete whale: comparison of the inner ear of Zygorhiza kochii with that of crown Mysticeti

The evolution of hearing in cetaceans is a matter of current interest given that odontocetes (toothed whales) are sensitive to high frequency sounds and mysticetes (baleen whales) are sensitive to low and potentially infrasonic noises. Earlier diverging stem cetaceans (archaeocetes) were hypothesized to have had either low or high frequency sensitivity. Through CT scanning, the morphology of the bony labyrinth of the basilosaurid archaeocete Zygorhiza kochii is described and compared to novel information from the inner ears of mysticetes, which are less known than the inner ears of odontocetes. Further comparisons are made with published information for other cetaceans. The anatomy of the cochlea of Zygorhiza is in line with mysticetes and supports the hypothesis that Zygorhiza was sensitive to low frequency noises. Morphological features that support the low frequency hypothesis and are shared by Zygorhiza and mysticetes include a long cochlear canal with a high number of turns, steeply graded curvature of the cochlear spiral in which the apical turn is coiled tighter than the basal turn, thin walls separating successive turns that overlap in vestibular view, and reduction of the secondary bony lamina. Additional morphology of the vestibular system indicates that Zygorhiza was more sensitive to head rotations than extant mysticetes are, which likely indicates higher agility in the ancestral taxon.

Morphometric study of phylogenetic and ecologic signals in procyonid (mammalia: carnivora) endocasts

Endocasts provide a proxy for brain morphology but are rarely incorporated in phylogenetic analyses despite the potential for new suites of characters. The phylogeny of Procyonidae, a carnivoran family with relatively limited taxonomic diversity, is not well resolved because morphological and molecular data yield conflicting topologies. The presence of phylogenetic and ecologic signals in the endocasts of procyonids will be determined using three-dimensional geometric morphometrics. Endocasts of seven ingroup species and four outgroup species were digitally rendered and 21 landmarks were collected from the endocast surface. Two phylogenetic hypotheses of Procyonidae will be examined using methods testing for phylogenetic signal in morphometric data. In analyses of all taxa, there is significant phylogenetic signal in brain shape for both the morphological and molecular topologies. However, the analyses of ingroup taxa recover a significant phylogenetic signal for the morphological topology only. These results indicate support for the molecular outgroup topology, but not the ingroup topology given the brain shape data. Further examination of brain shape using principal components analysis and wireframe comparisons suggests procyonids possess more developed areas of the brain associated with motor control, spatial perception, and balance relative to the basal musteloid condition. Within Procyonidae, similar patterns of variation are present, and may be associated with increased arboreality in certain taxa. Thus, brain shape derived from endocasts may be used to test for phylogenetic signal and preliminary analyses suggest an association with behavior and ecology.

Implications of the “initial brain” concept for brain evolution in Cetacea

We review the evidence for the concept of the “initial” or prototype brain. We outline four possible modes of brain evolution suggested by our new findings on the evolutionary status of the dolphin brain. The four modes involve various forms of deviation from and conformity to the hypothesized initial brain type. These include examples of conservative evolution, progressive evolution, and combinations of the two in which features of one or the other become dominant. The four types of neocortical organization in extant mammals may be the result of selective pressures on sensory/motor systems resulting in divergent patterns of brain phylogenesis. A modular “modification/multiplication” hypothesis is proposed as a mechanism of neocortical evolution in eutherians. Representative models of the initial ancestral group of mammals include not only extant basal Insectivora but also Chiroptera; we have found that dolphins and large whales have also retained many features of the archetypal or initial brain. This group evolved from the initial mammalian stock and returned to the aquatic environment some 50 million years ago. This unique experiment of nature shows the effects of radical changes in environment on brain-body adaptations and specializations. Although the dolphin brain has certain quantitative characteristics of the evolutionary changes seen in the higher terrestrial mammals, it has also retained many of the conservative structural features of the initial brain. Its neocortical organization is accordingly different, largely in a quantitative sense, from that of terrestrial models of the initial brain such as the hedgehog.

Description of a Cranial Endocast from the Fossil MammalVincelestes neuquenianus (Theriiformes) and its Relevance to the Evolution of Endocranial Characters in Therians

We generated a digital cranial endocast (infilling of the braincase) of Vincelestes neuquenianus, a Cretaceous theriiform mammal from Argentina, to achieve two goals. First, we described this endocast of Vincelestes to reconstruct the brain, associated soft-tissue structures, and internal osteological features. This report represents the first description of an endocast from a stem therian that is near crown group Theria (marsupials, placentals, and all descendants of that ancestor). Second, we examined 21 morphological characters related to the morphology of endocasts and endocranial osteology across 19 taxa (including Vincelestes) in the context of a current hypothesis about mammal phylogeny to identify potential synapomorphies for Theria. The digital endocast of Vincelestes is mostly complete, facilitating description in all views and allowing collection of accurate linear and volumetric measurements. However, it is unclear if the midbrain is exposed on the dorsal surface of the brain because of damage to this region of the endocast. Other portions of this specimen are extraordinarily well preserved, allowing identification of the accessory olfactory bulbs on the endocast, an ossified falx cerebri, and an osseous tentorium. The encephalization quotient (EQ) calculated for Vincelestes falls within the range of EQs of crown therians. Comparison of the endocranial characters across different mammalian taxa did not reveal any new synapomorphies for the clade Theria.

Cranial endocasts from a growth series ofMonodelphis domestica (Didelphidae, Marsupialia): A study of individual and ontogenetic variation

Intraspecific variation (e.g., ontogenetic, individual, sexual dimorphic) is rarely examined among cranial endocasts (infillings of the braincase cavity) because of the difficulty in obtaining multiple specimens of a species, particularly fossil taxa. We extracted digital cranial endocasts from CT scans of a growth series of skulls of Monodelphis domestica, the gray short-tailed opossum, as a preliminary assessment of the amount of intraspecific variation in mammalian endocranial morphology. The goals of this study were 1) to provide an anatomical description to document developmental changes in endocranial morphology of M. domestica and 2) to examine ontogenetic and individual variation with respect to phylogenetic characters of endocranial cavities that are known to be variable between different mammalian taxa. In this study, "ontogenetic variation" refers to variation between specimens of different ages whereas "individual variation" (i.e., polymorphism) is restricted to variation between specimens of comparable age. Aside from size, changes in shape account for the greatest amount of morphological variation between the endocasts of different ages. Endocast length, width, and volume increase with age for the growth series. Relative olfactory bulb cast size increases with age in the growth series, but the relative size of the parafloccular casts shows a slight negative allometric trend through ontogeny. More than one-third of the phylogenetic characters of the endocranial cavity we examined showed some sort of variation (ontogenetic, individual, or both). This suggests that although endocasts are potentially informative for systematics, both ontogenetic and individual variation affect how endocranial characters are scored for phylogenetic analysis. Further studies such as this are necessary to determine the taxonomic extent of significant intraspecific variation of these endocranial characters.

An examination of cetacean brain structure with a novel hypothesis correlating thermogenesis to the evolution of a big brain

This review examines aspects of cetacean brain structure related to behaviour and evolution. Major considerations include cetacean brain-body allometry, structure of the cerebral cortex, the hippocampal formation, specialisations of the cetacean brain related to vocalisations and sleep phenomenology, paleoneurology, and brain-body allometry during cetacean evolution. These data are assimilated to demonstrate that there is no neural basis for the often-asserted high intellectual abilities of cetaceans. Despite this, the cetaceans do have volumetrically large brains. A novel hypothesis regarding the evolution of large brain size in cetaceans is put forward. It is shown that a combination of an unusually high number of glial cells and unihemispheric sleep phenomenology make the cetacean brain an efficient thermogenetic organ, which is needed to counteract heat loss to the water. It is demonstrated that water temperature is the major selection pressure driving an altered scaling of brain and body size and an increased actual brain size in cetaceans. A point in the evolutionary history of cetaceans is identified as the moment in which water temperature became a significant selection pressure in cetacean brain evolution. This occurred at the Archaeoceti - modern cetacean faunal transition. The size, structure and scaling of the cetacean brain continues to be shaped by water temperature in extant cetaceans. The alterations in cetacean brain structure, function and scaling, combined with the imperative of producing offspring that can withstand the rate of heat loss experienced in water, within the genetic confines of eutherian mammal reproductive constraints, provides an explanation for the evolution of the large size of the cetacean brain. These observations provide an alternative to the widely held belief of a correlation between brain size and intelligence in cetaceans.

Reconstructing cetacean brain evolution using computed tomography

Until recently, there have been relatively few studies of brain mass and morphology in fossil cetaceans (dolphins, whales, and porpoises) because of difficulty accessing the matrix that fills the endocranial cavity of fossil cetacean skulls. As a result, our knowledge about cetacean brain evolution has been quite limited. By applying the noninvasive technique of computed tomography (CT) to visualize, measure, and reconstruct the endocranial morphology of fossil cetacean skulls, we can gain vastly more information at an unprecedented rate about cetacean brain evolution. Here, we discuss our method and demonstrate it with several examples from our fossil cetacean database. This approach will provide new insights into the little-known evolutionary history of cetacean brain evolution.

The gospel of the fossil brain: Tilly Edinger and the science of paleoneurology

Tilly Edinger (1897-1967) was a vertebrate paleontologist interested in the evolution of the central nervous system. By combining methods and insights gained from comparative neuroanatomy and paleontology, she almost single-handedly founded modern paleoneurology in the 1920s while working at the Senckenberg Museum in Frankfurt am Main. Edinger's early research was mostly descriptive and conducted within the theoretical framework of brain evolution formulated by O. C. Marsh in the late 19th century. Nevertheless, she became immediately known in 1929 after publishing an extensive review on "fossil brains." Reconstructing evolutionary history from the fossil record instead of from the comparative analysis of living forms allowed her to identify the sequence of neural innovations within several mammalian lineages. Anti-Jewish terrorism forced Edinger to leave Nazi Germany in 1939. After finding refuge first in England, she continued her career at Harvard's Museum of Comparative Zoology. There she documented the occurrence of gross neural correlates of specialized behavior in several vertebrate lineages, and identified parallel evolution in mammalian sulcation patterns. Her insight that neural innovations need not be "correlated" with either nonneural innovations or with evolutionary "success" led her to reject Marsh's theory of progressive increase in brain size over time and other "anthropocentric" understandings of brain evolution. Edinger's research, her insistence on a stratigraphic and evolutionary framework for interpretation, and her massive compilations of paleoneurological literature established her as the leading definer, practitioner, and chronicler of her field.