Phenotype constrains the vocal tract in the most dimorphic mammal, the southern elephant seal

The study of mammal acoustic communication was revolutionized by the application of the source-filter theory, originally developed for human speech. The theory states that the vocal tract is constrained by body anatomy and, therefore, creates a structural link between phenotype and acoustic formants, providing a basis for honest signalling. The phenotype-formants link was validated in many species, but the phenotype-vocal tract link was rarely assessed. We used 2D videogrammetry to estimate the vocal tract length of wild southern elephant seal males (Mirounga leonina Linnaeus, 1758) during their normal vocalization behaviour. We showed that: 1) the vocal tract can be measured non-invasively in a wild large mammal; 2) the vocal tract depends on the structural phenotype (age, body length, and skull size); 3) the nasal tract is more related to the structural phenotype than the buccal tract; 4) the dependence on size, and body length in particular, is stronger than the dependence on age. All together, the phenotypic constraint on vocal tract provides the anatomical basis for honest signalling in elephant seals.

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.

In-air and underwater sounds of hooded seals during the breeding season in the Gulf of St. Lawrence

The hooded seal is a migratory species inhabiting the North Atlantic. Passive acoustic monitoring (PAM) conducted over spatial scales consistent with their known and potential habitat could provide insight into seasonal and spatial occurrence patterns of this species. Hooded seal airborne and underwater acoustic signals were recorded during the breeding season on the pack ice in the Gulf of St. Lawrence in March 2018 to better characterize their acoustic repertoire (notably underwater calls). In-air and underwater signals were classified into 12 and 22 types, respectively. Signals produced by males through the inflation and deflation of the proboscis and septum were the predominant sounds heard on the ice surface. Five of the 22 underwater signals were proboscis and septum noises. The remaining underwater signals (17) were categorized as voiced calls and further analyzed using two classification methods. Agreement with the initial subjective classification of voiced calls was high (77% for classification tree analysis and 88% for random forest analysis), showing that 12-13 call types separated well. The hooded seal's underwater acoustic repertoire is larger and more diverse than has been previously described. This study provides important baseline information necessary to monitor hooded seals using PAM.

How small could a pup sound? The physical bases of signaling body size in harbor seals

Vocal communication is a crucial aspect of animal behavior. The mechanism which most mammals use to vocalize relies on three anatomical components. First, air overpressure is generated inside the lower vocal tract. Second, as the airstream goes through the glottis, sound is produced via vocal fold vibration. Third, this sound is further filtered by the geometry and length of the upper vocal tract. Evidence from mammalian anatomy and bioacoustics suggests that some of these three components may covary with an animal's body size. The framework provided by acoustic allometry suggests that, because vocal tract length (VTL) is more strongly constrained by the growth of the body than vocal fold length (VFL), VTL generates more reliable acoustic cues to an animal's size. This hypothesis is often tested acoustically but rarely anatomically, especially in pinnipeds. Here, we test the anatomical bases of the acoustic allometry hypothesis in harbor seal pups Phoca vitulina. We dissected and measured vocal tract, vocal folds, and other anatomical features of 15 harbor seals post-mortem. We found that, while VTL correlates with body size, VFL does not. This suggests that, while body growth puts anatomical constraints on how vocalizations are filtered by harbor seals' vocal tract, no such constraints appear to exist on vocal folds, at least during puppyhood. It is particularly interesting to find anatomical constraints on harbor seals' vocal tracts, the same anatomical region partially enabling pups to produce individually distinctive vocalizations.

Impact of the terrestrial-aquatic transition on disparity and rates of evolution in the carnivoran skull

Background

Which factors influence the distribution patterns of morphological diversity among clades? The adaptive radiation model predicts that a clade entering new ecological niche will experience high rates of evolution early in its history, followed by a gradual slowing. Here we measure disparity and rates of evolution in Carnivora, specifically focusing on the terrestrial-aquatic transition in Pinnipedia. We analyze fissiped (mostly terrestrial, arboreal, and semi-arboreal, but also including the semi-aquatic otter) and pinniped (secondarily aquatic) carnivorans as a case study of an extreme ecological transition. We used 3D geometric morphometrics to quantify cranial shape in 151 carnivoran specimens (64 fissiped, 87 pinniped) and five exceptionally-preserved fossil pinnipeds, including the stem-pinniped Enaliarctos emlongi. Range-based and variance-based disparity measures were compared between pinnipeds and fissipeds. To distinguish between evolutionary modes, a Brownian motion model was compared to selective regime shifts associated with the terrestrial-aquatic transition and at the base of Pinnipedia. Further, evolutionary patterns were estimated on individual branches using both Ornstein-Uhlenbeck and Independent Evolution models, to examine the origin of pinniped diversity.

Results

Pinnipeds exhibit greater cranial disparity than fissipeds, even though they are less taxonomically diverse and, as a clade nested within fissipeds, phylogenetically younger. Despite this, there is no increase in the rate of morphological evolution at the base of Pinnipedia, as would be predicted by an adaptive radiation model, and a Brownian motion model of evolution is supported. Instead basal pinnipeds populated new areas of morphospace via low to moderate rates of evolution in new directions, followed by later bursts within the crown-group, potentially associated with ecological diversification within the marine realm.

Conclusion

The transition to an aquatic habitat in carnivorans resulted in a shift in cranial morphology without an increase in rate in the stem lineage, contra to the adaptive radiation model. Instead these data suggest a release from evolutionary constraint model, followed by aquatic diversifications within crown families.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0285-5) contains supplementary material, which is available to authorized users.

Structural and functional comparison of the proboscis between tapirs and other extant and extinct vertebrates

Tapirs (Perissodactyla: Tapiridae) are the only living vertebrates, beyond the order Proboscidea, found to possess a true proboscis, defined as a flexible tubular extension of the joint narial and upper labial musculature that serves, at least in part, to grasp food. Tapirs show only partial homology and analogy with elephants in the narial and upper labial structures, as well as in the skull bones and teeth. However, superficially similar extensions in other extant vertebrates differ greatly in anatomy and function. Therefore, they deserve new names: prorhiscis (e.g. Mammalia: Saiga tatarica), prorhinosis (e.g. Chondrichthyes: Callorhinchus spp.), prorhynchis (e.g. Osteichthyes: Campylomormyrus spp.) and progeneiontis (e.g. Osteichthyes: Gnathonemus spp.). Among non-mammalian vertebrates, no bird or reptile is known to possess a proboscis. Among fishes, there are various extensions of the rostrum, jaws, 'nose' and 'chin' that lack the required narial involvement. The skulls of extinct mammals within (e.g. deinotheres) and beyond (e.g. astrapotheres) the Proboscidea confirm that a proboscis evolved independently in several mammalian lineages before the Pliocene. This convergence with tapirs presumably reflects, in part, the advantages of concentrating the olfactory sensor on what is, effectively, the tip of a long mobile upper lip. However, the proboscis does not appear to have arisen de novo in any vertebrate post-Pliocene, and its continued evolution has apparently depended on the further development of its length, flexibility and innervations, as epitomized by elephants.

Mammalian sexual dimorphism

Sexual dimorphisms (SDs) have evolved in mammals to assure greater reproductive success for individuals, usually males. Secondary sexual characteristics (SSC) developed to further this objective, tending to be more pronounced in species which are polygynous, diurnal and open-habitat dwellers. Sexual selection has underpinned many of these changes, which are not necessarily advantageous for individual survival. Domestication has affected certain characteristics, more in terms of their quantitative rather than qualitative expression. However, restrictions imposed by domestication can also affect behaviors such as isolation and post-natal bonding while artificial selection can, by focusing on certain traits, cause unforeseen effects in genetically linked traits, which, when sex-specific or sex-linked, can be reflected in SD. On a global scale, environmental changes can have important phylogenetic implications for species which rely upon environmental cues for activities as migration, hibernation and breeding, especially when SD occurs in response to such cues. Understanding the evolutionary rationale behind the development of SDs, as well as the dynamics which occur at the interface between natural and artificial selection, allows positive insights into areas as diverse as wildlife preservation and livestock management. For both, greatest "success" should be achieved when artificial selection occurs in harmony with natural selection within a supportive environment. Thus the aim of this review is to discuss current knowledge relating to the evolution, benefits and costs of mammalian sexual dimorphisms and, where possible, draw conclusions that might be beneficial for the husbandry and propagation of mammals today.

A nose that roars: anatomical specializations and behavioural features of rutting male saiga

The involvement of the unique saiga nose in vocal production has been neglected so far. Rutting male saigas produce loud nasal roars. Prior to roaring, they tense and extend their noses in a highly stereotypic manner. This change of nose configuration includes dorsal folding and convex curving of the nasal vestibulum and is maintained until the roar ends. Red and fallow deer males that orally roar achieve a temporary increase of vocal tract length (vtl) by larynx retraction. Saiga males attain a similar effect by pulling their flexible nasal vestibulum rostrally, allowing for a temporary elongation of the nasal vocal tract by about 20%. Decrease of formant frequencies and formant dispersion, as acoustic effects of an increase of vtl, are assumed to convey important information on the quality of a dominant male to conspecifics, e.g. on body size and fighting ability. Nasal roaring in saiga may equally serve to deter rival males and to attract females. Anatomical constraints might have set a limit to the rostral pulling of the nasal vestibulum. It seems likely that the sexual dimorphism of the saiga nose was induced by sexual selection. Adult males of many mammalian species, after sniffing or licking female urine or genital secretions, raise their head and strongly retract their upper lip and small nasal vestibulum while inhalating orally. This flehmen behaviour is assumed to promote transport of non-volatile substances via the incisive ducts into the vomeronasal organs for pheromone detection. The flehmen aspect in saiga involves the extensive flexible walls of the greatly enlarged nasal vestibulum and is characterized by a distinctly concave configuration of the nose region, the reverse of that observed in nasal roaring. A step-by-step model for the gradual evolution of the saiga nose is presented here.