Inner ear morphology in wild versus laboratory house mice

The semicircular canals of the inner ear are involved in balance and velocity control. Being crucial to ensure efficient mobility, their morphology exhibits an evolutionary conservatism attributed to stabilizing selection. Release of selection in slow-moving animals has been argued to lead to morphological divergence and increased inter-individual variation. In its natural habitat, the house mouse Mus musculus moves in a tridimensional space where efficient balance is required. In contrast, laboratory mice in standard cages are severely restricted in their ability to move, which possibly reduces selection on the inner ear morphology. This effect was tested by comparing four groups of mice: several populations of wild mice trapped in commensal habitats in France; their second-generation laboratory offspring, to assess plastic effects related to breeding conditions; a standard laboratory strain (Swiss) that evolved for many generations in a regime of mobility reduction; and hybrids between wild offspring and Swiss mice. The morphology of the semicircular canals was quantified using a set of 3D landmarks and semi-landmarks analyzed using geometric morphometric protocols. Levels of inter-population, inter-individual (disparity) and intra-individual (asymmetry) variation were compared. All wild mice shared a similar inner ear morphology, in contrast to the important divergence of the Swiss strain. The release of selection in the laboratory strain obviously allowed for an important and rapid drift in the otherwise conserved structure. Shared traits between the inner ear of the lab strain and domestic pigs suggested a common response to mobility reduction in captivity. The lab-bred offspring of wild mice also differed from their wild relatives, suggesting plastic response related to maternal locomotory behavior, since inner ear morphology matures before birth in mammals. The signature observed in lab-bred wild mice and the lab strain was however not congruent, suggesting that plasticity did not participate to the divergence of the laboratory strain. However, contrary to the expectation, wild mice displayed slightly higher levels of inter-individual variation than laboratory mice, possibly due to the higher levels of genetic variance within and among wild populations compared to the lab strain. Differences in fluctuating asymmetry levels were detected, with the laboratory strain occasionally displaying higher asymmetry scores than its wild relatives. This suggests that there may indeed be a release of selection and/or a decrease in developmental stability in the laboratory strain.

Lufengpithecus inner ear provides evidence of a common locomotor repertoire ancestral to human bipedalism

Various lines of evidence have been used to infer the origin of human bipedalism, but the paucity of hominoid postcranial fossils and the diversity of inferred locomotor modes have tended to confound the reconstruction of ancestral morphotypes. Examination of the bony labyrinth morphology of the inner ear of extinct and living hominoids provides independent evidence for inferring the evolution of hominoid locomotor patterns. New computed tomography data and morphometric analyses of the Late Miocene ape Lufengpithecus indicate that it and other stem great apes possess labyrinths similar to one another and show that hominoids initially evolved from a positional repertoire that included orthogrady, below-branch forelimb suspension and progression, above-branch bipedalism, climbing, clambering, and leaping (hylobatid-like) to one that comprised above-branch quadrupedalism, below-branch forelimb suspension, vertical climbing, limited leaping, terrestrial quadrupedal running and walking, possibly with knuckle walking, and short bouts of bipedalism (chimpanzee-like). The bony labyrinth morphology of Lufengpithecus indicates that it probably conforms more closely to the last common ancestors of crown hominoids and hominids in its locomotor behavior than do other Miocene hominoids. Human bipedalism evolved from this common archetypal Lufengpithecus-like locomotor repertoire. The low evolutionary rate of semicircular canal morphology suggests that Lufengpithecus experienced a relative stasis in locomotor behavior, probably due to the uplift of the Tibetan Plateau, which created a stable environment in the Miocene of southwestern China.

Basicranial Modular Organization. A Study in the Araucanian Horse of Colombia

The skull is divided into neurocranium and splanchnocranium, and its variation allows ecomorphological studies to learn about possible evolutionary and adaptive characteristics. The basicranial organization of the neurocranium and splanchnocranium modules was studied in a sample of 31 skulls from adult Araucanian horses by means of 2D geometric morphometric techniques. The neurocranium and splanchnocranium modules on the ventral aspect were analyzed separately using a set of 31 landmarks. The RV coefficient (the multivariate analog of a correlation) was estimated to analyze the independence of these two parts, as well as their morphological integration, using a two-block analysis of least squares. The study results confirm the modular development of the neurocranium and the splanchnocranium, the former being more stable than the latter as well as low morphological integration between the two. The development between both parties is structured in a modular way but allows relative independence. Now it would be interesting for future studies to add muscles (those that connect the cranial parts, but also the cervical), the hyoid apparatus, and the ossicles of the internal ear and the jaw and analyze if they behave as integrated modules between them. Since this research has been conducted at the subspecific breed level, it could be plausible that in other breeds, this integrative development was different.

Morphology of the Bony Labyrinth Supports the Affinities of Paradolichopithecus with the Papionina

Discoveries in recent decades indicate that the large papionin monkeys Paradolipopithecus and Procynocephalus are key members of the Late Pliocene - Early Pleistocene mammalian faunas of Eurasia. However, their taxonomical status, phylogenetic relationships, and ecological profile remain unclear. Here we investigate the two latter aspects through the study of the inner ear anatomy, as revealed by applying micro-CT scan imaging techniques on the cranium LGPUT DFN3-150 of Paradolichopithecus from the lower Pleistocene (2.3 Ma) fossil site Dafnero-3 in Northwestern Greece. Using geometric morphometric methods, we quantified shape variation and the allometric and phylogenetic signals in extant cercopithecines (n = 80), and explored the morphological affinities of the fossil specimen with extant taxa. LGPUT DFN3-150 has a large centroid size similar to that of baboons and their relatives. It shares several shape features with Macacina and Cercopithecini, which we interpret as probable retention of a primitive morphology. Overall, its inner ear morphology is more consistent with a stem Papionini more closely related to Papionina than Macacina, or to a basal crown Papionina. Our results, along with morphometrical and ecological features from previous studies, call into question the traditional hypothesis of a Paradolichopithecus-Macacina clade, and provide alternative perspectives in the study of Eurasian primate evolution during the late Neogene-Quaternary.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10764-022-00329-4.