Evolution of Cranial Ontogeny in South American Canids (Carnivora: Canidae)

Fractal Dimension and Suture Complexity During Postnatal Ontogeny in Neotropical Deer in Relation to Sexual Dimorphism and Other Biological Features

In mammals, the increase in cranial suture complexity throughout postnatal ontogeny has been linked to mechanical forces that load on the skull, including compression forces from mastication and the presence and use of cranial appendages in ungulates. Suture complexity, or interdigitation, provides a large absorptive capacity for mechanical stress. Deer are unique among ungulates by the presence of antlers only in males. In particular, Neotropical species exhibit a great diversity in terms of morphology, weight, sexual size dimorphism (absence or presence), and ecology. To evaluate the relationship of suture interdigitation with the mechanical stress produced by antlers, we quantified the complexity of the interfrontal and coronal sutures using fractal dimension (D) in six Neotropical deer species. Utilizing the occlusal tooth area (OTA) of the first upper molar as a proxy of age, we analyze changes of D throughout postnatal ontogeny and test for sexual dimorphism. In all species, ontogenetic series indicate an increase of complexity of both sutures with OTA. Overall, the species with a significant sexual dimorphism in body size and antlers with more than one tine show the more complex sutures, with the presence of greater interdigitation in adult males than in females, regardless of the existence of intraspecific fights during the rut. This is the first study where sexual dimorphism in cranial suture complexity in relation to the presence of antlers in deer is reported, suggesting the role of the interfrontal and coronal sutures on the dissipation of mechanical stress forces produced by the presence of antlers.

The hind limb of Octodontidae (Rodentia, Mammalia): Functional implications for substrate preferences

Octodontids are South American caviomorph rodents endemic to mesic and arid biomes displaying a wide range of substrate preferences, from terrestrial to subterranean habits. However, the hind limb morphology of these rodents remain relatively poorly understudied, particularly from an ecomorphological perspective. To investigate the association between hind limb morphology and substrate preference-epigean, semifossorial, fossorial, and subterranean- this study analyzed six skeletal measurements of femur and tibia, along with five morphological indices. We employed phylogenetic mapping, allometry, and multivariate analyses (Phylogentic Flexible Discriminant and Principal Component analyses) on log-transformed variables and indices. The results suggest that the epigeans and subterraneans possess hind limb skeletal features that enhance their mechanical capabilities, which are advantageous for their respective lifestyles. However, in the absence of clear behavioral adaptations or associations, the functional habits of Octodontidae do not requires significant structural modifications of the proximal bones of the hind limbs. These results indicate that understanding the form-function relationship in octodontids requires direct field or laboratory observations of behavior and environmental interactions, highlighting the limitations of current research without such data.

Postnatal Skull Development Reveals a Conservative Pattern in Living and Fossil Vizcachas Genus Lagostomus (Rodentia, Chinchillidae)

The plains vizcacha, Lagostomus maximus, is the only living species in the genus, being notably larger than fossil congeneric species, such as Lagostomus incisus, from the Pliocene of Argentina and Uruguay. Here, we compare the skull growth allometric pattern and sexual dimorphism of L. maximus and L. incisus, relating shape and size changes with skull function. We also test whether the ontogenetic trajectories and allometric trends between both sexes of L. maximus follow the same pattern. A common allometric pattern between both species was the elongation of the skull, a product of the lengthening of rostrum, and chondrogenesis on the spheno-occipitalis synchondrosis and coronalis suture. We also detected a low proportion of skull suture fusion. In some variables, older male specimens did not represent a simple linear extension of female trajectory, and all dimorphic traits were related to the development of the masticatory muscles. Sexual dimorphism previously attributed to L. incisus would indicate that this phenomenon was present in the genus since the early Pliocene and suggests social behaviors such as polygyny and male-male competition. Ontogenetic changes in L. incisus were similar to L. maximus, showing a conservative condition of the genus. Only two changes were different in the ontogeny of both species, which appeared earlier in L. incisus compared to L. maximus: the development of the frontal process of the nasals in a square shape, and the straight shape of the occipital bone in lateral view. Juveniles of L. maximus were close to adult L. incisus in the morphospace, suggesting a peramorphic process. The sequence of suture and synchondroses fusion showed minor differences in temporozygomatica and frontonasalis sutures, indicating major mechanical stress in L. maximus related to size. We suggest a generalized growth path in Chinchillidae, but further analyses are necessary at an evolutionary level, including Lagidium and Chinchilla.

Lions & sea lions & bears, oh my: utilizing museum specimens to study the ossification sequence of carnivoran taxa

BACKGROUND

Mammalian skeletons are largely formed before birth. Heterochronic changes in skeletal formation can be investigated by comparing the order of ossification for different elements of the skeleton. Due to the challenge of collecting prenatal specimens in viviparous taxa, opportunistically collected museum specimens provide the best material for studying prenatal skeletal development across many mammalian species. Previous studies have investigated ossification sequence in a range of mammalian species, but little is known about the pattern of bone formation in Carnivora. Carnivorans have diverse ecologies, diets, and biomechanical specializations and are well-suited for investigating questions in evolutionary biology. Currently, developmental data on carnivorans is largely limited to domesticated species. To expand available data on carnivoran skeletal development, we used micro-computed tomography (micro-CT) to non-invasively evaluate the degree of ossification in all prenatal carnivoran specimens housed in the Harvard Museum of Comparative Zoology. By coding the presence or absence of bones in each specimen, we constructed ossification sequences for each species. Parsimov-based genetic inference (PGi) was then used to identify heterochronic shifts between carnivoran lineages and reconstruct the ancestral ossification sequence of Carnivora.

RESULTS

We used micro-CT to study prenatal ossification sequence in six carnivora species: Eumetopias jubatus (Steller sea lion, n = 6), Herpestes javanicus (small Indian mongoose, n = 1), Panthera leo (lion, n = 1), Urocyon cinereoargenteus (gray fox, n = 1), Ursus arctos arctos (Eurasian brown bear, n = 1), and Viverricula indica (small Indian civet, n = 5). Due to the relatively later stage of collection for the available specimens, few heterochronic shifts were identified. Ossification sequences of feliform species showed complete agreement with the domestic cat. In caniforms, the bear and fox ossification sequences largely matched the dog, but numerous heterochronic shifts were identified in the sea lion.

CONCLUSIONS

We use museum specimens to generate cranial and postcranial micro-CT data on six species split between the two major carnivoran clades: Caniformia and Feliformia. Our data suggest that the ossification sequence of domestic dogs and cats are likely good models for terrestrial caniforms and feliforms, respectively, but not pinnipeds.

Unexpectedly uneven distribution of functional trade-offs explains cranial morphological diversity in carnivores

Functional trade-offs can affect patterns of morphological and ecological evolution as well as the magnitude of morphological changes through evolutionary time. Using morpho-functional landscape modelling on the cranium of 132 carnivore species, we focused on the macroevolutionary effects of the trade-off between bite force and bite velocity. Here, we show that rates of evolution in form (morphology) are decoupled from rates of evolution in function. Further, we found theoretical morphologies optimising for velocity to be more diverse, while a much smaller phenotypic space was occupied by shapes optimising force. This pattern of differential representation of different functions in theoretical morphological space was highly correlated with patterns of actual morphological disparity. We hypothesise that many-to-one mapping of cranium shape on function may prevent the detection of direct relationships between form and function. As comparatively only few morphologies optimise bite force, species optimising this function may be less abundant because they are less likely to evolve. This, in turn, may explain why certain clades are less variable than others. Given the ubiquity of functional trade-offs in biological systems, these patterns may be general and may help to explain the unevenness of morphological and functional diversity across the tree of life.

Different, but the same: Inferring the hunting behaviour of the hypercarnivorous bush dog (Speothos venaticus) through finite element analysis

Cerdocyonina is a clade composed by the South-American canids in which the bush dog (Speothos venaticus) is one of the most elusive species. Known for its unique morphology within the group, this small, bear-like faced canid is the only member of the clade adapted to hypercarnivory, an almost exclusively meat-based diet currently present only in usually large, pack-hunting canids such as the grey wolf (Canis lupus). However, much of the biology of the bush dog is poorly understood, and inferences about its ecology, hunting strategies and diet are usually based on observation of captive individuals and anecdotal records, with reduced quantitative data to offer support. Here, we investigated the craniomandibular functional morphology of the bush dog through finite element analysis (FEA). FEA was employed to model the biting behaviour and to create extrinsic and intrinsic functional scenarios with different loads, corresponding to different bites used to subdue and process the prey. For comparison, the same modelling was applied to the skull of a grey wolf and a grey fox (Urocyon cinereoargenteus). Our analysis showed that the bush dog's responses to loading are more similar to the wolf's than to the fox's in most scenarios, suggesting a convergent craniomandibular functional morphology between these two hypercarnivorous species, despite their distinct phylogenetic positions and body sizes. Differences between the three taxa are noteworthy and suggested to be related to the size of the usual prey. The modelled bite force for the bush dog is relatively strong, about half of that estimated for the wolf and about 40% stronger than the fox's bite. The results strengthen with quantitative data the inferences of the bush dog as a pack-hunting predator with prey size similar to its own, such as large rodents and armadillos, being specialised in subduing and killing its prey using multiple bites. Its similarity to the wolf also confirms anecdotal accounts of predation on mammals that are much larger than itself, such as peccaries and tapirs. These data highlight the ecological specialisation of this small canid in a continent where large, pack-hunting canids are absent.

SPASOS 1.1: a program for the inference of ancestral shape ontogenies

We recently published a method to infer ancestral landmark-based shape ontogenies that takes into account the possible existence of changes in developmental timing. Here we describe SPASOS, a software to perform that analysis. SPASOS is an open-source Windows program written in C. Input data include landmark coordinates for each specimen -with the corresponding information about developmental timing- and a phylogenetic tree showing the relationships among the species sampled. As output, the program produces image files for an easy visualization of the results and data files useful for post-processing. The program incorporates an interpolating function, based on weighting moving averages, which allows analysis of data with scarce information along the ontogenetic trajectory. An empirical evaluation of this function showed its suitability to fill in incomplete ontogenetic trajectories. Finally, we present the results of a reanalysis in SPASOS of a published dataset, where changes in developmental timing were originally inferred by considering PCA scores as shape variables. Both approaches retrieved the same four largest changes in developmental timing, but differed in the ancestral shapes inferred.