Mandible morphology, dental microwear, and diet of the extinct giant rats Canariomys (Rodentia: Murinae) of the Canary Islands (Spain)

Genomic basis of insularity and ecological divergence in barn owls (Tyto alba) of the Canary Islands

Islands, and the particular organisms that populate them, have long fascinated biologists. Due to their isolation, islands offer unique opportunities to study the effect of neutral and adaptive mechanisms in determining genomic and phenotypical divergence. In the Canary Islands, an archipelago rich in endemics, the barn owl (Tyto alba), present in all the islands, is thought to have diverged into a subspecies (T. a. gracilirostris) on the eastern ones, Fuerteventura and Lanzarote. Taking advantage of 40 whole-genomes and modern population genomics tools, we provide the first look at the origin and genetic makeup of barn owls of this archipelago. We show that the Canaries hold diverse, long-standing and monophyletic populations with a neat distinction of gene pools from the different islands. Using a new method, less sensitive to structure than classical F_ST, to detect regions involved in local adaptation to insular environments, we identified a haplotype-like region likely under selection in all Canaries individuals and genes in this region suggest morphological adaptations to insularity. In the eastern islands, where the subspecies is present, genomic traces of selection pinpoint signs of adapted body proportions and blood pressure, consistent with the smaller size of this population living in a hot arid climate. In turn, genomic regions under selection in the western barn owls from Tenerife showed an enrichment in genes linked to hypoxia, a potential response to inhabiting a small island with a marked altitudinal gradient. Our results illustrate the interplay of neutral and adaptive forces in shaping divergence and early onset speciation.

Cheliceral chelal design in free-living astigmatid mites

Cheliceral chelal design in free-living astigmatid mites (Arthropoda: Acari) is reviewed within a mechanical model. Trophic access (body size and cheliceral reach) and food morsel handling (chelal gape and estimated static adductive crushing force) are morphologically investigated. Forty-seven commonly occurring astigmatid mite species from 20 genera (covering the Acaridae, Aeroglyphidae, Carpoglyphidae, Chortoglyphidae, Glycyphagidae, Lardoglyphidae, Pyroglyphidae, Suidasiidae, and Winterschmidtiidae) are categorised into functional groups using heuristics. Conclusions are confirmed with statistical tests and multivariate morphometrics. Despite these saprophagous acarines in general being simple 'shrunken/swollen' versions of each other, clear statistical correlations in the specifics of their mechanical design (cheliceral and chelal scale and general shape) with the type of habitat and food consumed (their 'biome') are found. Using multivariate analyses, macro- and microsaprophagous subtypes are delineated. Relative ratios of sizes on their own are not highly informative of adaptive syndromes. Sympatric resource competition is examined. Evidence for a maximum doubling of approximate body volume within nominal taxa is detected but larger mites are not more 'generalist' feeding types. Two contrasting types of basic 'Bauplan' are found differing in general scale: (i) a large, chunk-crunching, 'demolition'-feeding omnivore design (comprising 10 macrosaprophagous astigmatid species), and (ii) a small selective picking, squashing/slicing or fragmentary/'plankton' feeding design (which may indicate obligate fungivory/microbivory) comprising 20 microsaprophagous acarid-shaped species. Seventeen other species appear to be specialists. Eleven of these are either: small (interstitial/burrowing) omnivores-or a derived form designed for processing large hard food morsels (debris durophagy, typified by the pyroglyphid Dermatophagoides farinae), or a specialist sub-type of particular surface gleaning/scraping fragmentary feeding. Six possible other minor specialist gleaning/scraping fragmentary feeders types each comprising one to two species are described. Details of these astigmatid trophic-processing functional groups need field validation and more corroborative comparative enzymology. Chelal velocity ratio in itself is not highly predictive of habitat but with cheliceral aspect ratio (or chelal adductive force) is indicative of life-style. Herbivores and pest species are typified by a predicted large chelal adductive force. Pest species may be 'shredders' derived from protein-seeking necrophages. Carpoglyphus lactis typifies a mite with tweezer-like chelae of very feeble adductive force. It is suggested that possible zoophagy (hypocarnivory) is associated with low chelal adductive force together with a small or large gape depending upon the size of the nematode being consumed. Kuzinia laevis typifies an oophagous durophage. Functional form is correlated with taxonomic position within the Astigmata-pyroglyphids and glycyphagids being distinct from acarids. A synthesis with mesostigmatid and oribatid feeding types is offered together with clarification of terminologies. The chelal lyrifissure in the daintiest chelicerae of these astigmatids is located similar to where the action of the chelal moveable digit folds the cheliceral shaft in uropodoids, suggesting mechanical similarities of function. Acarid astigmatids are trophically structured like microphytophagous/fragmentary feeding oribatids. Some larger astigmatids (Aleuroglyphus ovatus, Kuzinia laevis, Tyroborus lini) approximate, and Neosuidasia sp. matches, the design of macrophytophagous oribatids. Most astigmatid species reviewed appear to be positioned with other oribatid secondary decomposers. Only Dermatophagoides microceras might be a primary decomposer approximating a lichenivorous oribatid (Austrachipteria sp.) in trophic form. Astigmatid differences are consilient with the morphological trend from micro- to macrophytophagy in oribatids. The key competency in these actinotrichid mites is a type of 'gnathosomisation' through increased chelal and cheliceral height (i.e., a shape change that adjusts the chelal input effort arm and input adductive force) unrestricted by the dorsal constraint of a mesostigmatid-like gnathotectum. A predictive nomogram for ecologists to use on field samples is included. Future work is proposed in detail.

Bite force and its relationship to jaw shape in domestic dogs

Previous studies based on two-dimensional methods have suggested that the great morphological variability of cranial shape in domestic dogs has impacted bite performance. Here, we used a three-dimensional biomechanical model based on dissection data to estimate the bite force of 47 dogs of various breeds at several bite points and gape angles. In vivo bite force for three Belgian shepherd dogs was used to validate our model. We then used three-dimensional geometric morphometrics to investigate the drivers of bite force variation and to describe the relationships between the overall shape of the jaws and bite force. The model output shows that bite force is rather variable in dogs and that dogs bite harder on the molar teeth and at lower gape angles. Half of the bite force is determined by the temporal muscle. Bite force also increased with size, and brachycephalic dogs showed higher bite forces for their size than mesocephalic dogs. We obtained significant covariation between the shape of the upper or lower jaw and absolute or residual bite force. Our results demonstrate that domestication has not resulted in a disruption of the functional links in the jaw system in dogs and that mandible shape is a good predictor of bite force.

Fasciola gigantica, F. hepatica and Fasciola intermediate forms: geometric morphometrics and an artificial neural network to help morphological identification

Background

Fasciola hepatica and F. gigantica cause fascioliasis in both humans and livestock. Some adult specimens of Fasciola sp. referred to as “intermediate forms” based on their genetic traits, are also frequently reported. Simple morphological criteria are unreliable for their specific identification. In previous studies, promising phenotypic identification scores were obtained using morphometrics based on linear measurements (distances, angles, curves) between anatomical features. Such an approach is commonly termed “traditional” morphometrics, as opposed to “modern” morphometrics, which is based on the coordinates of anatomical points.

Methods

Here, we explored the possible improvements that modern methods of morphometrics, including landmark-based and outline-based approaches, could bring to solving the problem of the non-molecular identification of these parasites. F. gigantica and Fasciola intermediate forms suitable for morphometric characterization were selected from Thai strains following their molecular identification. Specimens of F. hepatica were obtained from the Liverpool School of Tropical Medicine (UK). Using these three taxa, we tested the taxonomic signal embedded in traditional linear measurements versus the coordinates of anatomical points (landmark- and outline-based approaches). Various statistical techniques of validated reclassification were used, based on either the shortest Mahalanobis distance, the maximum likelihood, or the artificial neural network method.

Results

Our results revealed that both traditional and modern morphometric approaches can help in the morphological identification of Fasciola sp. We showed that the accuracy of the traditional approach could be improved by selecting a subset of characters among the most contributive ones. The influence of size on discrimination by shape was much more important in traditional than in modern analyses. In our study, the modern approach provided different results according to the type of data: satisfactory when using pseudolandmarks (outlines), less satisfactory when using landmarks. The different reclassification methods provided approximately similar scores, with a special mention to the neural network, which allowed improvements in accuracy by combining data from both morphometric approaches.

Conclusion

We conclude that morphometrics, whether traditional or modern, represent a valuable tool to assist in Fasciola species recognition. The general level of accuracy is comparable among the various methods, but their demands on skills and time differ. Based on the outline method, our study could provide the first description of the shape differences between species, highlighting the more globular contours of the intermediate forms.

Island rule and bone metabolism in fossil murines from Timor

Skeletal growth rates reconstructed from bone histology in extinct insular hippopotamids, elephants, bovids and sauropods have been used to infer dwarfism as a response to island conditions. Limited published records of osteocyte lacunae densities (Ot.Dn), a proxy for living osteocyte proliferation, have suggested a slower rate of bone metabolism in giant mammals. Here, we test whether insularity might have affected bone metabolism in a series of small to giant murine rodents from Timor. Ten adult femora were selected from a fossil assemblage dated to the Late Quaternary (~5000–18 000 years old). Femur morphometric data were used in computing phylogenetically informed body mass regressions, although the phylogenetic signal was very low (Pagel’s λ = 0.03). Estimates of body weight calculated from these femora ranged from 75 to 1188 g. Osteocyte lacunae densities from histological sections of the midshaft femur were evaluated against bone size and estimated body weight. Statistically significant (P < 0.05) and strongly negative relationships between Ot.Dn, femur size and estimated weight were found. Larger specimens were characterized by lower Ot.Dn, indicating that giant murines from Timor might have had a relatively slow pace of bone metabolic activity, consistent with predictions made by the island rule.

Dental microwear texture analysis and diet in caviomorphs (Rodentia) from the Serra do Mar Atlantic forest (Brazil)

The Serra do Mar Atlantic forest (Brazil) shelters about 15 different species of caviomorph rodents and thus represents a unique opportunity to explore resource partitioning. We studied 12 species with distinct diets using dental microwear texture analysis (DMTA). Our results revealed differences (complexity, textural fill volume, and heterogeneity of complexity) among species with different dietary preferences, and among taxa sharing the same primary dietary components but not those with similar secondary dietary preferences (heterogeneity of complexity). We found three main dietary tendencies characterized by distinct physical properties: consumers of young leaves had low complexity; bamboo specialists, fruit and seed eaters, and omnivorous species, had intermediate values for complexity; grass, leaf, and aquatic vegetation consumers, had highly complex dental microwear texture. Dietary preferences and body mass explained a major part of the resource partitioning that presumably enables coexistence among these rodent species. DMTA was useful in assessing what foods contributed to resource partitioning in caviomorphs. Our database for extant caviomorph rodents is a prerequisite for interpretation of dental microwear texture of extinct caviomorph taxa, and thus for reconstructing their diets and better understanding the resource partitioning in paleocommunities and its role in the successful evolutionary history of this rodent group.

Revisiting the Foraging Ecology and Extinction History of Two Endemic Vertebrates from Tenerife, Canary Islands

We used carbon (δ13C) and nitrogen (δ15N) isotopes to examine the foraging ecology of Tenerife giant rats (Canariomys bravoi) and lizards (Gallotia goliath) in northwestern Tenerife, which until recently, were the island’s largest terrestrial vertebrates. We combined new isotope data for 28 C. bravoi and 14 G. goliath with published regional data for both species and then compared these with data for co-occurring extant taxa and modern C3 plants. Isotope data suggest both extinct species relied primarily on C3 resources and were trophic omnivores. However, the two species appear to have partitioned their resources when living in sympatry. Isotopic overlap between C. bravoi and Rattus spp., and between G. goliath, extant Gallotia galloti, and introduced rabbits (Oryctolagus cuniculus) suggests reliance on similar foods. We radiocarbon dated four C. bravoi and two G. goliath with the most extreme isotope values. These new dates do not settle the question of what triggered the demise of either species. Nevertheless, the data are most consistent with anthropogenically-induced extinction. Temporal isotopic trends contradict expectations if regional climate were responsible, and confidence intervals for radiocarbon dates suggest it is highly likely that both species were present when humans first settled the island.

Dental microwear of sympatric rodent species sampled across habitats in southern Africa: Implications for environmental influence

Dental microwear textures have proven to be a valuable tool for reconstructing the diets of a wide assortment of fossil vertebrates. Nevertheless, some studies have recently questioned the efficacy of this approach, suggesting that aspects of habitat unrelated to food preference, especially environmental grit load, might have a confounding effect on microwear patterning that obscures the diet signal. Here we evaluate this hypothesis by examining microwear textures of 3 extant sympatric rodent species that vary in diet breadth and are found in a variety of habitat types: Mastomys coucha, Micaelamys namaquensis and Rhabdomys pumilio. We sample each of these species from 3 distinct environmental settings in southern Africa that differ in rainfall and vegetative cover: Nama-Karoo shrublands (semi-desert) and Dry Highveld grasslands in the Free State Province of South Africa, and Afromontane (wet) grasslands in the highlands of Lesotho. While differences between habitat types are evident for some of the species, inconsistency in the pattern suggests that the microwear signal is driven by variation in foods eaten rather than grit-level per se. It is clear that, at least for species and habitats sampled in the current study, environmental grit load does not swamp diet-related microwear signatures.

Outline-based morphometrics, an overlooked method in arthropod studies?

Modern methods allow a geometric representation of forms, separating size and shape. In entomology, as well as in many other fields involving arthropod studies, shape variation has proved useful for species identification and population characterization. In medical entomology, it has been applied to very specific questions such as population structure, reinfestation of insecticide-treated areas and cryptic species recognition. For shape comparisons, great importance is given to the quality of landmarks in terms of comparability. Two conceptually and statistically separate approaches are: (i) landmark-based morphometrics, based on the relative position of a few anatomical "true" or "traditional" landmarks, and (ii) outline-based morphometrics, which captures the contour of forms through a sequence of close "pseudo-landmarks". Most of the studies on insects of medical, veterinary or economic importance make use of the landmark approach. The present survey makes a case for the outline method, here based on elliptic Fourier analysis. The collection of pseudo-landmarks may require the manual digitization of many points and, for this reason, might appear less attractive. It, however, has the ability to compare homologous organs or structures having no landmarks at all. This strength offers the possibility to study a wider range of anatomical structures and thus, a larger range of arthropods. We present a few examples highlighting its interest for separating close or cryptic species, or characterizing conspecific geographic populations, in a series of different vector organisms. In this simple application, i.e. the recognition of close or cryptic forms, the outline approach provided similar scores as those obtained by the landmark-based approach.