Sympatric Distribution Research Articles

Some Aspects of Historical Biogeography of Asian Rock Agamids

The genesis of areals of Asian rock agamids belonging to the genus Laudakia Gray, 1845 is considered. The hypothesis about two routes of radiation of these lizards endemic for arid mountain regions of Palearctic is offered. The southern route from the hypothetical center of origin in the area of mountain systems of Hindukush and Himalayas passed in the conditions of southern subtropics with well developed belt of sclerophilous forests. This evolutional line with « caucasia » complex remains the relic areas in forest regions. Northern group of species with « himalayana » complex had its speciation in northern subtropic area with more severe climatic situation determined by strong continental and arid conditions. Altitudinal and biotopic differences of Laudakia species under their sympatric distribution are specially discussed.

Diversification of an endemic Southeast Asian genus: Phylogenetic relationships of the spiderhunters (Nectariniidae:Arachnothera)

ABSTRACT. The phylogeny of spiderhunters (Nectariniidae: Arachnothera) was reconstructed by comparing mitochondrial and nuclear DNA sequences of all currently recognized species and with broad geographic sampling of two particularly variable species complexes, the Little Spiderhunter (Arachnothera longirostra) and the streaky spiderhunters (A. modesta and A. affinis). It appears to be a relatively old group, whose diversification was not caused by recent sea-level changes. However, the modern, highly sympatric distribution of the large species in the Sunda lowlands was probably a result of dispersal via recent land bridges. Within the highly variable A. longirostra group, there are substantially diverged taxa in the Philippines that should be considered different species. Within the A. affinis—modesta complex, there are three distinct species and a closely related fourth, which describe a clear allopatric distribution: A. affinis in Java, A. modesta in the rest of the Sunda lowlands (except Sabah), A. mag...

Biological traits of naturally induced hybrid individuals of two gizzard shads, Nematalosa come and N. japonica, in coastal waters around Okinawa Island, Ryukyu Archipelago, southwestern Japan

The age, growth, reproductive condition, and occurrence of natural hybrids of two Nematalosa species around Okinawa Island were examined using 128 specimens obtained from April 2003 to June 2004. Standard length (SL) reached approximately 150–210 mm within the first 2 years, and then remained stagnant. The maximum age for both sexes was ca. 5 years old. Maturity sizes and ages were estimated to be at least 173.2 mm SL and 2 years old for females and 192.6 mm SL and 3 years old for males. Spawnable individuals were mainly observed from January to March based on histological observations of gonads. Natural hybrids appeared at all sampling sites except for the Haneji Inlet and were dominant at Makiminato (in south-central Okinawa Island). Their incidence was also quite high (66.9%) in the Makiminato population, when compared with records for other marine fishes around Japan. In Okinawa Island, these shallow areas are rapidly decreasing in size because of recent reclamation and land exploitation. Hybrid production may be caused by not only the reproductive biology and sympatric distributions of the parent species but also recent environmental changes.

Out of sight out of mind: current knowledge of Chinese cave fishes

Caves and karsts are among the most threatened ecosystems in the world. They are very fragile, balanced habitats with high levels of endemic species that are extremely sensitive to environmental changes. In recent decades, however, threats from rapid economic growth have increased the need for conservation efforts for cave-dwelling communities. In addition, difficulties in accessing and sampling these habitats mean that they remain as one of the least known ecosystems in the world with modern studies of cave fishes only starting in China during the 1980s. Here, the current status of cave fishes in China is reviewed. China is host to the highest number of cave fish species in the world, with 48 troglobite species out of a total of 101 cave fish species. All of these cave fish species (one order and three families) and half of the genera are endemic to China with Sinocyclocheilus being the most speciose cave fish genus. Species from this genus possess horns and humpbacks resulting from processes of parallel evolution, but the function of these features remains unknown. With the exception of Onychostoma macrolepis distributed in north China, all other species are found in the karst environment of the Yunnan-Guizhou Plateau. Sympatric distribution is common, and sometimes several different cave fish species can be found in the same cave or subterranean river. For this reason, Chinese cave fishes represent an important evolutionary framework.

Genetic and morphological divergence within the Sebastes pachycephalus complex (Scorpaeniformes: Scorpaenidae)

Genetic and morphological divergence among the four subspecies in the Sebastes pachycephalus complex (S. pachycephalus pachycephalus, S. p. nigricans, S. p.nudus and S. p. chalcogrammus) was clarified. Principal coordinate analysis (PCoA) based on AFLP clearly divided 55 specimens of the complex into two groups, the S. p. pachycephalus–S. p. nigricans group (P-Ni group) and the S. p. nudus–S. p. chalcogrammus group (Nu-C group), although three specimens occupied intermediate positions. The minimum spanning network (MSN) based on partial sequences of the mitochondrial control region (mtCR) failed to separate either the P-Ni and Nu-C groups or the four subspecies into distinct clades, although restricted gene flow and genetic differentiation between the former were indicated by the FST estimation. Differences in morphological characters, including counts of pectoral fin rays and counts of dorsal fin spines lacking basal scales, were also evident between the two groups. However, little or no genetic or morphological difference was found between the two subspecies within each group. It was concluded that the P-Ni and Nu-C groups of the S. pachycephalus complex actually represent two different species, which is further supported by their sympatric distribution. Differences in dorsal body coloration and the presence or absence of brown spots on the ventral surface, which were formerly used to discriminate between four “subspecies,” may simply represent intraspecific variation. The three specimens occupying intermediate positions in the AFLP PCoA also occupied equivocal positions between the two species in the principal component analysis (PCA) based on morphometric characters, suggesting that they were hybrids between the two species. The star-shaped MSN of mtCR, which lacks distinct clades representing the two species, may be due to not only interspecific hybridization but also the sharing of ancestral haplotypes.

Dobzhansky’s rule and reinforcement of prezygotic reproductive isolation in zones of secondary contact

It is well known that closely related sympatric species are usually more different in characters involved in species recognition (e.g., in visual and acoustic signals) than allopatric species of the same evolutionary age. In this article I call this phenomenon Dobzhansky's rule in accordance with the name of the scientist who first discovered it. There are two alternative explanations for this pattern. Under hypothesis of reinforcement by Dobzhansky, these species-specific differences evolve in situ, exactly in zone of overlap between two populations. Under hypothesis of differential fusion by Templeton, the differences originate in geographically separated regions, and only those populations that have evolved such differences can persist in secondary sympatry. These evolutionary scenarios are significantly different. The scenario by Dobzhansky is an essentially sympatric model, in which natural selection reinforces pre-zygotic isolation between divergent populations by selecting against unfit hybrids. The scenario by Templeton is based on classic allopatric speciation model that consider the formation of reproductive isolation to be a by-product of divergent evolution. In this work we show that the sympatric distribution of sister taxa of Agrodiaetus butterflies strongly correlates with differences in male wing colour. We also use a new quantitative phylogenetic test to distinguish between the models by Dobzhansky and by Templeton and to demonstrate that the pattern observed is, most likely, the result of reinforcement.

Helminth parasite communities in four species of sympatric macropodids in western Victoria

Helminth parasites of Macropus fuliginosus, M. giganteus, M. rufogriseus and Wallabia bicolor were examined in a region of western Victoria, Australia, where all four species of hosts are sympatric. M. fuliginosus and M. giganteus shared most of their parasites while the helminth communities of M. rufogriseus and W. bicolor were distinctive. The sympatric distribution of the host species studied provides evidence in support of the hypothesis that the differences between the parasite communities of M. fuliginosus–M. giganteus compared with those of M. rufogriseus and W. bicolor are due to parasite specificity rather than to host ecological differences. However, lack of detailed data on the ecological differences of these hosts in areas of sympatry prevents more precise conclusions being drawn on the reasons for the distinctiveness of the parasite communities.

Revision of the pantropical genus 'Treptopale' (Annelida: Phyllodocida: Chrysopetalidae): Redescription of 'Treptopale rudolphi' Perkins, 1985 and description of two new species including comparison of 'Treptopale' populations in northern Australia

The monotypic genus Treptopale, originally described from the Atlantic Ocean, is now extended - with the description of two new species with a broad and almost sympatric distribution - throughout the Indo-Pacific Ocean. The type species, Treptopale rudolphi Perkins, 1985, is redescribed from additional material from the type locality in the Florida Keys, western Atlantic Ocean, and from material examined from the Cape Verde Islands and Cape Town, South Africa. The morphology of the notochaetal lateral paleae group separates the Atlantic Treptopale rudolphi from a cryptic Treptopale species complex that extends from the Seychelles, western Indian Ocean, throughout the western Indo-Pacific to Hawai'i, eastern Pacific Ocean. Morphological analysis of tropical northern Australian populations of the two new Treptopale species, which are typically found among Halimeda algae on coral reefs, revealed two species separated primarily on the shape and insertion point of a transitional lateral palea and a marked difference in the degree of raised ribs of the main paleae: Treptopale homalos sp. nov. is predominantly found along the eastern Queensland coast from Heron Island north to New Guinea; T. paramolos sp. nov. is predominantly found along the Northern Territory coast. The two species occur sympatrically in a number of localities across tropical northern Australia and both are present on offshore and mainland reefs of northern Western Australia. Reproductive and larval states and chaetal patterns within and between species are presented. The two patterns of morphology and diversity present in these taxa, observed also across the Indo-Pacific, are plausibly related to ancient colonisation histories.

Fourier transform near- and mid-infrared spectroscopy can distinguish between the commercially important Pelargonium sidoides and its close taxonomic ally P. reniforme

Pelargonium sidoides is indigenous to South Africa and abundant in the Eastern Cape Province. Several herbal products have been formulated using P. sidoides of which Umckaloabo® is probably the most popular and successfully marketed in Germany. The objective of this study was to discriminate between P. sidoides and Pelargonium reniforme by FT-IR spectroscopy. Absorbance spectra were collected for P. sidoides (n=96) and its close taxonomic ally P. reniforme (n=57) in the near infrared (NIR) and mid infrared (MIR) regions. The spectroscopic data were analysed using chemometric computations including principal component analysis and orthogonal projections to latent structures discriminant analysis. Phytochemical variation of 5.79% in the NIR dataset (R2X(cum)=0.962; Q2(cum)=0.918) and 9.22% variation in the MIR dataset (R2X(cum)=0.497; Q2(cum)=0.658) was responsible for the separation of the two species. Seven absorption areas were identified as putative biomarkers responsible for the differences between the two species. These results indicate that FT-NIR and FT-MIR spectroscopy can be used to discriminate between these two closely related species which occupy a sympatric distribution in South Africa.

Morphology, distribution, and chromosome counts of two varieties of Hedychium villosum (Zingiberaceae)

Abstract Polyploidy is a major mechanism of adaptation and speciation in plants. Two varieties of Hedychium villosum, var. villosum and var. tenuiflorum, primarily differ in plant and flower size. Chromosome number suggests that var. tenuiflorum is diploid (2n= 34) and var. villosum is tetraploid (2n= 68). Although the flowers of the two varieties do not have any difference in floral shape, each can be easily distinguished morphologically because the shoots, leaves, and flowers of the tetraploid var. villosum are consistently larger than the diploid var. tenuiflorum. The two varieties each possess distinct geographic ranges and habitats, and no sympatric distribution has been found. The tetraploid var. villosum has a broader geographic distribution range and more diverse ecological habitats than the diploid var. tenuiflorum. The two varieties are also completely reproductively isolated due to the non-overlap of their flowering times. The two varieties should be recognized as two distinct species as they fulfill the requirements of various species concepts. Thus, we suggested that the tetraploid var. villosum should be kept as Hedychium villosum and the diploid var. tenuiflorum should be renewed to Hedychium tenuiflorum.

Hydrocotyle apolobambensis (Apiaceae), una Especie Nueva Andina del Noroeste de Bolivia

Se describe e ilustra Hydrocotyle apolobambensis M. Mend. & A. Fuentes (Apiaceae), una nueva especie de hojas trifolioladas, restringida a la cordillera de Apolobamba, en la cuesta oriental del noroeste de Bolivia. La especie es similar a H. palmata Mathias con la que comparte su área distribución natural, pero difiere de ella por tener un menor número de flores, pedicelos pubescentes, lóbulos de las hojas redondeados a obtusos y el indumento piloso. Se adiciona una clave y una tabla comparativa para distinguirla de otras especies trifolioladas de Hydrocotyle L. de América del Sur.

Trichoderma asperellum sensu lato consists of two cryptic species

Analysis of a worldwide collection of strains of Trichoderma asperellum sensu lato using multilocus genealogies of four genomic regions (tef1, rpb2, act, ITS1, 2 and 5.8s rRNA), sequence polymorphism-derived (SPD) markers, matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) of the proteome and classical mycological techniques revealed two morphologically cryptic sister species within T. asperellum, T. asperellum, T. asperelloides sp. nov. and a third closely related but morphologically distinct species. T. yunnanense. Trichoderma asperellum and T. asperelloides have wide sympatric distribution on multiple continents; T. yunnanense is represented by a single strain from China. Several strains reported in the literature or represented in GenBank as T. asperellum are re-identified as T. asperelloides. Four molecular SPD typing patterns (I–IV) were found over a large geographic range. Patterns I–III were produced only by T. asperellum and pattern IV by T. asperelloides and T. yunnanense. Pattern I was found in North America, South America, Africa and Europe and Asia (Saudi Arabia). Pattern III was found in Africa, North America, South America and Asia, not in Europe. Pattern II was found only in Cameroon (central Africa) and Peru. Pattern IV was found in all continents. All SPD II pattern strains formed a strongly supported subclade within the T. asperellum clade in the phylogenetic tree based on rpb2 and MLS (combined multilocus sequence). The diversity of DNA sequences, SPD markers and polypeptides in T. asperellum suggests that further speciation is under way within T. asperellum. MALDI-TOF MS distinguished T. yunnanense from related taxa by UPGMA clustering, but separation between T. asperellum and T. asperelloides was less clear.

Molecular phylogenetic analysis among species of Paridae, Remizidae and Aegithalos based on mtDNA sequences of COI and cyt b

The phylogeny of Paridae and allies has been studied intensively during past decades. However, the phylogenetic relationship among species tends to become increasingly controversial as different genetic markers emerge. In our study, the partial mitochondrial genes cytochrome b (cyt b) and cytochrome c oxidase subunit I (COI) were obtained from 15 species that included 10 tits, 4 long-tailed tits and a Chinese penduline tit. Analyses were conducted on the combined cyt b and COI sequences with maximum likelihood and Bayesian algorithms. Based on strong, congruent support among the different temporal partitions and models of sequence evolution, a highly resolved consensus of the relationships among Parids and their allies has been formed. The monophyly of Paridae and Remizidae is strongly supported. However, the monophyly of Paridae and Aegithalos is rejected. This agrees with previous studies using other molecular markers. Our results suggest the promotion of the subgenus Machlolophus from genus Parus to a separate genus. The phylogeny of Aegithalos is robust in the current study. However, by considering differences of both morphological and molecular characters within species, we conclude that more data are needed to define their phylogeny. Based on the patterns of taxonomic diversity and endemism, we suggest the southwestern mountain ranges of China might be the center of origin of the Aegithalos species. Divergence time estimates for the long-tailed tits range from the late Miocene to the Pleistocene (from 5.5 to 0.1 Mya) using a calibration of 2% divergence per million years. In a comparative sense, we found a congruent genetic differentiation among sympatric distribution taxa.

Transcriptomic analyses of the avirulent protozoan parasite Trypanosoma rangeli

Two species of the genus Trypanosoma infective to humans have been extensively studied at a cell and molecular level, but study of the third, Trypanosoma rangeli, remains in relative infancy. T. rangeli is non-pathogenic, but is frequently mistaken for the related Chagas disease agent Trypanosoma cruzi with which it shares vectors, hosts, significant antigenicity and a sympatric distribution over a wide geographical area. In this study, we present the T. rangeli gene expression profile as determined by the generation of ESTs (Expressed Sequence Tags) and ORESTES (Open Reading Frame ESTs). A total of 4208 unique high quality sequences were analyzed, composed from epimastigote and trypomastigote forms of SC-58 and Choachí strains, representing the two major phylogenetic lineages of this species. Comparative analyses with T. cruzi and other parasitic kinetoplastid species allowed the assignment of putative biological functions to most of the sequences generated and the establishment of an annotated T. rangeli gene expression database. Even though T. rangeli is apathogenic to mammals, genes associated with virulence in other pathogenic kinetoplastids were found. Transposable elements and genes associated mitochondrial gene expression, specifically RNA editing components, are also described for the first time. Our studies confirm the close phylogenetic relationship between T. cruzi and T. rangeli and enable us to make an estimate for the size of the T. rangeli genome repertoire (∼8500 genes).

Molecular phylogeny of Nearctic species of Rhynchelmis (Annelida)

Zhou, H., Fend, S. V., Gustafson, D. L., De Wit, P. & Erséus, C. (2010). Molecular phylogeny of Nearctic species of Rhynchelmis (Annelida). —Zoologica Scripta, 39, 378–393.The Nearctic species of Rhynchelmis (Clitellata, Lumbriculidae) are known primarily from cool‐water habitats in western North America. Their taxonomy has so far been based on limited collections from isolated localities, using intuitive assessment of morphological characters. This approach has proved unsatisfactory when additional populations of closely related species were sampled and scrutinized for incorporation in the present classification. Therefore, in this study, mitochondrial (cytochrome c oxidase subunit I and 16S rDNA) and nuclear internal transcriber spacer (ITS rDNA) genes were analysed as phylogenetic markers of Nearctic Rhynchelmis species. A combined approach with all the three gene regions provided a better resolution than any of the individual genes by itself. The genes demonstrated monophyly of all major groupings proposed on the morphological basis. Within the Rhynchelmis yakimorum complex, however, the genetic data and distribution suggested that two clades initially referred to as a ‘R. yakimorum variant 1’, one from the lower Snake River drainage in Idaho and one from southern coastal Oregon, might represent two separate species. On the other hand, the sympatric distribution and low genetic distance between Rhynchelmis gustafsoni and a form tentatively identified as ‘R. cf. yakimorum’ (both collected in eastern Idaho) indicated conspecific status. This study also showed that the cytochrome c oxidase subunit I (COI) gene, which may be informative of recent and on‐going speciation and useful for species discrimination (as a DNA barcode), is less suitable as a single molecular marker for phylogenetic inference. Regardless of whether one deals with very closely related species (such as those of the yakimorum complex), with taxa with a wide and disjunct distribution (such as Rhynchelmis rostrata), or with more distantly related species, COI data should be supplemented by other genetic markers as well as morphological and biogeographical information.

One species or at least eight? Delimitation and distribution of Frullania tamarisci (L.) Dumort. s. l. (Jungermanniopsida, Porellales) inferred from nuclear and chloroplast DNA markers

Frullania tamarisci is usually regarded as a polymorphic, holarctic-Asian liverwort species with four allopatric subspecies [subsp. asagrayana, moniliata, nisquallensis and tamarisci]. This hypothesis is examined using a dataset including sequences of the nuclear internal transcribed spacer region and the plastid trnL- trnF and atpB- rbcL regions of 88 accessions of F. tamarisci and putatively related taxa. Maximum parsimony and maximum likelihood analyses indicate the presence of at least eight main lineages within F. tamarisci s. l. The long branches leading to the tip nodes of the different F. tamarisci s. l. clades and their partly sympatric distribution reinforce species rank. Within F. tamarisci s. l. we recognize the Asian F. moniliata, the western North American F. californica and F. nisquallensis, the eastern North American F. asagrayana, the eastern North American–European F. tamarisci s. str., the Macaronesian F. sergiae, and two newly identified European lineages assigned to as F. calcarifera and F. tamarisci var. azorica. The considerable sequence differences are not reflected in conspicuous morphological disparities, rendering F. tamarisci s. l. the most explicit example of a complex of semi-cryptic and cryptic liverwort species. The temperate Frullania clades of this study likely went through recent extinction and expansion processes as indicated by the bottleneck pattern of genetic diversity. Species from tropical regions or regions with an Atlantic climate usually contain several geographical lineages. Our findings support frequent short-distance migration, rare successful long-distance dispersal events, extinction and recolonization as an explanation for the range formation in these Frullania species.

Origin of agriculture and plant domestication in West Mesoamerica

Recent paleoecological, archaeobotanical and genetic-molecular data are used to develop a hypothesis on the where, when, how and whom of plant domestication and the origin of agriculture in west Mesoamerica, and the formation of the maize-bean-squash multicrop milpa system and agro-food system which formed the base for development of ancient complex societies in this area. It is highly likely that about 10,000 before present (BP) human groups specializing in plant gathering and small game hunting in the dry tropical forest of the Balsas-Jalisco biotic morphotectonic province began the process of plant domestication and agriculture, using fire as a tool. Sympatric distribution of the putative wild ancestral populations of maize, beans and squash indicate the extreme northwest Balsas-Jalisco region as a possible locus of domestication. Diffusion of these domesticates to the rest of Mesoamerica would have occurred via existing biological-cultural corridors. The milpa agro-food system would have been established between 7,000 and 4,400 calendar years (cal) BP. The complex food technology developed in the northwest Balsas-Jalisco region between 4,500 and 3,500 BP, much more complex than in other areas at the time, also suggests this area as the origin of the milpa agro-food system. Further archaeobotanical research is needed to confirm this hypothesis. Exploratory, collection and conservation efforts are needed in these putative source populations, as well as studies on their adaptation to climatic, edaphic and biotic factors, before they are displaced by the African grasses and pesticides forming part of the region’s growing cattle industry.

Diet and Trophic Niche Overlap of Three Toad Species (Amphibia, Anura) from Poland

ABSTRACTDuring our study we identified 53 prey items in the trophic spectrum of Bombina bombina, 124 prey items in the diet of Bufo bufo and 95 prey items in the diet of Epidalea viridis. The average number of prey items per stomach is as follows: Bombina bombina - 7.57, Bufo bufo - 9.0 and Epidalea viridis - 13.57. In all studied species the most important prey category is Coleoptera. Other important prey animals are Hemiptera, Hymenoptera and Dermaptera as well as non-insect invertebrates (Gastropoda and Arachnida) which also play significant role. All toads consume almost only terrestrial prey. The trophic niche breadths for the three species are as follows: Bombina bombina - 5.40, Bufo bufo - 4.47 and Epidalea viridis - 2.42. The estimated trophic niche overlap between the species is moderate (58.82% - 63.72%) and probably there is no or insignificant competition for food resources between them in the places with sympatric distribution. All studied species are polyphagous zoophages, like other amphibian species and they are probably consuming all mobile objects which they come in contact with and can swallow.

Distribution patterns and trophic characteristics of salmonids and native species inhabiting high altitude rivers of Pampa de Achala region, Argentina

The Pampa de Achala in central Argentina is an area with low fish species richness where salmonids were first introduced at the beginning of the twentieth century. A total of 19 rivers of different order were sampled during the low water period by using portable electrofishing equipment. We covered different identified macrohabitats (pools, riffles, glides and cascades) in reaches of 300-500 m length. The native species Trichomycterus corduvensis and two exotic salmonids such as Salvelinus fontinalis and Oncorhynchus mykiss represented the most common species, showing allopatric and sympatric distribution in some streams. Stomach contents and diet overlapping were also analyzed. Cluster results showed a first main group comprised those streams with high diet similitude between T. corduvensis and O. mykiss whereas a second main group consisted of streams with high diet similitude between O. mykiss and S. fontinalis. Mean niche width was similar among all these species but T. corduvensis showed that widest range whereas S. fontinalis exhibited the narrowest trophic niche. We were able to document a direct predation on T. corduvensis by O. mykiss in two stream although distribution overlapping among native and exotic species were low. In turn diet overlapping among species were negligible. Trophic niche amplitude of S. fontinalis was inversely related to stream order, providing clues that geomorphology and hydrology may exert also influence of trophic characteristics and can be used for predicting the potential for food competition with native species. Analysis by Tokeshi plot revealed that T. corduvensis ranged from specialist to generalist whereas both salmonids showed generalist feeding habits with a heterogeneous and homogeneous diet.

Craniodental mechanics and the feeding ecology of two sympatric callosciurine squirrels in Vietnam

AbstractThis study explores the relationship between feeding habit and craniodental morphology in two closely related sympatric callosciurine squirrels in south‐east Asia: Callosciurus erythraeus and Dremomys rufigenis. Both C. erythraeus and D. rufigenis prefer fruits and insects, as generally seen in other tree squirrels. However, C. erythraeus also feeds extensively on hard seeds and tough tree bark, which are rarely included in the diet of D. rufigenis. We tested the hypothesis that C. erythraeus shows structural features in craniodental morphology that provide an advantage for feeding on hard seeds and tough tree bark. Results show that C. erythraeus possesses a greater mechanical advantage of the masseter and temporalis muscles for chewing, and a more robust zygomatic arch, compared with D. rufigenis. Callosciurus erythraeus is capable of generating a greater bite force at a given muscle force at the cheek teeth and its zygomatic arch is more resistant against the stresses generated by the masseter muscle, suggesting adaptation for feeding on hard seeds. Callosciurus erythraeus also exhibits greater leverage of the masseter and temporalis for incision and has a wider skull at the interorbital region, suggesting the capability for efficiently generating a greater bite force and a greater mechanical resistance to increased loads when biting into wood with its anterior dentition. Our findings indicate that the craniodental divergence between C. erythraeus and D. rufigenis may be linked functionally to the differences in their diets. We suggest that their morphological divergence may correlate with their dietary differences and contribute to the sympatric distribution of these closely related squirrels.