Diversification Of Rodents Research Articles

Development shapes the evolutionary diversification of rodent stripe patterns.

Vertebrate groups have evolved strikingly diverse color patterns. However, it remains unknown to what extent the diversification of such patterns has been shaped by the proximate, developmental mechanisms that regulate their formation. While these developmental mechanisms have long been inaccessible empirically, here we take advantage of recent insights into rodent pattern formation to investigate the role of development in shaping pattern diversification across rodents. Based on a broad survey of museum specimens, we first establish that various rodents have independently evolved diverse patterns consisting of longitudinal stripes, varying across species in number, color, and relative positioning. We then interrogate this diversity using a simple model that incorporates recent molecular and developmental insights into stripe formation in African striped mice. Our results suggest that, on the one hand, development has facilitated pattern diversification: The diversity of patterns seen across species can be generated by a single developmental process, and small changes in this process suffice to recapitulate observed evolutionary changes in pattern organization. On the other hand, development has constrained diversification: Constraints on stripe positioning limit the scope of evolvable patterns, and although pattern organization appears at first glance phylogenetically unconstrained, development turns out to impose a cryptic constraint. Altogether, this work reveals that pattern diversification in rodents can in part be explained by the underlying development and illustrates how pattern formation models can be leveraged to interpret pattern evolution.

Ecological correlates of the morphology of the auditory bulla in rodents: Application to the fossil record.

For rodents, hearing is essential to survival. It enables predator evasion, prey detection, and conspecific recognition; it is also likely to be constrained by the physical environment. The resulting hypothetical link between tympanic bulla morphology and ecology has never been investigated across a broad array of rodent species before. Such link may enable the determination of the ecological affinities of many fossil species only known from partial skulls. In this study, we used geometric morphometrics to quantify the shape of the auditory bulla of 197specimens representing 91species from 17 families of extant rodents across four different locomotory modes. We used landmarks and semi-landmarks on the ventral and lateral views of the skull to capture morphological characteristics of the bulla and external auditory meatus (EAM). Our results demonstrate an association between bullar morphology and locomotion in rodents. Bullar shape enables the correct classification of 76% of the species in our training set. Fossorial taxa, in particular, show a characteristic morphology including an asymmetric bulla with a dorsally located and laterally expanded EAM that has a small opening diameter. A phylogenetically informed flexible discriminant analysis shows a weak phylogenetic effect on tympanic morphology. There is no evidence for differences in bullar hypertrophy across locomotory categories. The application of this approach to select fossil rodents from the Oligo-Miocene shows broad agreements with prior studies and yields new locomotory inferences for 14 fossil species, including the first proposed locomotion for members of the family Florentiamyidae. Such results call for the timing of burrowing diversification in rodents to be reevaluated.

Tooth wear patterns in black rats (Rattus rattus) of Madagascar differ more in relation to human impact than to differences in natural habitats.

Dietary characteristics and environmental variables are important selective factors directing ecological diversification in rodents. On Madagascar, the introductions and spread of the commensal black rat (Rattus rattus) can be seen as example cases to study dietary niche occupation and dietary adaptation in an insular environment. We investigate how tooth wear as a measure of dietary adaptation of black rats differs between four distinct habitats (village, manioc fields, spiny forest, and rainforest) with different dietary resources. We use the 3D surface texture analysis (3DST, using 30 parameters according to ISO 25178) as a measure of dietary abrasiveness. 3DST is applied on the occlusal surface of the upper first molar of 37 black rat specimens. The rainforest sample displays less rough and less voluminous surface textures compared to the village samples as indicated by smaller values for height parameters (Sa, Sp, Sq), inverse areal material ratio (Smc), and volume parameters (Vm, Vmc, Vmp, Vv, and Vvc). We therefore rank sampling areas from highest to lowest abrasiveness (village>manioc fields/spiny forest>rainforest). The rats from villages and rainforest differ to such an extent that one could have interpreted them to belong to different species. This indicates a high degree of variability in terms of ingesta abrasiveness. Furthermore, the pronounced difference between rats from human habitations compared to rats from associated fields or natural vegetation is interpreted to clearly indicate shifts in dietary niche occupation in relation to human impact.

Comparison of Jaw Muscle Morphology in Two Sympatic Callosciurine Squirrels (Callosciurus ErythraeusandDremomys Rufigenis) in Vietnam

Laboratory of Anatomy, Department of Basic Veterinary Sciences, Obihiro University of Agriculture and Veterinary Medicine, Obihiro 080-8555, JapanIn this study, we focus on the jaw muscle morphol-ogy of two callosciurine squirrel species, Callosciuruserythraeus and Dremomys rufigenis. Callosciuruserythraeus (Callosciurini: Sciuridae: Callosciurinae) isan arboreal squirrel that inhabits Southeast Asia fromeastern India to the Malayan archipelago, Indochina,southern China, and Taiwan (Nowak 1999; Can et al.2008; Francis 2008; Smith and Xie 2008). D. rufigenis,another member of the tribe Callosciurini, is found sym-patrically with C. erythraeus throughout Southeast Asia(Nowak 1999; Can et al. 2008; Francis 2008; Smith andXie 2008). Koyabu et al. (2009) previously reported thatthe sympatric C. erythraeus and D. rufigenis displaynotable difference in craniodental mechanics. Both C.erythraeus and D. rufigenis are known to prefer fruitsand insects, as generally seen in other tree squirrels.However, C. erythraeus also feeds extensively on hardseeds and tough tree bark, which are rarely included inthe diet of D. rufigenis. For example, C. erythraeus isreported to frequently forage on the hard seeds ofPinus, Camellia, and Castanopsis tree (Chou et al.1985; Sonoda et al. 2001; Lu 2003; Men et al. 2007).Callosciurus erythraeus is also known to often strip toughbark to obtain sugar from sweet tree saps (Tamura andOhara 2005), whereas this dietary behavior seems to berare or not reported in D. rufigenis (Medway 1969;Nowak 1999; Smith and Xie 2008). Koyabu et al.(2009) demonstrated that C. erythraeus possesses agreater mechanical advantage of the masseter and tem-poralis muscles for chewing compared to D. rufigenis.Callosciurus erythraeus was also found to display greaterleverage of the masseter and temporalis for incision.Given these findings, it was suggested that C. erythraeusshould be capable of generating greater bite force for agiven muscle force at the cheek teeth and that the cranio-dental divergence between C. erythraeus and D. rufigenismay facilitate their trophic separation under sympatry.In their study, however, only the analysis of mechani-cal leverages was conducted, and comparisons of jawmuscle morphology were not incorporated. Althoughthe craniodental arrangements and mechanical advantageof the masticatory muscles are critical parameters thataffect the amount of force production, muscle mass isalso another essential contributor to the bite forces ananimal can generate (Raadsheer 1999). Since the biteforces an organism can generate should constrain therange of food items it can eat (Lucas 2004), variation inmasticatory muscle mass has undoubtedly been funda-mental for the dietary diversification of rodents. Giventhe great range of diet and phenomenal level of morpho-logical diversity found across rodents, studies on rodentmuscle mass variation should provide promising avenuesto study the ecomorphological patterns of mammals ingeneral. To date, reports on muscle mass are availablefor some muroids (e.g., Hiiemae and Houston 1971;Weijs and Dantuma 1975; Satoh 1997; Satoh and Iwaku2004) and New World squirrels (Ball and Roth 1995).Although Thorington and Darrow (1996) provideddetailed anatomical descriptions of jaw muscles for someOld World squirrel species, still very little is describedquantitatively for them. Here, to bridge the gaps in ourknowledge of Old World squirrel muscle morphology

Salivary glands, cellular evolution, and adaptive radiation in mammals.

Evolutionary theory provides the foundation for interpretation of the natural world, but one remaining major challenge is to link genetic variation and particular gene products to natural selection and adaptation. Another challenge is to describe the role of cells, especially secretory cells, in the evolutionary process. Comparative studies of mammalian salivary glands could serve as an insightful model. Our theoretical approach combines data on genomic and chromosomal evolution with data on secretory cells and proteins so that adaptation can be understood in context of these interrelated components. The present paper reviews patterns and types of interspecific salivary gland cell variation that we have documented at the ultrastructural level. This data set shows ways in which secretory cells and the secretory process may have been evolutionarily modified in mammals. As a further example of our approach we also review how proline-rich-proteins (PRPs) may have had adaptive significance in the evolution of mammals, especially rodents. Our working hypotheses are: that Ca+2-binding acidic PRPs are incompatible with ever-growing dentition; and that presence or absence of tannin-binding basic PRPs was a major factor in ecological diversification in rodents. Some rodents might even exhibit behavioral compensation for absence of basic PRPs in their saliva and this illustrates the complex alternatives available to natural selection.