Evolutionary Relationships among Rodents

Patterns of variation and covariation within populations can influence how characters respond to natural selection and random genetic drift and so constrain the ability of natural selection to modify the phenotype. We examined several potential developmental and functional explanations of character covariation throughout ontogeny using known-age samples of the cotton rat (Sigmodon fulviventer) to identify the causes of covariation and to assess the variability of patterns of covariation throughout postnatal growth. Competing developmental and functional models were fit to samples of orofacial and neurocranial measures by confirmatory factor analysis and evaluated for their ability to reconstruct observed variance-covariance matrices. Samples of successive ages were simultaneously fit to a common model to test the hypothesis that the patterns of developmental and functional integration were invariant between ages. Orofacial characters derived from the same branchial-arch primordium covary early in ontogeny. Subsequently, there is a repatterning of integration that may reflect a transition from developmental to functional sources of integration. Neurocranial characters exhibit even more variation in patterns of covariation: initially, characters appear to comprise a single integrated unit; before puberty, they appear to respond to localized bone growth; after puberty, they form separate calvarial and basicranial components. This ontogenetic variation in patterns of covariation suggests that developmental constraints are transient and flexible and that the consequences of selection may depend upon the age at which it acts.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1558-5646.1989.tb05179.x

Ontogenetic variation in patterns of developmental and functional integration in skulls of sigmodon fulviventer.

The diversity of mammalian masticatory systems has attracted considerable attention for a long time. In the first half of this century, numerous descriptive studies appeared, including comparative ones, e.g. Fiedler (1953, Insectivora); Starck (1933, platyrrhine primates; 1935, ursids); Storch (1968, Chiroptera); later, significant studies were published by Gaspard et al. (1976), Schumacher (1961) and Turnbull (1970). Bluntschli (1929) and followers (Muller 1933; Zey 1939) published morphological papers, taking into account postnatal development.

Evolutionary Approach of Masticatory Motor Patterns in Mammals

Aim Size and shape of the mandible are investigated across the latitudinal range of theEuropean wood mouse (Apodemus sylvaticus), in order to address the relative import-ance of genetic structure, insularity, and geographical gradient in patterning morpho-logical variation. Results are compared with those on two Asiatic species of wood mice,A. argenteus and A. speciosus.Location The European wood mouse is sampled by a set of trapping localities includingboth, islands and mainland populations, as well as the four genetic groups identiﬁed inprevious studies. The localities cover a latitudinal gradient from 55 Nto36 N.Methods Different Fourier methods are applied to the outlines of mandibles and theirresults compared in the case of A. sylvaticus. All provide similar results and allow aquantiﬁcation of the size and shape variations across the geographical range of theEuropean wood mouse. Using the method allowing for the best reduction of theinformative data set, a comparison of the European wood mouse with the two Asiaticspecies was performed.Results Within the European wood mouse A. sylvaticus, a strong latitudinal gradient inmandible shape overrides the inﬂuence of insularity and genetic structure. Yet, randommorphological divergence in insular conditions can be identiﬁed as a secondary processof shape differentiation. Size displays no obvious pattern of variation, neither withinsularity or latitude. A comparison with two other species of wood mice suggests that asimilar latitudinal gradient in mandible shape exists in different species, mandibles beingﬂatter in the north and wider in the south.Main conclusion The latitudinal gradient in mandible shape observed in the threespecies of wood mice is interpreted as an intraspeciﬁc adaptive response to gradualchanges in feeding behaviour.KeywordsShape, size, mandible, Apodemus, Rodentia, latitude, island biogeography, morpho-metrics, Fourier analysis.INTRODUCTIONSize of organisms has long been recognized to displayimportant biogeographical variations. The trend towards anincrease in size with latitude, or Bergmann’s rule (Bergmann,1847; Mayr, 1942), has been demonstrated in mammals andinvertebrates (Ray, 1960; Chapelle & Peck, 1999). Sizechanges can also be the consequence of isolation such as inthe islands, where small mammals like rodents will tend tobecome larger, whereas large mammals tend to be smaller(Foster, 1964; Lomolino, 1985; Michaux et al., 2002b).Apart from size, the shape of organs is of prime import-ance in the interaction of the organism with its environment.Taxonomic differentiation is commonly associated withmorphological divergence, but patterns of intraspeciﬁcvariation are less investigated. Still, genetic structure(Auffray et al., 1996; Corti & Rohlf, 2001) and insularity

Adaptive latitudinal trends in the mandible shape of Apodemus wood mice

Voles and lemmings are the most successful group of graminivorous rodents, but the adaptations allowing them to enter this niche are not fully known. Dissections of the masticatory musculature of the 12 genera and subgenera of North American microtines show an increase in the potential anterior vector component and in the potential vertical vector component of these muscles relative to the molar tooth row. The result is a separation of the compressive and propulsive functions of the masticatory muscles during the power stroke of mastication. This has led to the formulation of a propalinal “swing” hypothesis which is supported by vector analyses of the musculature.

M. H. Kesner

Papers

Functional morphology of the masticatory musculature of the rodent subfamily microtinae