Biology Reference
In-Depth Information
Environmental Correlates of Geographic Variation in Skull and Mandible Shape of a
Rodent, the Punar
´
Rat (
Thrichomys apereoides
)
In an investigation of the environmental sources of geographic variation in an echimyid
rodent sampled from Cear´ , Para´ba, Pernambuco, Alagoas, Bahia, Goi´s and Minas
Gerais in Brazil,
Monteiro and colleagues (2003)
examined the relationship between skull
and mandibular shape and a collection of environmental variables, including geographic,
climatic and biotic environmental variables. In this region, increases in latitude and longi-
tude are associated with increases in altitude and rainfall and decreases in mean annual
temperature and human density; moreover, the vegetation changes from the arid caatingas
to the cerrado savannas. The environmental block comprises geographic variables (1) lati-
tude and (2), longitude and (3) altitude plus the climatic variables (1) mean temperature
and (2) rainfall, and two biotic variables, (1) human population density and (2) vegetation
type (a categorical variable). The environmental data were obtained from the Enciclop´dia
dos Munic´pios Brasileiros. The skull was measured in three views (dorsal, ventral and lat-
eral) and the mandible was measured in lateral view.
SA1 explained from 69% to 90% of the covariation between shape and the environment.
When summarizing the results, we focus on the two views for which the association
between geographic variation shape and the environment is statistically significant. For
the ventral view, the environmental SA1 represents a contrast between latitude, longitude,
altitude, versus vegetation type and human density. With increasing scores (towards the
south), snouts shorten and narrow relative to the skull, jugals become relatively shorter
and the tympanic bullae and foramen magnum reduce, plus there are more localized
changes in basicranial bones. The distribution of the scores for shape SA1 on the environ-
mental SA1 suggests an environmental gradient from north to south, but shape appears to
follow that gradient only from north to the most northern of the southern populations.
The southern populations show increasing scores on the environmental SA1 but the scores
are nearly constant on the shape SA1 (with the exception of one population from Goi´s).
For the lateral view, the environmental SA1 represents a contrast between latitude, longitude,
altitude and rainfall versus temperature, vegetation type and human density. The associated
shape changes include a relative shortening of the snout and brain case, a decrease in relative
length of the jugal plus a general dorsovental shallowing of the skull. The scores on the paired
SA1 suggest two clusters of populations, northern and southern; the northern populations
vary little along either the shape or environmental SA1 whereas the southern populations,
which have higher scores on the shape SA1, vary along the environmental SA1 but not along
shape SA1.
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