Some specific characters are duplicated in each of the two subgenera;
that is to say, there are some parallel developments and they give the
common generic stock its biotypical variability. Among these parallel
developments are the width of the aristiforms, the amount of pigment in
the agouti-colored setiforms, and the shape of the nasal bones. Other
characters, however, appear in one subgeneric group and not in the
other. The specific variation will be discussed separately for each
subgenus.
The aristiforms are narrow and soft in _P. dimidiatus_ and in the other
species are wide and stiff, and on the outer thighs and rump some are
light-colored. _P. albispinus_ has the maximum number of light-colored
aristiforms; they are present over the sides and back. This species has,
however, a type of aristiforms unique in the genus--the clavate type.
The tail is longer in _P. iheringi_ and _P. setosus_ than in _P.
dimidiatus_ and _P. albispinus_; the longer type is associated with a
penicillate tip suggesting an adaptation to arboreal habit. The skull
and nasals are longer in _P. dimidiatus_ and _P. iheringi_ than in _P.
setosus_ and _P. albispinus_. In the latter two species the longitudinal
dorsal outline of the skull is conspicuously convex as opposed to
slightly convex in the other two species. The palate is longest in _P.
dimidiatus_ and _P. iheringi_ extending posteriorly to the level of the
second molars; it is slightly shorter in _P. setosus_ and shortest in
_P. albispinus_ where it does not extend behind the level of the first
molars. The incisors are opisthodont in _P. dimidiatus_ and _P.
iheringi_ and orthodont in _P. setosus_ and _P. albispinus_ and even
proodont in one part of the last species.
The molariform teeth have a large number of counterfolds in both _P.
dimidiatus_ and _P. iheringi_, although the number varies but little in
the first species and much in the second. The variation in _P. iheringi_
decreases in populations of increasingly more northern geographic
distribution; in both _P. setosus_ and _P. albispinus_ the number of
counterfolds is greatly reduced; there is only one in most specimens of
_P. albispinus_. The incisive foramen is small and nearly round in _P.
dimidiatus_, larger and elongate in _P. iheringi_, very narrow and
fissurelike in both _P. setosus_ and _P. albispinus_.
The characters of _Trinomys_, as briefly outlined above, seem to be the
result of one original species having split first into four species
which provide a gradient for certain characters. Subsequently one of
these four species, _P. iheringi_, split into six subspecies and
another gradient, parallel to the first, and involving the same
characters, is to be seen.
The interrelationship among the species is evident, not only because
they have the same subgeneric characters, but because the full species
themselves provide successive steps in a stairway of increasing
specialization from _P. dimidiatus_ to _P. albispinus_.
Morphologically _P. dimidiatus_ and _P. iheringi_ are sometimes
difficult to distinguish, especially on the basis of cranial features.
Nevertheless close attention to the small, nearly round, incisive
foramen of _P. dimidiatus_ versus the larger, more elongate foramen in
_P. iheringi_ will permit separation of the two. However, the two
species live in the same place and one is led to infer that there may be
greater differences in their physiology than in their morphology. In
fact Dr. H. W. Laemmert, from the Serviço de Estudos e Pesquisas Sobre a
Febre Amarela in Brazil, informs me that while _P. dimidiatus_ was
highly susceptible to the virus of yellow fever (18 out of 24 with virus
in circulation), _P. iheringi_ showed a lower rate of susceptibility (3
out of 25 with virus in circulation). _P. longicaudatus roberti_,
belonging in the other subgenus, showed no susceptibility at all.
At Teresópolis, Estado do Rio de Janeiro, the two species were found in
two different forests, only a few kilometers apart, but _dimidiatus_
lived at a higher elevation, where the humidity was remarkably higher.
Naturally the plant associations were different in the two forests. This
seeming ecological adaptation of the two kinds of _Proechimys_ may
explain why _P. iheringi_ ranges farther north; the forests to the
northward are less humid.
One of the four species, _P. setosus_, subspecies _elegans_, was used by
Winge (1941:80, 82) as representative of the genus _Proechimys_ when he
was estimating the relationships of that genus. Because _Cercomys_, with
four crests in each of its cheekteeth, was, on other grounds, regarded
by him (_op. cit._: 80) as "... the most primitive genus within the
group.", and because he noted in _P. s. elegans_ 4 crests in P4 and in
some first molars, he concluded that _Proechimys_ was "very closely
related to _Cercomys_." His conclusion seems to be correct, but actually
other species of _Proechimys_ (subgenus _Trinomys_), for example, _P.
dimidiatus_, have four or more crests in each cheektooth, and,
therefore, may be considered as more closely related to _Cercomys_ than
is _P. setosus_. If a large number of crests indicates primitiveness,
_P. dimidiatus_, always with four, is more primitive than any other
species in the subgenus _Trinomys_. Also, the large skull, long hind
foot, short tail and thin aristiforms of _P. dimidiatus_, in my opinion,
are primitive characters.