Proechimys semispinosus has its wide range in the mountainous, western area of South America, the headwaters of the Amazon drainage and northward in Central America and the nearby Pacific Islands. In these populations a gradient may exist in the number of counterfolds which is varied in every population. The highest number seems to occur in the populations from northern Peru and Ecuador, decreasing from there in all directions, except in the Panamanian and Columbian islands. In gross examination, it seems that the size of the animals increases to the northwards.
SPECIFIC VARIATION IN THE SUBGENUS
TRINOMYS
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.