Part 5

In spite of the lack of specimens from areas in which Proechimys certainly occurs, it is evident that the genus has great plasticity and that the number of subspecies will be greatly increased as additional material is studied. Only perfunctory examination of samples from outside the area of Brazil shows me that there are several unnamed subspecies there. My impression is that Allen's trinitatis, of Trinidad, the genotype of Proechimys, will eventually be split.

There are two main lines of subspeciation in Proechimys guyannensis. The one south of the Amazon River includes P. g. bolivianus, in Bolivia, P. g. villicauda, and P. g. ribeiroi occurring on the divide of the headwaters of the Amazon and Parana rivers, in Brazil, and P. g. hyleae in the lower Tapajoz and P. g. nesiotes in the lower Tocantins. All six subspecies have a large number of counterfolds in the molariform teeth. In these six subspecies, p4 has four counterfolds and the lower molars have three each. Toward the northeastern coast the number of counterfolds decreases to three in p4 and to two in the lower molars, as in P. g. arescens, P. g. leioprimna and P. g. oris.

In northern South America, north of the Amazon River, the subspecies with the greatest number of counterfolds is P. guyannensis warreni (known from only the Demerara River area); p4 has four counterfolds and the lower molars have three each. The number decreases in all the adjacent populations: P. g. guyannensis, in the Guianas, P. g. trinitatis, and P. g. urichi (going westward from the Guianas to Venezuela) have the counterfolds reduced to three in p4, but the lower molars still have the same number of counterfolds, namely, three, although there is a tendency for them to coalesce; farther west, on the coast, the number decreases to three counterfolds in p4 and to only two in the lower molars as in P. g. guairae and P. g. mincae. Subspecies south of the coast show the same reduction of counterfolds, P. g. cherriei and P. g. o'connelli being examples; P. g. ochraceus and P. g. poliopus have the reduction carried to the upper molars, M3 having usually only two counterfolds; P. g. chrysaeolus in the valley between the Madalena and the Cauca rivers seems to be somewhat isolated and shows reversion to three counterfolds in the lower molariform teeth; directly southward of the range of P. g. warreni the number of counterfolds decreases to three in all lower cheekteeth (population at Ayan-Tepuy, southern Venezuela), and then to three in p4 and to two in the lower molars, as in P. g. arabupu on the Brazilian side of Mount Roraima, and the reduction is extended to the upper molars in P. g. vacillator.

On the north bank of the Amazon, the only population of P. g. hyleae known to me (from Obidos) has four counterfolds in p4 and three in the lower molars; P. g. riparum, from Manaus, also on the north bank of the Amazon, has three counterfolds in p4 and two counterfolds in the lower molars. P. g. hyleae occurs also on the south bank of the Amazon. P. g. riparum, therefore, may be the northern part of the southern cline, instead of the southern end of the northern cline.

The whole picture, as outlined above, may be explained by assuming that the species P. guyannensis differentiated somewhere on the Central Plateau of South America, with three counterfolds in each upper molariform tooth, four counterfolds in the lower premolar and three counterfolds in the lower molars. The species might have extended its range to the Guianas and then all the biotypes with reduced number of counterfolds might have slowly developed by natural selection. The gradient is, broadly, from subspecies with greater number of counterfolds in more humid areas, to a gradually lessening number of counterfolds in less humid areas.

Proechimys longicaudatus is limited in the south to the headwaters of the Parana River drainage, where the subspecies P. l. roberti and P. l. longicaudatus are found. The species ranges northward through the Tapajoz drainage, with P. l. leucomystax in the headwaters and P. l. boimensis in the lower course. To the northwest and west the species is represented in Bolivia by P. l. securus; P. l. elassopus, P. l. simonsi, P. l. pachita, and P. l. hendeei occur in Peru and P. l. brevicauda in Peru and Brazil; and P. l. nigrofulvus occurs in Ecuador. Again in P. longicaudatus it seems that the number of counterfolds follows a gradient from more humid areas with four counterfolds in p4, as seen in nigrofulvus, pachita, simonsi, elassopus and brevicauda, decreasing to three or four in securus, to three in longicaudatus, but with m3 having only two counterfolds in leucomystax and roberti. P. l. boimensis, widely separated in the lower Tapajoz (no samples being known from the intervening range) may be the end of a cline started by leucomystax with only 2 counterfolds in m3 and ending to the northward with four counterfolds in m3. Over the same area the counterfolds in p4 increase from 3 to 4.

Of Proechimys goeldii I have had inadequate material but there seems to be a similar gradient in it which may be traced from P. g. steerei to P. g. goeldii. P. g. steerei has four counterfolds in more upper molars than occurs in the other subspecies.

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.

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.

SUBSPECIFIC VARIATION IN THE SUBGENUS TRINOMYS

One of the species of Trinomys, Proechimys iheringi, is here subdivided into six subspecies which show a clinal variation. P. i. iheringi, in the southernmost part of the range of the species (Ilha de São Sebastião), has three counterfolds in the upper cheekteeth of almost every young specimen but one of these counterfolds, since it is small, very shallow, and disappears after little wear, is probably in the process of disappearance; all lower cheekteeth have two counterfolds or, rarely, m3 has only one. P. i. bonafidei is the next subspecies northward, where it was collected at 850 m altitude (Fazenda Bõa Fé). This subspecies still has two counterfolds in all the upper cheekteeth; only 3 out of 16 specimens fail to have these counterfolds coalesced in one or more of the teeth. In the lower cheekteeth the coalescence is evident in 18 per cent of the specimens. P. i. gratiosus, from Floresta da Caixa Dagua (alt. 750 m), geographically is well removed from bonafidei (more than two degrees north), and no samples were obtained from the intervening area. It shows such great reduction in the counterfolds that the existence of intermediate populations is clearly suggested. Every upper cheektooth of this subspecies has the two counterfolds coalesced and in 40 per cent of the specimens M3 has only one counterfold; in the lower cheekteeth 60 per cent of the specimens have only one counterfold in m3. P. i. panema, occurring approximately 100 kilometers to the northward of P. i. gratiosus (lowland form), has one counterfold in M3 in only 20 per cent of the specimens but the lower third molar has only one counterfold in 80 per cent of the specimens. In P. i. denigratus, from about 3 degrees north of the range of P. i. panema, the reduction is proportionately greater: P4 now is the only upper cheektooth with two counterfolds in every specimen; all molars tend to have only one; p4 has also two counterfolds but all lower molars have only one.

The relative size of the tail also varies in a cline from south to north. Its length is approximately 87 per cent of the length of the head and body in P. iheringi; 88 per cent in bonafidei; 99 per cent in gratiosus; 100 per cent in panema; and 103 in denigratus.

One of the subspecies, P. i. paratus, however, seems to be completely out of the dental cline. It was collected in the near proximity of the type locality of P. i. gratiosus, at an elevation of 120 m lower. This subspecies has two counterfolds in all molariform teeth and only one of the two specimens known shows these counterfolds coalesced in P4 and M1. The sample, 2 specimens, is too small to be trustworthy; hence it is impossible satisfactorily to account for the break in the clinal variation. Conceivably two full species are involved, but I prefer at present to defer decision on this problem until such time as more evidence is accumulated.

P. setosus is poorly represented, both of the available skins being faded. Furthermore, no type locality is known for the subspecies P. s. setosus.

P. albispinus has only two known subspecies: P. a. albispinus, living in a region of higher humidity, is slightly the darker and has subapical zones of the setiforms on the sides Ochraceous-Tawny; P. a. sertonius, living in a much drier region, has the same subapical zone Ochraceous-Buff. The number of specimens of P. a. sertonius is so few that no gradient can be detected, even if one exists.

TAXONOMIC CHARACTERS

Size and Proportions of External Parts

Absolute size of head and body, tail, hind-foot and ear are useful in distinguishing subgenera and subspecies and to some extent in differentiating species.

The length of head and body is large to medium in Proechimys and medium to small in Trinomys. The tail is long to medium in Trinomys and short in Proechimys. The longest tail, 242 mm, is found in P. i. denigratus, and the shortest tail, 123 mm, in P. g. steerei. The relative length of tail also provides gradients or clines.

In every species, males surpass females in average size. Nevertheless, the largest animals are usually females. How this paradoxal fact is to be accounted for, I am not sure, but it may be that the animals grow as long as they live and that females have more chances to survive longer since the care of the young keeps them closer to shelter.