Part 4

Sometimes it is convenient to recognize species-groups, a systematic category without nomenclatural status, intermediate between the species and the subgenus. When there are two groups of species not sharply separated, including one species whose characters overlap those of each of the two groups, it would seem most appropriate to recognize only species-groups instead of subgenera. When, on the other hand, the two groups of species have mutually exclusive characters and a species with intermediate characters is unknown, the two groups of species can conveniently be accorded separate subgeneric rank.

SUBGENERIC VARIATION

A few characters are common to one group of species and other features are common to a second group. The most striking of these features is the character of the main fold in the molariform teeth. In one group the fold transversely crosses the crown of the tooth and in the other it extends scarcely halfway across. No specimen is intermediate in this respect. These two groups, furthermore, are separated geographically by an important barrier, the arid belt that starts in the northeastern littoral of Brazil (Ceará), and that extends south and southwesterly, more or less accompanying the São Francisco River in the Plateau, to about 20° S. Proechimys is thought not to inhabit this arid belt. At the latitude of 20° S the conditions become more suitable for Proechimys, especially along the rivers which flow eastward, but there the Plateau is replaced by mountains: the Serra Geral at the west, and Serra da Mantiqueira at the south; these ranges are bare of forests at higher elevations. Two groups of species of Proechimys are, therefore, kept geographically isolated: one group lives in southeastern Brazil, and the other lives in a large area to the west which starts at 21° S in Paraguay and Brazil and widens northward and includes, farther west, central and northern Brazil and all the South American countries above 21° S, as well as Central America northward to southern Nicaragua.

The two groups which are here treated as subgenera may be designated as follows:

Trinomys--main fold deep: aristiforms well-developed on the rump and outer thighs; tail no less than 75 per cent of length of head and body; skull without ridges across the parietals; no conspicuous groove for transmission of nerve inside infraorbital foramen; molariform teeth decreasing in size from premolar to third molar; 1 to 3 counterfolds in the molariform teeth.

Proechimys--main fold shallow: aristiforms not developed on rump and outer thighs; tail less than 75 per cent of length of head and body; groove for transmission of nerve present in infraorbital foramen of several subspecies; molariform teeth increasing in size from premolar to second molar; 2 to 5 counterfolds in molariform teeth.

Most of these characters vary but do not overlap. Subgeneric rank is here accorded to the two groups of Proechimys characterized immediately above.

The primary cause of the subgeneric differentiation is thought to have been geologic changes in the continental area. As already pointed out (see Paleontology), decreasing humidity in the Central Plateau of Brazil may have caused a migration southwestward of one or more of the species along with the forests. Once isolated geographically, the species probably differentiated at an accelerated rate.

The fact that a much larger number of subspecies occupies the larger geographic range of the subgenus Proechimys would not be sufficient to prove that this subgenus, Proechimys, is nearer to the primitive group than Trinomys, the subgenus occupying the smaller range with fewer subspecies. The paleontological evolution of the rodents, however, consistently points to teeth with a larger number of counterfolds (as seen in Proechimys) as the primitive condition. The extension of the main fold, tending to set apart one lamina in each upper molariform tooth, seems to be a specialization; reduction in the size of the head and body, increase in length of tail and decreasing size of molars posteriorly also may be specializations. The main point, however, is to establish if Trinomys is a relic group rather than a "differentiated" one. If an intermediate form were known which connected Trinomys with one species of Proechimys more than with another or even if Trinomys itself more closely resembled one of the groups of species of the subgenus Proechimys than it did another, we would assume that divergence and selection accounted for the subgeneric variation. The lack of any such connecting link favors the first idea, namely that Trinomys differentiated rapidly with the aid of geographic variation.

If Trinomys is, as I am inclined to consider it, the result of "differentiation," its subgeneric features are to be admitted as "new" and therefore the most primitive species in the genus should be found in the subgenus Proechimys.

It is a matter of common sense to admit the two groups considered above as subgenera rather than genera. Since the two structural plans were established they would, and do, act as different sources of variation. On the other hand, the morphological differences do not give the two groups an amount of morphological differences that would justify full generic rank for each.

SPECIFIC VARIATION IN THE SUBGENUS PROECHIMYS

Most of the described forms in the subgenus were initially named as distinct full species. More recently, however, in accordance with the ideas now prevalent in systematic work, many of the named kinds were reduced to the rank of subspecies. Tate first made a geographic arrangement (1935:399-400) and later (1939:177-178) provisionally synonymised several named kinds of Proechimys with Proechimys "cayennensis cayennensis." A similar tendency was clearly displayed by Ellerman (1940:115-122) who allocated 29 names, out of 33 (in the subgenus, as here understood), to the species Proechimys guyannensis and gave full specific rank to four other named kinds. Osgood (1944) also had the same viewpoint; that is to say, he appeared to have the idea that there were only two full species in the subgenus in Brazil--admitting this orally--and consequently he synonymised some full species where two or more occurred in the same place, thinking that he was dealing with individual, rather than specific, differences. Evidently the number of species in the subgenus cannot be great because the known kinds show few patterns worthy of specific designation and therefore the majority of the existing names should be suspected of having no more than subspecific value. Nevertheless none of the above writers presented real evidence in support of his arrangement.

Criteria for the recognition of full species are most easily recognized where two or more different species live together. In the literature, P. goeldii and P. "oris" were mentioned by Thomas (1912:89) as having been collected in the same place; P. mincae and P. canicollis, by H. H. Smith (in Allen, 1904:440); P. "leucomystax," from Utiarití, by Miranda Ribeiro (1914:42) and P. "longicaudatus," from the same place, by Allen (1916:569) were other examples. In these, and other alleged instances of two or more kinds occurring together, detailed study of the specimens concerned was necessary to learn the true facts. Also with the opportunity to compare collections from several different places, new facts emerged. P. longicaudatus, as it was conceived of by Allen, was a composite species, but in one locality, Utiarití, Ribeiro and Allen actually were dealing with two distinct species.

The species, or subspecies belonging to different species, living together are: goeldii and hyleae, at Fazenda Paraiso; goeldii and riparum in Manaus; boimensis and hyleae in Tauarí; leucomystax and villicauda in Utiarití; mincae and canicollis in Bonda; gularis and hendeei on the banks of Rio Napo ("same trap lines," according to P. Hershkovitz, In Litt.). Study of samples of the above named pairs of kinds of Proechimys showed the following specific differences: goeldii is large with narrow aristiforms, has a large and strongly built skull, with four counterfolds in one or more upper molars: hyleae is smaller, has wide aristiforms, smaller skull with less pronounced ridges, and never has more than three counterfolds in the upper molariform teeth; riparum closely resembles hyleae; boimensis has thin aristiforms, small skull and no more than three counterfolds in the upper molariform teeth in contrast to hyleae, already discussed; leucomystax closely resembles boimensis; villicauda closely resembles both hyleae and riparum; mincae is similar to hyleae-riparum-villicauda; canicollis has the number of counterfolds in all molars reduced to two; gularis is large, has a strongly built and ridged skull, some upper molariform teeth with four counterfolds and wide aristiforms; hendeei closely resembles leucomystax and boimensis.

The evidence obtained from study of specimens where two or more species occurred together was applied to the remaining samples and the geographic distribution was worked out. As a result the arrangement below was made, including all valid kinds already named and those here newly named from Brazil. The names of kinds I do not consider as belonging to the subgenus (and genus) are excluded. These are Echimys macrourus Jentink, not seen, and Proechimys cayennensis hoplomyoides Tate (= genus Hoplomys). The application of names is tentative, however, because the types deposited in Europe have not been seen. An asterisk denotes the forms not seen by me.

Proechimys guyannensis: arabupu, arescens, bolivianus, cherriei, chrysaeolus, guairae, o'connelli, guyannensis*, hyleae, leioprimna, mincae, nesiotes, ochraceus, oris, poliopus, ribeiroi, riparum, trinitatis, urichi, vacillator*, villicauda, warreni.

Proechimys longicaudatus: boimensis, brevicauda, elassopus, hendeei, leucomystax, longicaudatus, nigrofulvus, pachita, rattinus*, roberti, securus, simonsi.

Proechimys semispinosus: amphichoricus, burrus, calidior, centralis, chiriquinus, colombianus, decumanus, goldmani*, gorgonae, gularis, hilda*, ignotus, kermiti, liminalis, panamensis, rosa*, rubellus, semispinosus.

Proechimys goeldii: goeldii, steerei.

Proechimys canicollis.

Proechimys guyannensis appears to be more plastic than any other species. In size of animal, width of aristiforms, color and number of counterfolds in the cheekteeth, it shows marked response to variations in geographic conditions. Proechimys longicaudatus is apparently less plastic; only the number of counterfolds shows marked variation. Proechimys semispinosus varies much within its range. Proechimys goeldii seems to be relatively uniform. Proechimys canicollis shows relatively little variation throughout its range but probably is divisible into two or more subspecies.

The primitive Proechimys probably was large with a short tail, narrow aristiforms, strongly built skull, and five counterfolds in each molariform tooth. Primitiveness here is inferred from characters which now are of general occurrence in the whole group as opposed to those restricted in geographic occurrence.

It is a curious fact that in this genus, populations from small islands are more primitive than populations on the mainland. Apparently a small population restricted to a small island tends to revert to the primitive type. The homozygous condition will tend toward a generalized genotype and the disappearance of secondary biotypes. P. i. iheringi on the Island of São Sebastião averages larger, has thinner aristiforms, and a stronger skull than the same subspecies on the mainland, and the cheekteeth usually have two and three counterfolds. The same subspecies on the mainland has no more than two counterfolds. Proechimys semispinosus gorgonae and Proechimys semispinosus ignotus, living on Gorgona and San José islands, respectively, are both characterized by large size, short tails, strong and conspicuously ridged skulls, and cheekteeth frequently with four and five counterfolds. On the mainland, closely related subspecies, like P. s. panamensis, chiriquinus and gularis, far less frequently have four counterfolds in more than one or two teeth. More striking still is the population-sample of gularis from the island of Llunchi, in the Rio Napo, eastern Ecuador. In it there is a higher ratio of cheekteeth with four counterfolds than there is in the samples from the banks of the river.

The two insular forms, P. s. gorgonae and P. s. ignotus, referred to as primitive in the discussion above, have wide aristiforms, which is contrary to what would be expected in a primitive Proechimys. Supposing, however, as actually seems to be the fact, that narrowness of the aristiforms depends on an increased number of genes, we deduce that the population from the mainland, that gave rise to the populations of the islands, did not have all of the genes necessary to make the aristiforms narrow. In fact the subspecies known on the mainland, near the aforementioned islands, have wide aristiforms.

Another point which favors the idea that narrow aristiforms result from an increased number of genes is that, generally, the aristiforms are narrow in any species whose geographic range is extensive and relatively uniform.

Proechimys goeldii is the species which has the largest number of characters that are judged to be primitive, and it may be the oldest stock. P. semispinosus, P. longicaudatus and P. guyannensis may have been derived from an early splitting of the genus or they may have branched off the main stem at different times. P. canicollis, however, seems clearly to be an offshoot of P. guyannensis; canicollis shows greater resemblance to guyannensis than to any other species. P. g. vacillator is another close relative of P. guyannensis with the number of counterfolds almost as much reduced as in P. canicollis. Conceivably, vacillator is a full species, but the reduction in number of counterfolds in the teeth more probably expresses only one extreme of a gradient, as will be discussed below.

SUBSPECIFIC VARIATION IN THE SUBGENUS PROECHIMYS