Speciation in the Brazilian Spiny Rats
(Genus Proechimys, Family Echimyidae)
BY
JOÃO MOOJEN
University of Kansas Publications
Museum of Natural History
Volume 1, No. 19, pp. 301-406, 140 figures in text
December 10, 1948
University of Kansas
LAWRENCE
1948
University of Kansas Publications, Museum of Natural History
Editors: E. Raymond Hall, Chairman, A. Byron Leonard,
Edward H. Taylor
Volume 1, No. 19, pp. 301-406, 1 plate, 140 figures in text
Published December 10, 1948
University of Kansas
Lawrence, Kansas
PRINTED BY
FERD VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1948
22-3343
Speciation in the Brazilian Spiny Rats
(Genus Proechimys, Family Echimyidae)
By
JOÃO MOOJEN
[CONTENTS]
| PAGE | |||
| Introduction | [305] | ||
| Methods and Terminology | [305] | ||
| Acknowledgments | [308] | ||
| Paleontology | [308] | ||
| Speciation | [311] | ||
| Subgeneric variation | [312] | ||
| Specific variation in the subgenus Proechimys | [314] | ||
| Subspecific variation in the subgenus Proechimys | [317] | ||
| Specific variation in the subgenus Trinomys | [320] | ||
| Subspecific variation in the subgenus Trinomys | [322] | ||
| Taxonomic Characters | [323] | ||
| Size and proportions of external parts | [323] | ||
| Pelage | [324] | ||
| Skull | [326] | ||
| Incisive foramen | [326] | ||
| Teeth | [327] | ||
| Habits | [330] | ||
| Changes with Age | [331] | ||
| Genus Proechimys | [333] | ||
| Artificial Key to Subgenera and Species | [334] | ||
| Subgenus Proechimys | [338] | ||
| Proechimys goeldii | [338] | ||
| Proechimys goeldii steerei | [338] | ||
| Proechimys goeldii goeldii | [340] | ||
| Proechimys semispinosus | [342] | ||
| Proechimys semispinosus liminalis | [343] | ||
| Proechimys semispinosus amphichoricus | [344] | ||
| Proechimys semispinosus kermiti | [345] | ||
| Proechimys longicaudatus | [346] | ||
| Proechimys semispinosus brevicauda | [349] | ||
| Proechimys semispinosus boimensis | [350] | ||
| Proechimys semispinosus longicaudatus | [351] | ||
| Proechimys semispinosus leucomystax | [352] | ||
| Proechimys semispinosus roberti | [353] | ||
| Proechimys guyannensis | [355] | ||
| Proechimys guyannensis villicauda | [355] | ||
| Proechimys guyannensis ribeiroi | [361] | ||
| Proechimys guyannensis hyleae | [361] | ||
| Proechimys guyannensis nesiotes | [363] | ||
| Proechimys guyannensis leioprimna | [364] | ||
| Proechimys guyannensis oris | [365] | ||
| Proechimys guyannensis arescens | [366] | ||
| Proechimys guyannensis riparum | [367] | ||
| Proechimys guyannensis arabupu | [369] | ||
| Subgenus Trinomys | [369] | ||
| Proechimys dimidiatus | [371] | ||
| Proechimys iheringi | [373] | ||
| Proechimys iheringi iheringi | [378] | ||
| Proechimys iheringi bonafidei | [378] | ||
| Proechimys iheringi gratiosus | [379] | ||
| Proechimys iheringi panema | [380] | ||
| Proechimys iheringi denigratus | [381] | ||
| Proechimys iheringi paratus | [382] | ||
| Proechimys setosus | [384] | ||
| Proechimys setosus setosus | [385] | ||
| Proechimys setosus elegans | [387] | ||
| Proechimys albispinus | [388] | ||
| Proechimys albispinus albispinus | [390] | ||
| Proechimys albispinus sertonius | [391] | ||
| Incerta Sedis | [392] | ||
| Proechimys myosuros | [392] | ||
| Conclusions | [393] | ||
| Table of Measurements | [395] | ||
| Literature Cited | [400] | ||
Fig. 1. Proechimys dimidiatus (Günther). Live female on left and male on right. × ½. From Tingua, Nova Iguassú, Rio de Janeiro, Brazil. Photographed in spring (August or September) of 1942 by author.
[INTRODUCTION]
The spiny-rats included in the genus Proechimys are common in almost every forest of South America above the Tropic of Capricorn, and in Central America northward to approximately 12° N, in Nicaragua. In size and proportions they are similar to the brown rat Rattus norvegicus but actually they belong to a very different suborder of rodents—the Hystricomorpha. The hystricomorphs are represented in South America by a large variety of animals, of which capybaras, agoutis and cavies are common representatives.
The pelage of the spiny-rats has a large number of flattened, spinelike hairs, especially on the back. The color ranges through different tints and shades of reddish-brown more or less evenly distributed on the upper parts; the underparts are usually pure white, sharply contrasting with the brown color above. The tail is bicolored, brown above and white below.
The spiny-rats live in forests of different types, generally in the proximity of water. Shelter is usually procured under boulders, stumps or masses of roots. The reproductive rate is low; on the average, there are only two young per litter and only two litters per year.
Sixty-odd names have been given to species and subspecies of Proechimys in the last hundred and fifty years and no serious revision of the taxonomy of the genus was undertaken in the last century. The purpose of the present work is to provide means of understanding species and subspecies within the genus and to describe the different kinds known to occur within the confines of Brazil.
[METHODS AND TERMINOLOGY]
Pelage.—It was found advisable to use a standardized nomenclature for hairs. The names here proposed are a choice of those used in the literature, with the suffix "form" as an element of uniformity. I feel that it would be advantageous if everyone adopted a similar universal system in mammalogy.
The names listed below are used as nouns and are considered as English versions which could easily be adapted to different languages. These names may be complemented with adjectives as needed. Examples are lanceolate aristiforms, spinous aristiforms, and woolly setiforms.
Aristiforms: The most conspicuously developed hairs in a three-layered pelage or the corresponding hairs in a simpler pelage. Names previously used for these hairs are: guard hair, leithaar and jarre.
Setiforms: Common to all species and most numerous throughout the pelage; second in conspicuousness, being the dominant hairs in the middle layer. Synonyms are: over hairs, grannenhaare and soies.
Villiforms: The smallest hairs in the three-layered pelage. Synonyms are: underfur, wollhaar and duvet.
Vibrissiforms: The vibrissae proper, or any typically sensory hair.
Teeth.—The tritubercular nomenclature was abandoned because of overwhelming difficulties; more research on the Hystricomorpha is certainly needed before the tritubercular nomenclature can be applied with confidence. The following names are used for features of the molariform teeth:
Main fold: The inner or lingual fold in the upper molariform teeth and outer or labial fold in the lower molariform teeth.
Counterfold: Any outer or labial fold in the upper or any inner or lingual fold in the lower molariform teeth.
For incisors [Thomas] (1921:141) is followed: opisthodont, orthodont and proodont depending on the angle between the exposed part of incisors and the ventral surface of the rostrum.
The capital letters P and M designate premolars and molars, respectively, of the upper jaws; lower case letters p and m designate corresponding teeth in the lower jaws.
Measurements.—Measurements of skins were used only when provided by the collector. The length of the hind-foot is intended to be always cum unguis, but in a few instances it is impossible to be sure whether the collector included the nail. Length of tail was used only when the tail seemed not to be mutilated. Ear measurements taken by collectors are scarce. In spite of the apparent usefulness of length of ear, it was found to be inadvisable to take the measurement on the dry skins.
The following measurements of the skull are used in the tables:
Greatest length: From the anteriormost part of the nasals to the posteriormost part of the supraoccipital.
Condylo-incisive length: From the anterior face of one incisor, at the alveolus, to the posteriormost part of the exoccipital condyle of the same side.
Zygomatic breadth: Maximum distance across zygomata in a plane perpendicular to longitudinal axis of the skull.
Length of nasals: Maximum length of one or both, whichever is the greater.
Interorbital constriction: Least width between the orbits on top of the skull.
Palatilar length: From the posterior face of an incisor, at the alveolus, to the nearest part of the posterior edge of the palatine bone.
Crown length of cheekteeth: From the anterior border of P4 to the posterior border of M3.
In the accounts of species, measurements of aristiforms and setiforms are used. The hairs measured were taken from the middorsal region and outer thighs, and the measurements are means.
All specimens of which measurements are here recorded, as for example in the tables, are fully adult; each specimen shows some wear on each of the four upper molariform teeth unless otherwise indicated.
Capitalized color terms are after [Ridgway] "Color Standards and Color Nomenclature," Washington, D. C., U. S. A., 1912. One setiform was taken from the animal and placed over the rectangles in [Ridgway's] charts and the examination made under a microscope with low (×7) magnification and natural light. This method was found to give the most satisfactory results.
The following abbreviations are used for names of institutions:
AMNH—American Museum of Natural History.
CNHM—Chicago Natural History Museum.
DZ—Departamento de Zoologia da Secretaria de Agricultura, São Paulo, Brazil.
MCZ—Museum of Comparative Zoology at Harvard College.
MN—Museu Nacional, Brazil.
MZ—Museum of Zoology, University of Michigan.
SEPFA—**Serviço de Estudos e Pesquisas sobre a Febre Amarela, Brazil.
USNM—United States National Museum.
UZM—Universitets Zoologiske Museum, Copenhagen.
[ACKNOWLEDGMENTS]
Approximately two thousand skins and skulls were assembled at the Museum of Natural History, University of Kansas, through the coöperation of the authorities in the various institutions of North America, Brazil and Denmark, as listed immediately above. This comprehensive material was used to obtain a more complete understanding of the group, and for the loan of these specimens I am extremely grateful to the authorities of each of the institutions.
First of all I acknowledge the encouragement given me in the Proechimys project by Heloisa Alberto Torres, Director of the Museu Nacional, Rio de Janeiro. I extend my thanks also to Stephen D. Durrant, of the University of Utah, for helpful corrections in the preparation of the manuscript; to Mrs. Virginia Cassell Unruh, for the preparation of the drawings of the skulls; to Miss Alice M. Bruce for assistance in drawing the maps; and to my daughter, Julieta, for help in assembling data and for typing.
Dr. Remington Kellogg, Curator of Mammals in the United States National Museum, and the late Dr. Wilfred H. [Osgood], formerly Curator Emeritus of the Department of Zoölogy in the Chicago Natural History Museum, generously permitted me to use their private lists of South American mammals. These lists contain much unpublished data, as for example, proof, in Kellogg's list, that Proechimys guyannensis (E. [Geoffroy] Saint-Hilaire, 1803) antedates P. cayennensis ([Desmarest], 1817). I register here my gratitude to both these zoölogists and acknowledge other critical assistance from Dr. Kellogg.
The John Simon Guggenheim Memorial Foundation awarded me a fellowship for which I am deeply grateful. This expression of the Foundation's interest in education and good neighborliness made possible the completion of the present paper.
Finally I desire to express my deepest gratitude to Professor E. Raymond Hall, Director of the Museum of Natural History and Chairman of the Department of Zoölogy at the University of Kansas whose untiring aid and guidance has enabled me to terminate this study.
[PALEONTOLOGY]
The only known, significant, fossil Proechimys comes from deposits in the limestone caves of Lagoa Santa, Minas Gerais, Brazil. These deposits, of Late Pleistocene or Recent age, were extensively studied by P. W. [Lund] and the results published in a series of French and Danish papers. F. [Ameghino] (1934:110) studied another fauna from a deposit of similar age in the cave of Iporanga, São Paulo, Brazil. Proechimys is recorded in his account under the inclusive specific name fuliginosus.
The molariform teeth of the fossil described by [Lund] (1841:pl. 21, fig. 14) shows its close relationship to the living form P. s. elegans ([Lund]) which still inhabits the same region. It belongs in the more specialized subgenus Trinomys which seems to have been derived from Proechimys. Trinomys has the main fold in the molars always greatly developed and the fold tends to set apart one lamina in the occlusal surface. The Lagoa Santa fossil, like some specimens of the living subspecies, has a small main fold in P4. However, the main fold is large in all upper molars and in the lower molariform teeth which are notably specialized in the extreme reduction of the number of counterfolds to only one.
One hypothesis concerning the evolution of the genus is that a more primitive group of Proechimys lived in all of the Central Plateau of Brazil in the Pleistocene Time. The climatic conditions at that time might have been such as to support large forests but, since the Pleistocene, these climatic conditions may have changed from humid to the present drier conditions, which support the dominant, savanna, floral climax. Actually the extinct fauna from the caves includes animals which have disappeared from the area and now live only in more humid areas, as for example Myocastor, which has shifted to the lowlands to the west and south.
Possibly climatic changes were responsible for the faunal shift from the region that is now a plateau in Central Brazil. This climatic change may have resulted from the gradual uplift of the eastern part of the continent. This uplift prevents part of the trade winds which come from the east from carrying the same amount of moisture inland as they did previously. In fact, the Andean revolution, even if it occurred as late as Late Tertiary, would have had no perceptible influence on the amount of water precipitated on the more eastern parts of the continent. [Oliveira] and [Leonardos] (1943:617) point out that after a Cretaceous submersion of the central part of Brazil, there was a general uplift. The authors (op. cit.:689) mention the presence of continental Cretaceous deposits in the Central Plateau of Brazil, in support of these changes, and state that "pelo menos em certas zonas do litoral a elevação do continente prolongou-se até o Pleistoceno."
[Berry] (1942:373) concluded, among other things, that there was a southward extension "in South America of equatorial floras in the lower Miocene," and (op. cit.:372) that ... "east of the Andean Axis in the south temperate zone there was a normal mesophytic flora ... instead ... of present day large steppes."
My idea is that a tropical forest still covered the Central Plateau of Brazil in (early?) Pleistocene times and that populations of Proechimys of a primitive type, similar to P. g. steerei, for example, lived in that extensive forest-climax. The gradual uplift of the plateau, however, gradually brought about drier conditions in this region. As a result a large cliseral change was initiated, which shifted the forest-climax to the more humid eastern escarpments and lowlands that were gradually being developed, while the savanna climax was being established on the plateau. Eventually the effect of the decreasing moisture was locally accentuated by the erosion of the sandstones ([Oliveira] and [Leonardos], 1943:690) in northeastern Brazil, thus depriving it of a natural reservoir of rain water. An arid belt was developed which now constitutes an efficient geographic barrier to the distribution of many kinds of animals.
One marginal species may have shifted eastward with the forest-climax to effect the Recent distribution. The eastern species became completely isolated from the main group, accumulated mutations, and evolved into the subgeneric type Trinomys. The generic trend that gave rise to Trinomys probably remained more stable as far as supraspecific changes are concerned. The lack of barriers in the distributional area of the original group favored the dispersal and submergence of mutations and, therefore, there was but little further supraspecific evolution. The speciation in both subgenera finally resulted from gradual differentiation of varying populations since they show combinations of the generic biotypes and possess few truly qualitative characters.
The cliseral changes in the Central Plateau, which developed the dry belt, a barrier, might explain the evolution of a few more supraspecific groups of mammals, as indicated by the presence of similar forms in the Amazonian region and in Southeastern Brazil. Among these Echimys and Phyllomys, in the same family with Proechimys, show differences that are parallel to those observed in Proechimys. One of these parallel changes is the increased lamination of the cheekteeth. Although Echimys, from the Amazonian region, has upper molariform teeth with the four laminae fused, Phyllomys has the four laminae completely separated.
None of the genera known from the Upper Oligocene and Miocene of Argentine deposits seems to be directly ancestral to Proechimys.
[SPECIATION]
The detection of differences of systematic worth between populations of animals, represented by skins and skulls, is a step preliminary to deducing the factors responsible for the differences. Ordinarily the factors which cause heritable differences have to do with geographic isolation and adaptation to ecological conditions. When differences in the structure of the animal are known, a person is led to speculate on the factors which could cause them. For one thing, does the observed degree of difference tend to isolate animals possessing the "new" character from the other animals? It would seem to me that the isolation once started by one of these differences tends to be accentuated with time and the difference itself thus then becomes a factor responsible for further differentiation.
Whether or not transition from one character to another occurs gradually, in its geographic expression, and thus whether or not intergradation occurs between two subspecies, can be ascertained by the analysis of a series of population-samples appropriately distributed geographically. If two characters of systematic worth are known to blend in one part of the geographic range of a subgenus, and if the same two characters are seen in two other populations, far removed geographically from each other and without any samples of annectent populations to provide actual evidence of intergradation, then such intergradation is to be inferred.
The available collections of Proechimys mostly were made haphazardly with the result that there are extensive areas from which no specimens as yet are available. Thus, actual proof of intergradation is often lacking in areas where it almost certainly occurs. In some extensive areas, however, many samples, from relatively regular intervals, have been available and they provide genuine proof of intergradation. These instances have served as a guide for estimating whether other samples should be considered to be full species or instead merely subspecies of the same species.
Lack of intergradation in any of the characters may be accepted as the criterion of full species. Where two populations occupying the same range (sympatric populations) show different qualitative characters, they almost certainly do not crossbreed. Furthermore the characters that distinguish such kinds of nonintergrading animals are likely to be considered as of full specific value when detected in far distant parts of the range of the subgenus.
In a genus that is widespread and continuously distributed, it is useful to know which characters always distinguish full species and which ones, sometimes or always, distinguish only subspecies, since in a population from a small island, there is, ordinarily, less individual variation than in a corresponding population from the mainland or a larger island; under certain circumstances a person might be tempted to give specific rank to the population when its characters actually are analogous to those separating subspecies elsewhere.
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.