Read Ebook: Speciation in the Brazilian Spiny Rats by Moojen Jo O Hall E Raymond Eugene Raymond Editor

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PAGE INTRODUCTION 305

METHODS AND TERMINOLOGY 305

ACKNOWLEDGMENTS 308

PALEONTOLOGY 308

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

CONCLUSIONS 393

TABLE OF MEASUREMENTS 395

LITERATURE CITED 400

INTRODUCTION

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.

METHODS AND TERMINOLOGY

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.

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:

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.

The following measurements of the skull are used in the tables:

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.

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, S?o Paulo, Brazil. MCZ--Museum of Comparative Zoology at Harvard College. MN--Museu Nacional, Brazil. MZ--Museum of Zoology, University of Michigan. SEPFA--Servi?o 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 co?peration 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.

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 Zoology at the University of Kansas whose untiring aid and guidance has enabled me to terminate this study.

PALEONTOLOGY

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 point out that after a Cretaceous submersion of the central part of Brazil, there was a general uplift. The authors 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 eleva??o do continente prolongou-se at? o Pleistoceno."

Berry concluded, among other things, that there was a southward extension "in South America of equatorial floras in the lower Miocene," and that ... "east of the Andean Axis in the south temperate zone there was a normal mesophytic flora ... instead ... of present day large steppes."

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.

Lack of intergradation in any of the characters may be accepted as the criterion of full species. Where two populations occupying the same range 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.

SUBGENERIC VARIATION

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

The primary cause of the subgeneric differentiation is thought to have been geologic changes in the continental area. As already pointed out , 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.

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

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.

SUBSPECIFIC VARIATION IN THE SUBGENUS PROECHIMYS

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.

SUBSPECIFIC VARIATION IN THE SUBGENUS TRINOMYS

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.

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.

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