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The dynamic, rapidly changing state of mammalian taxonomy, documented by an enormous literature, long hampered the compilation of a detailed, complete world checklist. It was only about a century ago that the first complete appraisal of all mammals of the world was produced by Trouessart (1898-99, 1904-05). A more modern compilation was provided when E. P. Walker and colleagues brought out, in 1964, the first edition of Mammals of the World. This compendium, now in its sixth edition (Nowak, 1999), is arranged systematically (with considerable supplementary natural history data) to the generic level, and later editions list the species in each genus in addition to furnishing an illustration of at least one member of each genus. V. E. Sokolov based his Systematics of Mammals, published in Russian (1973-79), on Walker's Mammals of the World. He provided a list of species with a brief summary of geographic distribution in each generic account. Corbett and Hill (1980, 1991) listed the species of the world, abbreviated distributions, common names, literature citations to major regional distributional works, and some additional revisionary works where appropriate. McKenna and Bell (1997) provided a complete phylogeny of mammals above the species level, including fossil as well as recent forms. That work provided a starting point for this edition of Mammal Species of the World, and deviations from their arrangement are noted in the comment sections of the accounts that follow. In the short time since the publication of McKenna and Bell (1997), an explosion of literature based on new techniques of molecular systematics has resulted in wholesale changes in our thinking about mammalian phylogeny. Those changes are reflected in the following pages, but this work is primarily a checklist at the species level, and higher-level relationships are used primarily to provide structure rather than to reflect phylogeny.

This volume, like previous editions (1982, 1993), will undoubtedly be used by many readers who are not systematic mammalogists. Do not be alarmed or disheartened by the debate over definition of species limits within many groups of mammals. Differences of opinion are aired in the comments sections in order to emphasize areas needing additional taxonomic study. Mammals are no worse off in this regard than other groups of animals, and in fact are probably better known than most, with the possible exception of birds. One recurring suggestion from users of previous editions spurred us to include common names in this edition. The publication of the first complete list of common names of mammal species of the world (Wilson and Cole, 2000) made this possible. Contributors to this edition used those names as a starting point, but were urged to adopt alternatives if there were compelling reasons to do so. As a result, this volume can be viewed as a second edition of Wilson and Cole (2000).

Process of Compilation

Knowledge of the systematics of mammals is distributed over an extensive assortment of works. The process of compiling and editing the information contained in this edition drew heavily on lessons learned from prior editions.

First Edition

The first edition (Honacki et al., 1982) evolved from three sources: a manuscript written by Kenneth E. Kinman, a preliminary list developed by the U.S. Fish and Wildlife Service, and the International Species Inventory (ISIS) List (Seal and Makey, 1974). A draft checklist developed from those sources was made available to the professional mammalogy community. Members of the American Society of Mammalogists were invited to contribute to the list at whatever level they wished. As a result, appropriate portions of the draft checklist were sent to 255 individuals. These reviewers were asked to provide the following information: author of the scientific name of the species, and citation; type locality; distribution; citations of revisions or reviews; important synonyms; and explanatory comments if necessary. One hundred fifty mammalogists provided reviews, covering 85 percent of the species in the list. Additional reviewers were eventually found for the remaining species. Compiling the various drafts included the necessity of comparing the various contributions and including information on which there was agreement. In cases where there was not agreement, errors may have been retained. To determine valid names of taxa in cases of disagreement, the most recent reviewer was followed. Some of these names have stood the test of time, and others have not.

The penultimate draft was forwarded to the Checklist Committee of the American Society of Mammalogists. This group (Robert L. Brownell, Jr., Alfred L. Gardner [Chair] Robert S. Hoffmann, Karl F. Koopman, Guy G. Musser, and Duane A. Schlitter) furnished the final review of the text and provided much useful additional information. The committee detected many problems resulting from the egalitarian effort, not all of which were satisfactorily resolved in the first edition.

Two categories of information were added after the review process. The protected status of affected taxa was listed, based on information from the Federal Register. The ISIS numbers for each species were listed, based on Seal and Makey (1974). In the second edition the protected status was updated and retained, but the ISIS numbers were not, due to their limited usefulness to most users.

Second Edition

The second edition was compiled and edited in a distinctly different fashion from the first. Although the original decision to consult many professional mammalogists was theoretically sensible, it introduced a practical information management dilemma for the first edition. The wealth of material and the challenge of uniting numerous different, frequently conflicting opinions caused weaknesses in the original volume. Nevertheless, the taxonomic hypotheses outlined in that volume provided the basis for the refinements apparent in the second edition. The second edition was envisioned as the second step in organizing a continuing taxonomic database for mammals.

Because comprehensive survey of the literature is now prohibitive for a single mammalogist (or perhaps because they don't make us like they used to), the labor of composing the second edition was partitioned among 20 authors, specialists on their respective taxa. Although some authors had been maintaining records for their groups of interest for some time, preparation of the second edition began in earnest in the fall of 1990, when electronic and paper copies of the appropriate portions of the original text were distributed to the members of the committee along with a mandate to produce an up-to-date revision. Most of 1991 was spent in the production of that updated text. The authors examined the literature on mammals, from the initial works to the most recent, and provided references published in many languages. As each portion was received from the authors, it was converted to an electronic database, edited, regenerated in word processing format, and returned to the authors for revisions. After a second round of editing, reviews were sought from other members of the community of systematic mammalogists. Although both editors and reviewers made suggestions to the authors, each section was the product of an individual author's scholarship, and represented that author's best hypothesis of the relationships of a particular group at that time.

The information leading to that edition was compiled with computer technology that permitted rapid manuscript revision and organization. This allowed automatic indexing of the many names used and standardization of literature citations, as well as considerably more consistency in citations, punctuation, format, and of spelling of place names. In spite of this, infelicities crept in, and the third printing of the second edition contained corrections of some typographical mistakes, misspellings, incorrect dates, and overlooked synonyms. Some literature citations were further standardized at that time.

Details on the 4,629 species of mammals covered by the second edition (up from 4,170 in the first edition) were the product of myriad former scholars united by a common curiosity about the diversity of mammals (Table 1). A major feature of the second edition, like the original, was that it identified gaps in our knowledge in need of further study, and served as a starting point for the third edition.

Third Edition

The third edition was prepared in much the same manner as the second, although the number of contributors was increased slightly. During the past decade, information on the systematic relationships of mammals has continued to increase. The advent of modern molecular techniques has allowed increasingly detailed comparisons of species limits and evolutionary relationships. The number of species represented in the current edition is 5,416, up from 4,629 in the second edition. Although most of this increase is due to taxonomic revision, a significant proportion is due to newly described species. In addition to currently recognized species, this edition contains 37,378 synonyms and full references to 9,373 scientific publications.

Fourth Edition

The fourth edition is currently being compiled by multiple authors from around the world. The database will be updated when revisions are complete. This edition is being edited by DeeAnn M. Reeder and Kristofer M. Helgen.

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