EVOLUTIONARY SIGNIFICANCE OF ADAPTATIONS IN HETEROMYID RODENTS IN BAJA CALIFORNIA, MEXICO
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Authors Roth, Edward Lee, 1944-
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University Microfilms International 300 North Zeeb Road Ann Arbor, Michigan 48106 USA St. John's Road, Tyler's Green High Wycombe, Bucks, England HP10 8HR 77-6047 ROTH, Edward Lee, 1944- EVOLUTIONARY SIGNIFICANCE OF ADAPTATIONS IN HETEROMYTD RODENTS IN BAJA CALIFORNIA, MEXICO.
The University o£ Arizona, Ph.D., 1976 Zoology
Xerox University Microfilms, Ann Arbor, Michigan 48106 EVOLUTIONARY SIGNIFICANCE OF ADAPTATIONS IN HETEROMYID RODENTS IN BAJA CALIFORNIA, MEXICO
by Edward Lee Roth
A Dissertation Submitted to the Faculty of the DEPARTMENT OF ECOLOGY AND EVOLUTIONARY BIOLOGY In Partial Fulfillment of the Requirements For the Degree of DOCTOR OF PHILOSOPHY WITH A MAJOR IN ZOOLOGY In the Graduate College THE UNIVERSITY OF ARIZONA
19 7 6 THE UNIVERSITY OF ARIZONA
GRADUATE COLLEGE
I hereby recommend that this dissertation prepared under my direction by Edward Lee Roth entitled Evolutionary Significance of Adaptations In Heteromyld Rodents In Baja California, Mexico be accepted as fulfilling the dissertation requirement for the degree of Doctor of Philosophy
' lit Lwv- I £ a-u-Cj /r( Dissertation Director Date
As members of the Final Examination Committee, we certify that we have read this dissertation and agree that it may be presented for final defense.
'C~ /h ft /? 7 (, a J? /^7C /V /?7L Lui 'Ms^JLjJa^, /L-J. A- /?76 Final approval and acceptance of this dissertation is contingent on the candidate's adequate performance and defense thereof at the final oral examination. STATEMENT BY AUTHOR This dissertation has been submitted in partial ful fillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this dissertation are allow able without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manu script in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. SIGNED: ACKNOWLEDGMENTS I wish to thank Dr. E. Lendell Cockrum for his help ful advice and many hours of discussion throughout my graduate work at The University of Arizona. I am especially grateful for his assistance with the manuscript. I also thank Dr. Stephen M. Russell, Dr. Robert B. Chiasson, Dr. Charles Mason and Dr. Willard Van Asdall for reading the manuscript and offering many helpful suggestions. During the course of this investigation I came in contact with many citizens of the Republic of Mexico. In all cases they were most courteous and helpful. To these many people I extend a very sincere thanks. For allowing me to examine specimens in their care I would like to thank the following persons and their institu tions: Dr. Sydney Anderson, American Museum of Natural History; Dr. Charles 0. Handley and Dr. Don E. Wilson, U. S. National Museum; Dr. Emmet T. Hooper, University of Michigan Dr. Robert Hoffman, University of Kansas; and Dr. Joseph t Jehl, San Diego Society of Natural History. I would also like to thank Mario Luis Cossio, Direccion General de la Fauna Silvestre, for providing the necessary permits for collecting in the Republic of Mexico. iii iv Special appreciation is extended to Yar Petryszyn for his help and companionship during the many weeks spent in Baja California. His assistance was invaluable. This research was supported in part by a grant from the Theodore Roosevelt Memorial Fund of the American Museum of Natural History. Without this support the necessary travel to museums would have been difficult. Finally, I am especially grateful to my wife, Beth, for her support and encouragement throughout this study and for her valuable assistance in recording the data taken from hundreds of specimens. TABLE OF CONTENTS Page LIST OF TABLES vi LIST OF ILLUSTRATIONS vii ABSTRACT viii INTRODUCTION 1 METHODS AND MATERIALS . 4 DESCRIPTION OF BAJA CALIFORNIA AND ACCOUNTS OF SPECIES 6 Lower Colorado Valley 8 Central Gulf Coast 11 Magdalena Province 12 Vizcaino Province 13 San Pedro Martir Province 14 Giganta-Laguna Province 14 Perognathus arenarius Merriam 18 Perognathus baileyi Merriam 22 Perognathus spinatus Merriam 27 Perognathus formosus Merriam 31 Perognathus fallax "Merriam 35 Dipodomys merriami Mearns 38 Dipodomys" peninsularis (Merriam) 42 Dipodomyi" agilis Gambel 46 ANCESTRAL HETEROMYID MORPHOLOGY AND HABITATS ..... 50 DISCUSSION 53 CONCLUSIONS 70 APPENDIX A: SKIN AND SKULL MEASUREMENTS FOR SPECIMENS USED IN THIS STUDY 72 REFERENCES CITED 78 v LIST OF TABLES Table Page 1. Summary of some characteristics of the six phytogeographical provinces described in text . 10 2. Selected measurements and indices for Perognathus arenarius 21 3. Selected measurements and indices for Perognathus baileyi 25 4. Selected measurements and indices for Perognathus spinatus 30 5. Selected measurements and indices for Perognathus formosus 34 6. Selected measurements and indices for Perognathus fallax 37 7. Selected measurements and indices for Dipodomys merriami 41 8. Selected measurements and indices for Dipodomys peninsularis 45 9. Selected measurements and indices for Dipodomys agilis 48 10. Number of individuals and biomass of rodents for two localities in Baja California, Mexico . 59 11. Species diversity of heteromyid and cricetid rodents for selected areas in North America . . 62 12. Average number of species of heteromyidae and cricetidae present in the non-desert and desert areas listed in Table 11 63 13. Relative specialization of Perognathus and Dipodomys from Baja California, Mexico .... 68 vi LIST OF ILLUSTRATIONS Figure Page 1. Map of Baja California, Mexico 9 2. Map showing the distribution of Perognathus arenarius in Baja California 19 3. Map showing the distribution of Perognathus baileyi in Baja California 24 4. Map showing the distribution of Perognathus spinatus in Baja California 28 5. Map showing the distribution of Perognathus formosus in Baja California 32 6. Map showing the distribution of Perognathus fall ax in Baja California 36 7. Map showing the distribution of Dipodomys merriami in Baja California 39 8. Map showing the distribution of Dipodomys peninsularis in Baja California 43 9. Map showing the distribution of Dipodomys agilis in Baja California 47 \ vii ABSTRACT Rodents of the family heteromyidae primarily inhabit arid and semi-arid regions in the southwestern United States and northwestern Mexico. In the past, evolutionary relation ships within this family have been based on specializations of the auditory and locomotor systems. These specializations were thought to have arisen as a response to increased desertification during the Miocene and Pliocene which pro duced a more arid climate and a more sparse, open vegetation. Examinations of those features of the auditory and i locomotor systems thought to be adaptive in nature and thus to have evolutionary implications are presented for five species of Perognathus and three species of Dipodomys occur ring in Baja California, Mexico. Morphological features for 1296 specimens from 118 localities are quantified and com pared with the various degrees of desertification found in Baja California. In Baja California where there is an increase in openness of 20 to 30 percent, a greater development of the morphological specialization of the auditory bullae, hind feet, and tail is seen in heteromyid population. Further increase in openness (to 88 percent or more) does not necessarily result in additional specialization. viii ix Paleoecology of fossil faunas containing heteromyid rodents is summarized. This record indicates that hetero- myids with well developed "desert adaptations" lived during the Miocene, long before desert communities evolved. Grass land and open savannah or mesic forests seem to have been the dominant habitat of ancestral heteromyids. Pre-adaptation, and not in situ evolution, appears to have been the source of the morphological adaptations in the heteromyid genera Perognathus and Dipodomys that in the past have been attributed to increased desertification. INTRODUCTION Among North American rodents, members of the family heteromyidae appear to be best adapted to the arid and semi- arid conditions of the southwestern deserts. Within a given area of the North American desert there are generally sev eral species of pocket mice (Perognathus) and kangaroo rats (Dipodomys). Populations of these two genera are usually large, often making up most of the biomass of rodents of an area. It is generally accepted in the literature that Perognathus and Dipodomys show morphological features that have resulted from in situ evolution resulting from deserti fication, especially sparcity (openness) of vegetation. Further, the.currently understood phylogeny and expressed relationships within these genera have been based upon this assumption (Osgood 1900, Grinnell 1922, Setzer 1949, Lidicker 1960). Apparently no one has attempted to correlate such quantitative aspects of desert habitat as temperature, pre cipitation and openness with morphological features of heteromyids. Such a study might demonstrate whether in situ evolution has played any significant role in heteromyid 1 2 evolution. However, an examination of the fossil record suggests that heteromyids have been able to colonize deserts as the result of adaptations acquired in another environment. The following study utilizes the natural laboratory afforded by the various habitats in the Baja California por tion of the Sonoran Desert. This area is extensive (1400 km in north-south extent and 45-200 km in east-west extent) and varied physically and climatically. For example, average rainfall varies from 30 to 747 mm and average annual tempera tures range from 10 to 25°C with average monthly extremes of 5 and 34^0. Soil conditions range from large areas of bare rock (such as the volcanic hills near Puertocitos), to moving san dunes (e.g., near San Felipe and Guerrero Negro). Plant cover varies from three percent in the northeast to more than 90 percent in the Cape Region (Shreve 1951). The modern flora of this area has been a topic of intense investigation in recent years (Shreve and Wiggins 1964; Hastings, Turner and Warren 1972), and many details of the flora and vegetation types are now known. Some of this information, for example plant cover, while perhaps quanti tatively imprecise is undoubtedly correct qualitatively. The climate of Baja California has also received consider able attention. Precipitation regimes have been described in detail by Hastings and Turner (1965) and other climatic 3 data have been published (Hastings 1964, Hastings and Hum phrey 1969) for 109 stations in Baja. In this region a wide variety of heteromyid rodents occur. As currently understood, two genera (Dipodomys and Perognathus), 16 species and 68 named subspecies of hetero myid rodents have been recorded. The present study is an attempt to determine whether the morphological variability to be seen in heteromyid rodents has evolved in response to increased desertification. METHODS AND MATERIALS In the present study a total of 1296 specimens of Perognathus and Dipodomys were examined from 118 localities in Baja California. These include representatives of all currently recognized heteromyid species in the region. The following information was recorded from the specimen tag for every specimen: collection, number, locality, sex, total length, tail length, hind foot length, and, when available, weight. Body length was obtained by subtracting the tail length from the total length. Seven cranial measure ments were taken on mature specimens: total length of skull, basal length, nasal length, maxillary breadth, breadth of bullae, depth of cranium, and depth of skull at bullae. All skull measurements were taken by the writer with the same pair of dial calipers, measuring accurately to 0.1 mm and were read to the nearest 0.1 mm. These measurements are recorded in Appendix A. Since no specimens were of known'age, maturity was arbitrarily defined in morphological terms as follows: com plete permanent dentition; all cheek teeth showing some wear; auditory bullae smooth and non-porous; adult pelage and coloration. Only such specimens judged mature by all 4 5 criteria were used in the analysis of variation and adaptive features. The following indices were calculated for each spe cies of Perognathus and Dipodomys within each phytogeographic subdivision of Baja California: Bullar Index (= breadth of bullae/greatest length of skull), Pedal Index (= hind foot length/body length), and Tail Index (= tail length/body length). Total specialization was then calculated by summing the three indices. Specimens utilized in this study are housed in the scientific collections of a number of institutions (United States National Museum, American Museum of Natural History, The University of Kansas, The University of Michigan, The University of Arizona, and the San Diego Society of Natural History). In addition, during the years 1971-1976 I spent a total of 27 weeks in the Sonoran Desert of Baja California collecting data on habitat variation and specimens from localities from which adequate samples were not available. DESCRIPTION OF BAJA CALIFORNIA AND ACCOUNTS OF SPECIES Nearly 1400 km long, 32° 30' N latitude to 22° 50' N latitude, and from 45 to 200 km wide, the peninsula of Baja California comprises 143,600 sq km. The geologic history of the Gulf of California and Baja California has been the sub ject of intensive research interest in recent years. Work by Larson, Menard and Smith (1968), Moore and Buffington (1968), and Anderson (1971) indicate a rather recent origin resulting from continental drift. In this process Baja California has moved 200 km west and 270 km north. This movement occurred primarily during the Pliocene. Prior to this time the peninsula lay adjacent to the mainland, extend ing as far south as Puerto Vallarta, Jalisco. Today, most of Baja is covered with desert thomscrub communities de rived from thomscrub of the Madrotertiary Geoflora (Axelrod 1950). Axelrod (1958) has shown that the major evolutionary events in the derivation of both the plant and animal com munities occurred during the Pliocene, largely before the present extent of the geographic and climatic configuration of the Gulf of California was realized. The topography of Baja California includes rugged mountain chains, broad plains and deep valleys. The 6 7 mountains are massed along the eastern coast, a continuation of the mountain system of western North America. Most ranges are less than 1500 m in elevation, but both in the extreme north and in the extreme south they are higher, reaching elevations of 3100 m and 2100 m respectively. Western Baja California is a broad, relatively low, plain with only a few low mountain ranges. Average annual precipitation for the entire peninsula is 153 mm ranging from 30 mm at Bataques in the north to 747 mm at Sierra de la Laguna in the south. Mean annual pre cipitation for an area as arid as Baja California can be misleading due to the variability of precipitation from year to year. For example, in 1953 Batiques recorded no rainfall; in 1954, 3 mm; and in 1955, 84 mm. In 1958 the Sierra de la Laguna station recorded 1407 mm of precipita tion, almost twice the annual mean. For a detailed account of precipitation regimes in Baja California, see Hastings and Turner (1965). Shreve and Wiggins (1964) and Nelson (1922) conducted detailed geographic studies on the plants and animals of this region. Their accounts contain a wealth of descriptive in formation on the natural history of Baja California. Their observations along with mine made during the years 1971-1975, together with climatic data from Hastings and Turner (1965) 8 and Hastings and Humphrey (1969) are combined in the follow ing descriptive synopsis. The Sonoran Desert is one of the four subdivision of the North American Desert (Shreve 1942). Approximately 100,000 sq km (70 percent) of Baja California are part of the Sonoran Desert. Based on characteristic vegetation, this desert was subdivided into seven phytogeographic divisions by Shreve (1951): Lower Colorado Valley, Arizona Upland, Plains of Sonora, Foothills of Sonora, Central Gulf Coast, Vizcaino Region and Magdalena Region. Four of these (Lower Colorado Valley, Central Gulf Coast, Vizcaino and Magdalena Regions) occur in Baja California. Hastings and Turner (1965) named the two non-desert areas of Baja California: San Pedro Martir (northern) and Giganta-Laguna (southern). The bound aries of- these six phytogeographical units ("provinces") are shown in Figure 1. The climate of each is distinctive (see Table 1, adapted from Hastings and Turner 1965). Nelson (1922) recognized five "faunal districts" based on indigenous birds and mammals. These roughly coincide with the phytogeographic units as shown in Figure 1. Brief de scriptions of the phytogeographical subdivision of Baja California follow. Lower Colorado Valley In Baja California this province occupies the coastal region from Bahia de los Angeles north to the International 9 \<1 3 *0 O c no o Q(fl w / A B Figure 1. Map of Baja California, Mexico. -- (A) Phyto- geographic subdivisions of Baja California (after Shreve 1951, Hastings and Turner 1965). (B) Faunal Districts of Baja California (after Nelson 1922). 10 Table 1. Summary of some characteristics of the six phyto- geographical provinces, described in text. -- Pre cipitation is expressed in average mean annual in millimeters. Temperature (mean annual) is in de grees centigrade. Climatological data from Hastings and Turner (1965) and Hastings and Humphrey (1969). Plant coverage from Shreve and Wiggins (1964). Province Precipitation Temperature % Plant Cover Lower Colorado Valley 61 22 3-12 Central Gulf Coast 168 24 10-20 Magdalena 125 22 40-50 Vizcaino 105 19 35-60 San Pedro Martir 178 20 60-70 Giganta- Laguna 296 22 60-80 Boundary. It is the most arid province, averaging only 61 mm in total annual precipitation with no season of relatively reliable precipitation from year to year. This area coin cides with the Colorado Desert Faunal District of Nelson (1922). In Baja California the Lower Colorado Valley Pro vince is an area of volcanic hills, bajadas, and plains. Few streamways traverse the area., most drainageways being reticulate in nature. Plants are slightly larger and more abundant along these drainages than in adjacent areas. Be tween Puertocitos and Bahia de los Angeles a series of low volcanic hills extend up to the Gulf of California, with small areas of shallow soils intervening. For the most part surfaces are bare rock with isolated small areas of poorly developed soils. The vegetation is open and simple. Plant cover is the lowest in the Sonoran Desert, averaging only 3 to 12 percent. To the west, this area is bounded by the high mountains of the San Pedro Martir Province and to the south by the relative absence of frost. Central Gulf Coast This province is situated along the Gulf of Cali fornia in a narrow strip from Bahia de los Angeles to just south of La Paz, and from Bahia de las Palmas to just north west of San Jose del Cabo. These two regions of the Central Gulf Province in Baja California are separated by a portion of the Giganta-Laguna Province. Nelson did not distinguish 12 this region as a faunal district, including it as parts of his Vizcaino Desert and Cape Faunal Districts. Precipita tion is the highest of the four desert provinces, averaging 168 mm per year with relatively reliable rainfall only in stammer (June, July and August). The area consists of rugged volcanic hills and low mountains that provide little suit able habitat for plants. The soils are thin and rocky and steep gradients result in rapid running and deeply cut streamways. The streamways broaden as they reach sea level, crossing the narrow coastal lains as a series of small anastomosing streams. Despite the significantly higher rainfall, the substrate characteristics just described re sult in low vegetation cover. Plant cover only slightly exceeds that of the Lower Colorado Valley, averaging 10-20 percent north of La Paz and 10-15 percent south of there. Magdalena Province The Magdalena Province is the southernmost province of the Sonoran Desert, and is entirely contained in Nelson's Cape Faunal District. The northern part contains extensive malpais and volcanic mesas. Throughout the soil is thin with only scattered areas of deeper soils. The southern portion is dominated by the Magdalena Plain, a very uniform area 180 km long having numerous playas and very few stream- ways. Extensive fogs occur in all months. Near the coast, lichens cover most of the perennial vegetation. Rainfall 13 averages 125 mm per year and reliable precipitation occurs only in the fall (September, October and November). Plant cover averages 40-50 percent but reaches 80 percent in a few isolated locations. The Province is bounded on the east by the Sierra de la Giganta and merges with the Vizcaino Desert to the north and the thornscrub of the Cape region to the south. Vizcaino Province This constitutes the Pacific drainage of the penin sula from the Sierra San Pedro Martir to La Purisma near latitude 26° N. With the exception of a narrow coastal strip between Bahia de los Angeles and Santa Rosalia, it corresponds with the Vizcaino Desert Faunal District of Nelson. Its eastern portion is the mountainous central divide of Baja California where some peaks exceed 1500 m in elevation. West of this divide stretches the Vizcaino Desert, a broad monotonous plain of alluvial deposits. Halophytes lightly cover large areas of the Vizcaino Desert. Plant cover averages 35-60 percent outside the desert and only 20-30 percent in it. Northern coastal parts are fre quently foggy. There Tillandsia (ball moss) covers much of the vegetation. The province receives an average of 105 mm of rainfall and is reliable only in winter (December, January and February). As described by Shreve (1936) the Vizcaino 14 Province merges with the chaparral of the San Pedro Martir Province to the north. San Pedro Martir Province Northwestern Baja California including both the high mountains and the coastal chaparral is in this province. Plant cover ranges from 60 to 70 percent and the average annual rainfall is 178 mm with reliable rainfall in winter and summer. Even though deserts have been arbitrarily de fined (see Odum 1959) as regions with less than 250 mm (ten inches) of average annual rainfall the entire area is non- desert. Moderate temperatures and suitable soils result in more effective rainfall use by plants. Nelson (1922) defined two faunal districts in this area: the San Pedro Martir District in the higher elevations and the San Diegan District to the west. Giganta-Laguna Province Also non-desert, this province includes the Cape region as well as a tongue that extends north along the Sierra de la Giganta to latitude 26° N. This mountain range and the Sierra Victoria are by far the dominating features of this province. The area is almost entirely thorn forest with an average plant cover of 60 to 80 percent in the lower foothills and lowlands to the west of the Sierra Victoria. Locally cover is even greater. Tall shrubs and small trees 15 are abundant, forming an irregular and somewhat open canopy. Here is the highest rainfall in Baja California, averaging 296 mm per year and with reliable rainfall in both summer and fall. The Cape Faunal District of Nelson includes that part of the Central Gulf Coast Province south of Santa Rosalia, the Magdalena Province and" all of the Giganta- Laguna Province. This faunal district contains two distinct parts: a northern one containing the coastal Magdalena Plain, the lava plateau to the east and the Sierra de la Giganta as well as a southern Cape region. These two parts correspond with the Magdalena and the Giganta-Laguna Provinces. Heteromyid rodents are restricted to the Western Hemisphere and appear to have evolved from Sciuromorph stock during the early Oligocene. They are most closely related to geomyids (Hill 1937) and Shotwell (1967) considered the geomyidae to be a subfamily of the heteromyidae. In the present work the heteromyidae are considered to be a family distinct from the geomyidae. Five living genera are currently recognized as being in the family Heteromyidae: two of these, Liomys (five species) and Heteromys (ten species), are tropical and sub tropical in distribution, and the remainder, Microdipodops (two species), Perognathus (27 species), and Dipodomys 16 (21 species) are generally restricted to arid and semi-arid regions. Within the family Heteromyidae there can be little doubt that Dipodomys and Perognathus are highly specialized forms. However, the question as to hox? these specializations were acquired has never been fully examined. Functional and structural modifications of these two genera generally indi cated by past authors as having evolved in response to an arid desert environment include: the vertebral column (Hatt 1932), locomotor apparatus (Howell 1932, Bartholomew and Caswell 1951, Bartholomew and Cary 1954), middle ear ap paratus (Webster 1961, 1962), excretory system (Schmidt- Nielsen and Schmidt-Nielsen 1951, 1952), and ability to with stand high body temperatures (Dawson 1955). Many of the above studies were concerned only with kangaroo rats, Dipodomys. However, pocket mice, Perognathus, have many similar modifications, but usually developed to a lesser degree. Only those adaptive features of the verte bral column, locomotor apparatus and middle ear are consid ered here in detail. It is generally accepted in the literature that these modifications are adaptive in nature and that all three are responses to the same parameter, openness of the environment. Some have indicated that the increased susceptibility to predation resulting as a conse quence of this parameter has been the selecting force of evolution. 17 The following analysis of adaptations of extant Baja California heteromyids indicates the degree of specialization (morphological features generally assumed to be related to openness of habitat) within each of the various levels of desertification evident in Baja California. At present nine species of Perognathus and seven species of Dipodomys are known to occur on the peninsula of Baja California. Of these 16 species, only four--Perognathus baileyi, P. arenarius, P. spinatus, and Dipodomys merriami-- inhabit all six of the phytogeographic provinces described above. Seven—three Perognathus and four Dipodomys--occur only in one province, generally the San Pedro Martir or Lower Colorado Valley. For each of these species the following information is summarized: current geographic distribution, ecological data, relative degree of specialization, actual variation, and a list of localities from which specimens were examined. For the purpose of analyzing morphological variation and attempting to correlate this with environmental factors, collecting localities for each species were separated accord ing to the provinces in which they occurred. Specimens within a given province were pooled and indices of speciali zation were computed as described earlier. 18 In Tables 1 through 9, provinces are listed in the left-hand column according to increasing plant cover. A slightly different order would result if temperature or pre cipitation had been used (see Table 1). Perognathus arenarius Merriam With the exception of Perognathus longimembris, P. arenarius is the smallest pocket mouse in Baja California. This species is found throughout Baja except for the higher mountains in the north and in the Cape region (see Figure 2). This mouse seems to prefer areas where the substrate is loose sand or light gravel, and shuns those areas with a rocky- surface. P. arenarius is locally abundant in areas of dense vegetation as well as very open areas such as dunes. Lack of suitable sandy habitat appears to restrict its distribution locally. As in most other species of Perognathus examines, P. arenarius shows little actual variation between provinces in those features thought to be. adaptations to desert conditions. According to the morphological parameters outlined above, the most specialized populations P. arenarius inhabit the Magdalena Province and the most generalized are found in the Giganta-Laguna, the area having the greatest plant cover and highest annual precipitation. The next most generalized population of P. arenarius is found in the lower Colorado 19 118 114 33 - -32 ARIZONA 31 - - 30 SONORA 29 - -28 27- — 26 spacimans axamincd 23 - - 24 milts 90 100 > 90 100 Mill 1 km 117 113 113 Figure 2. Map showing the distribution of Perognathus arenarius in Baja California. -- See text and Figure 1 for description of provinces. 20 Valley Province—an area with only 3 to 12 percent cover and receiving an average of 61 mm of precipitation annually. The extent to which the bullae, hind foot and tail have developed in response to plant cover and other desert conditions appears to be random in P. arenarius. There is no correlation or pattern evident to support the idea that different populations of P. arenarius are either more or less adapted than others to a particular environment. .This data is summarized in Table 2 and Appendix A. Specimens examined. Total, 400, distributed as follows: Lower Colorado Valley (Total 78): Bahia de los Angeles, 9 (1, AM; 8, SDSNH); 12 mi N Puertocitos, 11 (UA); San Felipe, 42 (SDSNH); Gaskills Tank near Laguna Salada, 11 (SDSNH); De Mara's Well, Laguna Salada, 5 (SDSNH). Central Gulf Coast (Total 42): 1 mi E Rancho la Junitas, 1 (SDSNH); La Paz, 12 (BS); San Bruno, 8 (BS); San Francisquito Bay, 3 (1, AM; 2, SDSNH); 7 mi W San Francisquito Bay, 1 (SDSNH); Barril, 17 (SDSNH). Magdalena (Total 91): 20 mi W San Ignacio, 19 (UA); i San Ignacio, 8 (3, BS; 5, SDSNH); San Jorge, 34 (1, KU; 10, BS; 11, SDSNH; 12, UA); 9 mi S El Refugio, 18 (SDSNH); Santo Domingo, 21 (SDSNH). Table 2. Selected measurements and indices for Perognathus arenarius. -- See section on Methods and Materials for definitions of terms used. Indices are computed in such a manner that lower numbers are judged to be the least specialized. All measurements are in millimeters. N Cranial Bullar Hind Pedal Tail Tail Index of Area Breadth Index Foot Index Length Index Specialization Lower Colorado Valley 78 12.3 .52 21.8 .30 89.6 1.23 2.04 Central Gulf Coast 42 12.3 .52 21.9 .31 88.6 1.27 2.10 Magdalena 91 12.4 .54 21.4 .31 87.3 1.27 2.12 Vizcaino 89 12.2 .53 20.9 .30 87.5 1.26 2.08 San Pedro Martir 67 11.9 .53 20.5 .30 84.6 1.25 2.08 Giganta- Laguna 33 12.1 .52 21.1 .31 84.0 1.21 2.03 22 Vizcaino (Total 89): 27 mi SE Guerrero Negro, 1 (UA); San Bartolome Bay, 1 (AM); Turtle Bay, 2 (AM); Santo Domingo Landing, 15 (SDSNH); 7 mi N Santa Catarina, 4 (SDSNH); Santa Catarina Landing, 2 (SDSNH); 1.5 mi N Las Arrastras, 1 (UA); 25 mi N Punta Prieta, 33 (SDSNH); San Andres, 2 (SDSNH); 12 mi E Arco, 9 (SDSNH); La Lomita Maria, 4 (SDSNH); Mesquital, 1 (SDSNH); Rancho Ramona, 13 mi SW San Agustin, 4 (SDSNH); San Agustin, 8 (SDSNH); 13 mi E Punta Prieta, 2 (SDSNH). San Pedro Martir (Total 67): San Pedro Martir Moun tains, 1 (BS); Esperanza Canyon, 1 (BS); 2 mi N San Ramon, 1 (SDSNH); 2 mi W San Ramon, 1 (SDSNH); Mouth San Juan Dios Canyon, 2 (SDSNH); San Quintin, 15 (2 BS; 13 SDSNH); 5 mi E San Quintin, 1 (SDSNH); 10 mi E San Quintin, 5 (SDSNH); San Simon River, San Quintin, 9 (BS); N end San Quintin Plain, 9 (SDSNH); N side San Quintin Bay, 4 (SDSNH); Santa Maria, 14 (SDSNH); Santo Domingo, 4 (SDSNH). Giganta Laguna (Total 33): Tree Pachitas, 4 (BS); 10 mi S Todos Santos, 1 (BS); Pescadero, 4 (BS); 4 mi SE Migrino, 2 (UA); 5 mi N Cunano, 21 (UA). Perognathus baileyi Merriam Perognathus baileyi, the largest pocket mouse in Baja California, is found in suitable habitat throughout most of the peninsula. It is not known from the northwest chapparal 23 region, that part of the Vizcaino Desert west of 113° W longitude, nor in the eastern Cape south of the Tropic of Cancer (Figure 3). In North America Perognathus baileyi has a distribution that roughly coincides with the boundary of the Sonoran Desert. P. baileyi seems to tolerate a wider range of habitats and substrates than either P. arenarius or P. spinatus which ar6 the other wide ranging species of'Perognathus in Baja California. Bailey's pocket mouse is most often found in open gravelly or stony areas and also inhabits those areas where rocks cover a considerable portion of the surface. They are not present if the rocky cover is continuous. Dense brushy areas are generally avoided but scattered trees and small bushes are tolerated. The pattern cf morphological specialization in P. baileyi is not the same as in P. arenarius. The most spe cialized P. baileyi populations are found in the Lower Colorado Valley Province and the least specialized in the Giganta-Laguna Province. P. baileyi from the San Pedro Martir Province are almost as specialized as those from the Lower Colorado Valley and populations in the Magdalena Province are as generalized as those in the Giganta-Laguna Other than the extremes of specialization, there is no con sistent pattern of adaptation in P. baileyi (Table 3) to either plant cover or aridity of the environment. 24 118 114 112 33 32 ARIZONA 30 • •• SONORA 29 28 '27 26 • specimens examinsd 29 24 miles 50 100 •• km 117 IIS 113 Figure 3. Map showing the distribution of Perognathus baileyi in Baja California. -- See text and Figure 1 for description of provinces. Table 3. Selected measurements and indices for Perognathus baileyi. -- See sec tion on Methods and Materials for definitions of terms used. Indices are computed in such a manner that lower numbers are judged to be the least specialized. All measurements are in millimeters. Index of Area Cranial Bullar Hind Pedal Tail Tail N Breadth Index Foot Index Length Index Specialization Lower Colorado Valley 50 14.9 .53 24.9 .29 112.5 1.31 2.13 Central Gulf Coast 40 15.2 .54 25.0 .28 109.7 1.25 2.07 Magdalena 21 15.2 .53 25.1 .27 115.4 1.24 2.04 Vizcaino 53 15.3 .54 25.0 .28 110.2 1.24 2.06 San Pedro Martir 7 15.2 .54 23.5 .27 113.0 1.30 2.11 Giganga- Laguna 42 14.8 .52 25.2 .29 108.2 1.23 2.04 26 Morphological data for P. baileyi is summarized in Appendix A. Specimens examined. Total, 213, distributed as follows: Lower Colorado Valley (Total 50): Gaskills Tank, Laguna Salada, 24 (SDSNH); Signal Mountain, 1 (SDSNH); San Felipe, 19 (SDSNH); 28 mi S Puertocitos, 5 (UA); 12 mi N Puertocitos, 1 (UA). Central Gulf Coast (Total 40): Pichilinque Bay, 3 (AM); Concepcion Bay, 1 (AM); San Bruno, 1 (BS); 7 mi SE Muelge, 6 (UA); 2 mi N Las Cruces, 1 (UA); 10 mi W Santa Rosalia, 1 (UA); 3 mi W Santa Aguida, 15 (UA); San Francisquito, 1 (BS); 7 mi W San Francisquito Bay, 4 (SDSNH); Barril, 7 (SDSNH). Magdalena (Total 21): San Jorge, 10 (1 BS; 9 UA); Matanciti, 1 (BS); San Ignacio, 4 (SDSNH); 20 mi W San Ignacio, 6 (UA). Vizcaino (Total 53): Comondu, 1 (BS); Calamuhue, 4 (BS); Santa Catarina Landing, 3 (1 SDSNH; 2 US); 4 mi W Santa Catarina Landing,5 (SDSNH); 8 mi W Santa Catarina Landing, 7 (SDSNH); Onyx, 2 (SDSNH); 6 mi N Punta Prieta, 2 (UA); 8 mi SW El Marmolito, 1 (UA); 1.5 mi N Las Arrastras, 7 (UA); San Agustin, 5 (SDSNH); San Fernando, 15 (1 UA; 14 SDSNH); 10 mi E El Rosario, 1 (SDSNH). 27 San Pedro Martir (Total 7): 3 mi E Ajuaita, 1 (SDSNH); San Quintin, 6 (UA). Giganta-Laguna (Total 42): Tres Pachitas, 36 mi S La Paz, 4 (BS) ; El Sauz, E Base Sierra de la Giganta, 1 (BS); 4 mi NE San Antonio, 2 (UA); 5 mi N Cunano, 32 (UA) ; 6 mi S La Paz, 3 (UA). Perognathus spinatus Merriam Perognathus spinatus is a medium sized pocket mouse that is almost entirely restricted to rocky situations. The spiny pocket mouse is found throughout Baja California except for the sandy Vizcaino Desert and the Magdalena Plain. P. spinatus is known from Las Cabras in the San Pedro Martir Province (Huey 1964) but I have not examined specimens from this province. P. spinatus ranges into southern California and extreme southwestern Arizona (Turkowski and Lewis 1974). The distribution of P. spinatus in Baja California is shown in Figure 4. This pocket mouse prefers habitats where rocks form a continuous or nearly continuous surface and is seldom found in other situations. An indication of its fondness for rocks is the fact that it is the only Perognathus that in habits most of the many rocky islands in the Gulf of Cali fornia (Hall and Kelson 1959). In the Lower Colorado Valley this mouse was found in rocky situations where the only plant cover was exceedingly sparse and composed almost 28 118 114 112 33 •• 32 ARIZONA 30 SONORA 29 28 27 26 • spaeimans «xamin«d 25 24 tnilu 90 100 0 SO 100 Imil J km 117 IIS 113 Figure 4. Map showing the distribution of Perognathus spinatus in Baja California. -- See text and Figure 1 for description of provinces. 29 entirely of well-spaced Encelia. In the Cape region P. spinatus was collected in an almost closed canopy, semi- tropical forest. The preference for rocky habitat probably makes this species less dependent on plant cover for pro tection from predators. The pattern of specialization for Pspinatus is very similar to that for P. arenarius. The most specialized populations are in the Magdalena Province and the most generalized in the Giganta-Laguna, Inspection of Table 4 will show that, as in P. arenarius and P. baileyi, there is no consistent correlation between morphological adaptation and the environment. Morphological parameters used for P; spinatus are summarized in Appendix A. Specimens examined. Total, 183, distributed as follows; Lower Colorado Valley (Total 17): 12 mi N Puertocitos f 3 CUA); 28 mi S Puertocitos, 1 (UA); Bahia de los Angeles, 5 (UMMZ); Base of Cocopah Mountains, 4 CBS); E Base of Concopah Mountains, 3 (BS); La Provedincea Can yon, E Base of San Pedro Martir Mountains, 1 (BS) , Central Gulf Coast CTotal 60): Los Frailes, 3 (AM); 3 mi W Santa Aguida, 11 (UA); 7 mi SE Mulege, 27 CUA); Mulege, 1 (AM); Concepcion Bay, 1 CAM); Las Cruces, 5 Table 4. Selected measurements and indices for Perognathus spinatus. See sec tion on Methods and Materials for definitions of terms used. Indices are computed in such a manner that lower numbers are judged to be the least specialized. All measurements are in millimeters. j. Cranial Bullar Hind Pedal Tail Tail Index of Breadth Index Foot Index Length Index Specialization Lower Colorado Valley 17 12.6 .51 21.7 .29 103.4 1.38 2.18 Central Gulf Coast 60 12.6 .51 24.2 .30 111.3 1.40 2.21 Magdalena 20 12.6 .51 22.3 .30 106.6 1.42 2.23 Vizcaino 8 12.6 .51 20.4 .28 99.4 1.37 2.16 Giganta- Laguna 78 13.1 .51 23.6 .29 104.4 1.30 2.10 31 (UMMZ); Agua Verde, 1 (AM) ; Pichilinque Bay, 3 (AM); 2 mi N Las Cruces, 1 (UA); San Francisquito, 7 (BS). Magdalena (Total 20): San Ignacio, 9 (BS); 20 mi W San Ignacio, 8 (UA) ; San Jorge, 3 (UA). Vizcaino (Total 8): 1.5 mi N Las Arrastras, 5 (UA); Calamahue, 1 (BS) ; Poso Altamirano, 1 (BS); 12.5 mi SW Punta Prieta, 1 (UA). Giganta-Laguna (Total 78): Santa Rosa, 2 (UA); Cabo San Lucas, 8 (AM) ; 7 mi N Cabo San Lucas, 6 (UA); Santa Anita, 1 (AM); 4 mi E Santa Anita, 6 (UA) ; San Jose del Cabo, 4 (AM); San Bernardo Mountains, 1 (AM); Miraflores, 4 (AM); Migrino, 1 (UA) ; 4 mi SE Migrino, 12 (IJA) ; 4 mi NE San Antonio, 20 (UA); 6 mi S La Paz, 1 (UA); 7 mi S Bahia de las Palmas, 5 (UA); 3 mi SE San Pedro, 2 (UA); 5 mi N Cunano, 5 (UA). Perognathus formosus Merriam The long-tailed pocket mouse has a limited distribu tion in Baja California being found east of the central divide and north of Concepcion Bay (Figure 5). It occurs in three of the six phytogeographic provinces. P. formosus has a much wider distribution in the United States, primarily west and north of the Colorado River in California, Nevada, Utah, and Arizona (Hall and Kelson 1959). Perognathus formosus seems to prefer the same type of habitat as P. baileyi. It was most often found on well 32 118 114 112 33 32 ARIZONA 30 SONORA 29 28 '27 26 • spacimtna axaminad 25 24 90 100 km 117 IIS 113 Figure 5. Map showing the distribution of Perognathus formosus in Baja California. — See text and Figure 1 for description of provinces. 33 drained gravelly or stony slopes with sparse vegetation and s was not taken in sandy .washes nor on flats, the preferred habitat of P. arenarius. Near Mulege P. formosus was taken in the same rocky areas as P. spinatus. As can be seen in Table 5, the pattern of specializa tion for P. formosus correlates well with openness of the environment although Central Gulf Coast and Vizcaino popula tions are almost equally specialized. The greatest variation in P. formosus between provinces is in the relative length of the tail. Other than tail length, actual and relative variation between provinces is about the same for this spe cies as in other Perognathus examined (Table 5 and Appen dix A). Specimens examined. Total, 37, distributed as follows: Lower Colorado Valley (Total 12): San Felipe, 2 (BS); 12 mi N Puertocitos, 10 (UA). Central Gulf Coast (Total 17): 7 mi SE Mulege, 12 (UA); Barril, 4 (SDSNH); 7 mi W San Francisquito Bay, 1 (SDSNH). Vizcaino (Total 8): 3 mi W El Marmol, 3 (SDSNH); Catarina, 1 (SDSNH); 2 mi NW Chapala, 3 (SDSNH); Santa Gertrudis Mission, 1 (SDSNH). Table 5. Selected measurements and indices for Perognathus formosus. — See section on Methods and Materials for definitions of terms used. Indices are computed in such a manner that lower numbers are judged to be the least specialized. All measurements are in millimeters. Area Cranial Bullar Hind Pedal Tail Tail Index of N Breadth Index Foot Index Length Index Specialization Lower Colorado Valley 12 13.8 .54 23.1 .30 104.9 1.37 2.21 Central Gulf Coast 17 14.7 .53 23.5 .27 109.7 1.26 2.06 Vizcaino 8 14.4 .53 23.9 .28 106.0 1.23 2.04 35 Perogriathus fallax Merriam The San Diego Pocket Mouse is distributed from south western California along the western slopes of the San Pedro Martir Mountains and coastal plains, south to the Vizcaino Desert near 28° N latitude (Figure 6). At San Fernando (near 30° N, 115° W) P. fallax was common in sandy washes with dense vegetation and was also found in lesser numbers on adjacent rocky slopes with sparse vegetation. This was typical for most situations where P, fallax was encountered and no definite habitat preference was noted. P. fallax inhabits the Vizcaino and San Pedro Martir Provinces and there is almost no actual or relative mor phological variation between populations in these provinces (see Table 6 and Appendix A). Specimens examined. Total, 33, distributed as follows: Vizcaino (Total 23): Turtle Bay, 7 (1 BS; 1 UMMZ; 5 AM); Santa Catarina Landing, 2 (UA); San Fernando, 6 (2 BS; 4 UA); 20 mi E San Fernando, 2 (BS); Calamahue, 1 (BS); 6 mi N Punta Prieta, 3 (UA); 30 mi SE Calamahue, 1 (BS); San Andres, 1 (BS). San Pedro Martir (Total 10): San Quintin, 4 (1 KU; 3 BS); San Simon River, San Quintin, 1 (BS); Santo Domingo, 4 (UMMZ); Hamilton Ranch, 1 (UMMZ). 36 118 114 112 33 32 ARIZONA 30 SONORA 29 '27 26 • sp«cim«ns «xamined 25 24 milas 30 100 SO 100 km 117 113 113 Figure 6. Map showing the distribution of Perognathus fallax in Baja California. -- See text and Figure 1 for description of provinces. Table 6. Selected measurements and indices for Perognathus fallax. -- See section qn Methods and Materials for definitions of terms used. Indices are computed in such a manner that lower numbers are judged to be the least specialized. All measurements are in millimeters. Cranial Bullar Hind Pedal Tail Tail Index of Area N Breadth Index Foot Index Length Index Specialization Vizcaino 23 13.8 .52 23.4 .29 103.4 1.28 2.09 San Pedro Martir 10 13.7 .52 24.0 .29 106.1 1.28 2.09 38 Four species of Perognathus are of such limited distribution in Baja California that calculation of com parative specialization is not feasible. One of these species, P. penicillatus, is confined to the northern Lower Colorado Valley. Another, P. californicus, is known only from the San Pedro Martir Province, and a third, P. dalquesti, is known only from the Pacific coast area of the Cape region (Giganta-Laguna Province; see Roth, in press). The fourth species in this group, P. longimembris, is known from the Lower Colorado Valley and a few localities in the San Pedro Martir Valley (Huey 1964). P. longimembris seems seems to be uncommon in Baja California, at least during the months I was working in the area (December to April), and no specimens were collected or studied from the San Pedro Martir Province. Dipodomys merriami Mearns The smallest kangaroo rat in Baja California, Dipodomys merriami, is a wide ranging species in North America found from northern Nevada south to Zacatecas and Aguascalientes in Mexico and as far east as West Texas and Tamaulipas, Mexico. In Baja California it occurs in all but the extreme northwest chaparral region and the coastal region of the Vizcaino Desert south of Bahia Tortugas (Figure 7). As might be inferred from its wide distribution, Dipodomys merriami tolerates a wide variety of habitats. 39 118 114 112 33 32 ARIZONA 30 30N0RA 29 28 '27 26 • sp*cim«ns examined 25 24 30 100 km •• 117 119 113 Figure 7. Map showing the distribution of Dipodomys merriami in Baja California. -- See text and Figure 1 for description of provinces. 40 The only factors that appear to limit its distribution locally are the lack of suitable open areas and the presence of soils that are rocky or contain large amounts of coarse gravel. However, sandy soils in open habitat are apparently preferred. Near Cabo San Lucas D. merriami was taken in closed canopy forest with very little underbrush. Open habi tat appears to be required at or near the surface. Like the pocket mice, kangaroo rats exhibit little morphological variation between provinces and their degree of adaptation seems random. Dipodomys merriami is no excep tion: the most specialized populations of this species are found in the Central Gulf Coast and Magdalena Provinces and the most generalized in the Giganta-Laguna and Lower Colorado Valley. Indices of specialization and morphological data for D. merriami are summarized in Table 7 and Appendix A. Specimens examined. Total 191, distributed as follows: Lower Colorado Valley (Total 26): 5 mi N San Felipe, 1 (UA); Valley at east base of San Pedro Martir Moun tains , 17 (BS); 12 mi N Puertocitos, 8 (UA). Central Gulf Coast (Total 30): San Bruno, 7 (BS); 7 mi S Mulege, 6 (UA); San Francisquito Bay, 10 (3 AM; 7 UA); San Francisquito, 6 (BS); Barril, 1 (UA). Table 7. Selected measurements and indices for Dipodomys merriami. -- See section on Methods and Materials for definitions of terms used. Indices are computed in such a manner that lower numbers are judged to be the least specialized. All measurements are in millimeters. Cranial Bullar Hind Pedal Tail Tail Index oj Area N Breadth Index Foot Index Length Index Special: Lower Colorado Valley 26 22.7 .64 36.6 .38 144.9 1.50 2.52 Central Gulf Coast 30 23.2 .63 38.4 .40 154.4 1.61 2.64 Magdalena 25 23.2 .64 36.0 .40 150.0 1.66 2.70 Vizcaino 36 22.8 .63 37.8 .39 147.4 1.52 2.54 San Pedro Martir 26 22.5 .65 37.3 .39 148.3 1.57 2.61 Giganta- Laguna 48 23.1 .64 36.4 .37 144.3 1.46 2.47 42 Magdalena (Total 25): 20 mi W San Ignacio, 10 (UA); San Jorge, 14 (UA); Matanciti, 1 (BS); 3 mi SE San Pedro, 2 (UA); Migrino, 1 (UA). Vizcaino (Total 36): 1.2 mi S Las Arrastras, 2 (UA); San Fernando, 2 (1 UA; 1 BS); 20 mi E San Fernando, 4 (BS); Esperanza, 9 (UA); 27 mi SE Guerrero Negro, 3 (UA); Mouth of Calamahue Canyon, 8 (BS); Yubay, 2 (BS); San Andres, 2 (BS); Poso Altamirano, 1 (BS); 12.5 mi SW Punta Prieta, 3 (UA). San Pedro Martir (Total 26): Santo Domingo, 5 (1 UMMZ; 4 BS); San Quintin, 18 (1 KU; 6 UA; 11 BS); La Huerta, 1 (BS); Trinidad Valley, 2 (BS). Giganta-Laguna (Total 48): 7 mi S Bahia de las Palmas, 2 (UA); 6 mi S La Paz, 11 (UA); Cabo San Lucas, 1 (BS); 7 mi N Cabo San Lucas, 3 (UA); San Jose del Cabo, 13 (BS); Santa Anita, 2 (BS); Tres Pachitas, 1 (BS); 5 mi N Cunano, 12 (UA). Dipodomys peninsularis (Merriam) Dipodomys peninsularis is a large kangaroo rat whose distribution is confined to Baja California between 24° and 30° N latitude and west of 1110 30' W longitude. It occurs in the Central Gulf Coast, Magdalena and Vizcaino Provinces (Figure 8). D. peninsularis is not found in the wide range of habitats that D. merriami tolerates. This kangaroo rat appears to be most abundant in very large areas of sparse 43 118 114 112 33 32 ARIZONA 30 SONORA 29 28 •• '27 26 • specimens examined 29 24 miles 50 100 50 100 km 117 115 113 Figure 8. Map showing the distribution of Dipodomys peninsularis in Baja California. -- See text and Figure 1 for description of provinces. 44 vegetation. Near Santo Domingo Landing (Lat 28° 51' N) these rats are abundant in such situations and rapidly dis appear as the vegetation becomes more brushy and dense. The presence of coarse gravelly soils does not affect the local distribution of D. peninsularis as it does D. merriami. This may reflect the capability of the larger D. peninsularis to burrow in gravelly soils. Morphological specialization to the severely demanding desert environment is not evident in Dipodomys peninsularis, in fact, the trend is exactly oppo site of what would be expected. The most specialized populations are found in the most densely vegetated provinces. Morphological data are shown in Table 8 and Appen dix A. Specimens examined. Total 70, distributed as follows: Central Gulf Coast (Total 18): 7 mi W San Francis- quito Bay, 18 (SDSNH). Magdalena (Total 25): San Ignacio, 4 (BS); 20 mi W San Ignacio, 1 (UA); 7 mi S El Refugio, 1 (SDSNH); 9 mi S El Refugio, 4 (SDSNH); Santo Domingo, 12 (SDSNH); San Jorge, 1 (SDSNH); Matancita, 2 (BS). Vizcaino (Total 27): 9 mi S Esperanza, 3 (UA); 12.5 mi S Punta Prieta, 1 (UA); San Fernando, 3 (BS); 20 mi E San Fernando, 5 (BS); Yubay, 1 (BS); San Andres, 5 (BS); Santo Domingo, 6 (BS); Calmalli, 2 (BS); Poso Altamirano, 1 (BS). Table 8. Selected measurements and indices for Dipodomys peninsularis. -- See section on Methods and Materials for definitions of terms used. Indices are computed in such a manner that lower numbers are judged to be the least specialized. All measurements are in millimeters. Area M Cranial Bullar Hind Pedal Tail Tail Index of Breadth Index Foot Index Length Index Specialization Central Gulf Coast 18 25.1 .64 42.5 .37 167.9 1.46 2.47 Magdalena 25 25.7 .64 42.0 .37 170.8 1.50 2.51 Vizcaino 27 24.7 .64 41.9 .39 172.6 1.62 2.65 46 Dipbdomys agilis Gambel This kangaroo rat has a limited range in southern California and northwestern Baja California (Figure 9). It is almost entirely restricted to areas of chaparral and dense brush, and this alone seems to restrict its distribu tion throughout most of its range. An exception is in the extreme northern part of the Vizcaino Province near Santa Catarina Landing where small numbers of D. agilis occupy an area of little brush. For the most part, D. agilis is con fined to the San Pedro Martir Province in Baja California (see Huey 1951). Like D. merriami and D. peninsularis, D. agilis does not fit the pattern of specialization, but has an extremely limited distribution in one of the two provinces in which it is found. Comparative morphological data for D. agilis will be found in Table 9 and Appendix A. Specimens examined. Total 57, distributed as follows: Vizcaino (Total 22): Santa Catarina Landing, 4 (UA); 4 mi N Santa Catarina Landing, 4 (SDSNH); 7 mi N Santa Catarina, 6 (SDSNH); 5 mi SE San Fernando, 8 (SDSNH). I San Pedro Martir (Total 35): Jacume, 3 (UA); Pinon, 6 (BS); 15 mi E Alamos, Rancho Viejo, 5 (BS); La Huerta, 4 (BS); San Vicente, 2 (UMMZ); San Telmo, 1 (BS); 47 114 112 33 - -32 ARIZONA 31 - - 30 SONORA 29 - -28 '27— - 26 • speciman* axaminad 23 ~ - 24 uu30 100 km 117 113 113 Figure 9. Map showing the distribution of Dipodomys agilis in Baja California. -- See text and FigureHT:for description of provinces. Table 9. Selected measurements and indices for Dipodomys agilis. -- See section on Methods and Materials for definitions of terms used. Indices are computed in such a manner that lower numbers are judged to be the least specialized. All measurements are in millimeters. Cranial Bullar Hind Pedal Tail Tail Index of Area N Breadth Index Foot Index Length Index Specialization Vizcaino 22 24.5 .62 40.7 .36 164.8 1.45 2.43 San Pedro Martir 35 24.3 .62 42.1 .38 168.7 1.51 2.51 49 Ensenada, 6 (BS); Santo Domingo, 1 '(BS) ; San Quintin, 5 (BS); Tecate Valley, 1 (BS); Tiajuana, 1 (BS). In addition to the three kangaroo ra-s discussed above, four additional species are of limited distribution in Baja California. Each is confined to a single phytogec- graphic province and three are endemic to the peninsula. Dipodomys deserti ranges from the southwestern United States into northeastern Baja California as far south as San Felipe in the Lower Colorado Province. Of the three endemic species, two, D. antiquarius and D. paralius, are confined to the northern Vizcaino Province, and the third, D. grapipes, to the San Pedro Martir Province. Species of such limited distribution provide little information on comparative adaptation as is being investigated in this work. For a detailed account of Dipodomys in Baja Cali fornia, see Huey (1951). ANCESTRAL HETEROMYID MORPHOLOGY AND HABITATS The paleontological record now suggests the possi bility that heteromyids with many of the same morphological adaptations as seen in living members of the family did not evolve in desert regions or under desert conditions. Rather, substantial evidence indicates that kangaroo rats and pocket mice with saltatorial locomotion and inflated auditory bullae were in existence as early as the middle Tertiary. At that time deserts had not evolved and the present desert areas of North America were occupied by a much more mesic vegetation (Axelrod 1950). A brief synopsis of this evidence follows. In Oregon, Shotwell (1956) reported Perognathus sargenti Shotwell from a Middle Pliocene (Hemphillian) fauna in the northeastern part of that state. Available evidence indicated that during that time this area was a grassland- woodland habitat. Also in Oregon, Downs (1956) reported Prodipodomys? mascallensis Downs and Peridiomys oregonensis (Gazin) from the Middle (Hemingfordian) and Late (Barstovian) Miocene Mascall fauna. Based on a floral and faunal analysis of material in this deposit Downs (1956, p. 335) concluded "forests and grasslands were probably present, with swamp conditions in the region." 50 51 The Hagermati local fauna of Idaho contained three heteromyids: Perognathus magnus Zakrzewski (as large as modern P. hispidus), Perognathus maldel Zakzrewski (near P. parvus) and the kangaroo rat Prodipodomys idahoensis Hibbard (Zakrzewski 1969). This fauna, Upper Pliocene in age, seemed to have lived in a marsh meadow habitat with somewhat drier uplands and gallery forests along stream borders. Dalquest (1975) found Perognathus cf. rexroadensis Hibbard, Perognathus cf. pearlettensis Hibbard and Prodipo domys centralis (Hibbard) in the Blanco local fauna of the Texas Panhandle (latest Pliocene or earliest Pleistocene). Dalquest (1975, p. 46) concluded that this fauna lived in "one rather uniform habitat: grassy plains with narrow belts of trees fringing watercourses and in the shaded parts of valleys." Nebraska appears to have yielded the most important heteromyid fossils pertinent to the origin of morphological adaptations in these rodents. The Norden Bridge local fauna (Klingener 1968) contained Perognathus sp., Perognathoides cuyamensis Wood, Cupidinimus nebraskensis Wood and Diprio- nomys agrarius Wood. The Norden Bridge local fauna lived in a moist lowland habitat during the Upper Miocene and Lower Pliocene. The earliest record of a dipodomyine known to have markedly inflated bullae and saltatorial capabilities is Eodipodomys celtiservator Voorhies from the Late 52 Clarendonian (Miocene) of Nebraska. Far from being a desert, the habitat of Eodipodomys was an evergreen-oak woodland or sub-tropical forest (Voorhies 1975). In summary, there is no evidence that early saltator- ial heteromyid rodents inhabited deserts. Perhaps, as Voorhies (1975, p. 173) suggested, "saltatorial heteromyids first acquired their specializations in what is now a peri pheral part of their range (the Eastern Great Plains) and only later invaded the more rigorous desert habitat." However, as already indicated, even if the ancestral heteromyids were inhabitants of present desert areas, con ditions at that time were much more mesic. It is generally agreed that desert grasslands and deserts evolved during the Miocene (Axelrod 1950). DISCUSSION As was noted in the introduction, it appears to have been generally accepted by past workers that heteromyid rodents have evolved morphological features that better adapt them to life in the arid, open environments of American deserts (Osgood 1900, Setzer 1949, Lidicker 1960). Morpho logical features generally assumed to reflect such adapta tions include increased bullar inflation and increased length of the hind foot and tail. Some workers have speculated that predation has been the selective force and that these adaptations better equip heteromyids to escape capture in the more open, arid habitats. Further general assumption has been that the more arid, the more open the habitat, the greater the development of these features. Pocket mice and kangaroo rats are almost entirely seed eaters (Reichman 1975) in areas where vegetation is often sparse and widely spaced. In such situations most larger seeds produced would also be in widely spaced clumps in a pattern similar to that of the vegetation. Thus seed eating rodents would be forced to cross large open areas to obtain enough seeds for their food requirements. Obviously, small wind-borne seeds that might be widely and relatively 53 54 uniformly distributed would not be suitable food, simply because the energy required for collection by the relatively large heteromyids would exceed the food value' of the small seeds. However, if a heteromyid encounters such seeds they are collected and utilized. Even if the larger seeds were not concentrated in scattered clumps but were in fact uniformly distributed throughout the area (in the open as well as near the source), the effect on the rodents would be the same. They would be forced into open areas where presumably they would be exposed to greater predation pressures from owls, snakes, coyotes and other mammalian predators. In either situation, a seed eating rodent would often be far from shelters from predators (in underground burrows or in dense vegetation where concealment and maneuverability would favor the rodent). However, in such situations, the path to a relatively safe retreat is generally unobstructed. Obviously, then, any rodent that had evolved both a sensory system to detect potential predators at some distance and a locomotive mechanism that combined both short range evasive features and rapid escape to safety (underground burrow or dense vegetation) would have a survival advantage. As already seen, heteromyids appear to have such features. Both their short range evasive abilities and their rapid retreat to safety is well known to anyone who has 55 attempted to capture these rodents at night utilizing a spotlight and a hand net. Of the three sensory systems available to heteromyids for predator detection, visual, olfactory, and auditory, the auditory apparatus seems to have undergone the greatest modification. The auditory apparatus in Dipodomys has been studied in great detail by Webster (1961, 1962). He found that a reduction of the middle-ear volume reduced the cochlear microphonics and increased susceptibility to preda- tion by owls and rattlesnakes, but had no affect on the animal's locomotion. Further, Webster (1962, p. 254) demon strated that sounds produced by such potential predators as rattlesnakes and owls "fall within the frequency ranges where the kangaroo rat's hearing is especially acute due to the resonance phenomena facilitated by the hypertrophied mastoid bullae." Perognathus also have large bullae, as compared to those of most other rodents. It is reasonable to assume that their bullae function much the same as in Dipodomys. As already seen, the two genera are sympatric throughout most of their range and several species of both genera often occur sympatrically. Modifications of the locomotor apparatus and verte bral column are directly related. Pocket mice under normal circumstances appear to be entirely quadrapedal (Bartholomew 56 and Cary 1954). However, when frightened they are capable of powerful erratic leaps, changing direction rapidly after landing. During this type of movement the tail is used as an organ of balance. Even though Perognathus are quadra- pedal, unlike Dipodomys which are bipedal (Bartholomew and Caswell 1951), their large hind feet and legs facilitate leaping movements of the type necessary to avoid predators. Kangaroo rats react in much the same manner when frightened and are capable of sustained bipedal locomotion. They appear to use quadrapedal locomotion only rarely. Hatt (1932) has discussed the role of the tail as an organ of balance in richochetal rodents. The long, often terminally tufted, tail of Perognathus and Dipodomys acts as a counterbalance providing directional and trajectory control during rapid movement and leaping, making it possible to land with the desired orientation in preparation for the next leap. The importance of this mechanism is seen in animals that have lost part of their tail in some way. The result of such an injury is the development of a "brush" of coarse hairs on what is now the distal end of the tail. This "brush" is much larger than the tuft that was formally present and would seem to compensate for the loss in length of the tail. There can be little doubt that the heteromyid genera Dipodomys and Perognathus are morphologically better adapted 57 for survival (and thriving in terms of both relative numbers and total biomass) than other North American rodents in the open habitats associated with desert and sub-desert environ ments . However, actual data concerning relative numbers and total biomass of rodents clearly show that heteromyids are not necessarily always dominant. In Baja California, often considered to include the hottest, driest, and most sparsely vegetated desert areas in North America, there are two genera and 16 species of hetero- myid rodents. In the same area are found six genera and 13 <. species of cricetids. Three of these, Peromyscus eremicus, Peromyscus maniculatus, and Neotoma lepida are among the most widely ranging rodent species in Baja California and make up a considerable portion of the rodent biomass and often locally outnumber the heteromyids. None of these cricetids have enlarged bullae or elongated hind feet or legs; saltation is not developed, and the tails, while long, have little terminal tufting and never develop significant terminal "brushes" when various natural "accidents" result in the amputation of part or most of the tail. In December 1972, trapping regimes designed to re veal both numbers and total biomass of rodents in a given region were conducted at two localities in Baja California. At each site a total of 64 traps were set in two parallel 58 lines with two traps per station and 15 meters between stations and lines. Thus there were 16 stations per line. These traps were run for three nights with an evening and morning check each night. One of the attempts at estimating relative (if not actual) numbers and biomass was carried out on December 27, 1972, at 3 mi W Santa Aguida in the Central Gulf Coast Province. As already indicated, this is in an area with, on the average, only 10 to 20 percent plant cover. As shown in Table 10, two separate lines were set at this locality. The first (line 1) was set on a gently sloping hillside in an area surrounded by low rocky hills. The habitat was uniform throughout and consisted of sparse perennial vegetation and soil that was very rocky with a few small areas (less than one m in diameter) of bare soil. Line 2 was in a relatively flat area in the same general region as line 1. The vege tation was similar in species composition and density. The soil conditions were quite different, however. Here large areas of light sandy soil were dominant and rocky areas rarely "exceeded 1-2 meters in diameter. The second attempt was conducted on January 2, 1972, at 5 mi N Cunano, in the Giganta-Laguna Province where 60-80 percent plant coverage of the ground surface is character istic. Notes concerning the micro-habitats sampled include: soil light and sandy with few rocks and no rocky outcrops; Table 10. Number of individuals and biomass of rodents for two localities in Baja California, Mexico. Species No. Individuals Biomass (g) 3 mi.W Santa Aguida, Baja California (Central Gulf Coast Province) Line 1: Perognathus spinatus 2 30 Peromyscus eremicus 12 171 Line 2: Perognathus spinatus 4 63 Peromyscus eremicus 19 239 5 mi N Cunano, Baja California (Giganta-Laguna Province) Perognathus baileyi 5 140 Perognathus arenarius 12 131 Dipodomys merriami 5 215 Totals 22 486 Peromyscus maniculatus 23 508 Peromyscus eva 5 98 Totals 28 616 60 vegetation diverse and dense (up to 100 percent plant cover) with smaller areas of sparse (10-20 percent) plant cover; and flat terrain. It was assumed that (a) the actual numbers and (b) the total weight (= biomass) of animals captured on the three traplines should yield meaningful relative results, even if actual biomass could not be calculated. Therefore the results of these efforts were compiled in terms of (a) total number and (b) total weight (biomass) of each species obtained in the three traplines (during all three nights) in each of the two contrasting provinces. Results (Table 10) reveal that in all traplines, cricetid rodents outnumbered and weighed more than the heteromyids. In an effort to determine the relative diversity of cricetids and heteromyids in various plant associations where both occur, members of species of each were compiled for four geographic areas of North America: west Kansas grassland; west Texas desert grassland; northern Sinaloa thorn forest; and Baja California. All but the last of these is considered to be non-desert. The four desert phyto- geographic provinces of Baja California (Lower Colorado Valley, Central Gulf Coast, Vizcaino, and Magdalena) were considered separately from the non-desert provinces (San Pedro Martir and Giganta-Laguna). Distribution data 61 (Tables 11 and 12) were compiled from published sources, primarily Hall and Kelson (1959). Examination of Table 11 shows that in the non-desert areas heteromyid rodents account for 27-45 percent of the species (relative to cricetids) while in the desert area of Baja.California they account for 50-77 percent of the species present. These data alone indicate that heteromyids are much more diverse relative to cricetids in desert than non- desert areas, but they say nothing of the source of the diversity. Table 12 shows the average number of cricetids and heteromyids for the desert and non-desert areas listed in Table 11. These data indicate that heteromyid species do in fact out-number cricetids in desert areas but they also show that while heteromyid diversity increases at an approximate rate of 20 percent as desertification increases, cricetid diversity decreases at a rate of more than 30 percent. Cricetids, which presumably existed in this modern desert area prior to its transition to a desert environment, are typically animals of the more moderate climatic condi tions that existed before the formation of deserts. As desertification progressed, cricetids living in the area would be subjected to the same climatic variations as the invading heteromyids. Unlike heteromyids, however, they were adapted to a more stable climate and not to the changes that were occurring. Table 11. Species diversity of heteromyid and cricetid rodents for selected areas in North America. -- Numbers refer to species within each genus listed. Ba.ja California N. Giganta- San Magda- Viz- Cent. Lower Species W. W. Sina- Laguna Pedro lena caino Gulf Colo. Kans. Texas loa Martir Coast Valley Non-desert Desert Heteromyidae: Liomys 1 Perognathus 4 3 3 4 5 3 5 4 6 Dipodomys 1 3 1 1 3 2 5 2 2 Totals 5 6 5 5 8 5 10 6 8 Cricetidae: Oryzomys 2 Reithro- dontomys 2 3 2 1 1 Peromyscus 2 6 2 4 5 3 2 3 3 Baiomys 1 Onychomys 1 1 1 1 Sigmodon 1 2 1 1 Neotoma 2 3 1 1 2 1 1 1 2 Microtus 1 1 1 Totals 9 16 9 6 10 4 3 4 8 % Hetero myidae 36 27 36 45 44 56 77 60 50 63 Table 12. Average number of species of heteromyidae and cricetidae present in the non-desert and desert areas listed in Table 11. Non-desert Non-desert Desert Species (Excluding Baja) (Baja) (Baja) Heteromyidae 5.3 6.5 7. 25 Cricetidae 11.3 8.0 4. 75 % Heteromyidae 32 49 60 If kangaroo rats and pocket mice evolved their adap tations in response to desertification and the development of the habitats described above, it is expected that rodents in open areas would be more highly adapted than those in areas providing more cover. Lidicker (1960, p. 163), working with Dipodomys merriami, stated, "It is difficult to correlate many quanti tative characters with selective forces of the environment. However, the degree of mastoid inflation, as reflected in the greatest cranial breadth, can easily be correlated with openness of the habitat." Lidicker did not present evidence to support this statement. In fact, I have been unable to find a published study that attempts to correlate mastoid 64 inflation or any other adaptive feature with openness of habitat. However, the analysis provided in the species ac counts and elsewhere in the present work shows no meaningful correlation between more extreme heteromyid adaptation (e.g., bigger bullae, etc.) and increased openness of vegetation. Only two serious alternatives to the above hypothesis have been expressed and these do not appear to be mutually exclusive. First was the suggestion by Bartholomew and Cary (1954, p. 392) that "the evolution of bipedalism in hetero- myids was basically a by-product of the specialized foraging habits and that strictly locomotor factors well may have been of secondary significance." The second alternative, already mentioned, was Voorhies (1975) suggestion that Dipodomys and Perognathus did not evolve in deserts at all and that "accumulations of shifting sand along rivers may have been the ancestral dipodomyine habitat." In terms of total area occupied, modern heteromyids are primarily non-desert in distribution, however, a greater diversity is found in desert areas. Setzer (1949) and Lidicker (1960) have interpreted this as evidence that dipo- domyides evolved in deserts. Perhaps this diversity is a result of climatic variability in desert areas. It seems probable these organisms developed adaptations in one en vironment and as a result of this were able to invade a 65 second environment of a more variable nature. Adaptations of this kind are called "preadaptation" or "prospective adaptation" (Simpson 1953). As stated earlier, heteromyids are primarily seed eaters. Furthermore, ephemeral vegetation (annuals) is the primary source of seeds for these rodents (Reichtnan 1975) . The growth of ephemerals and the consequent production of seeds is directly related to the quantity and temporal distri bution of rains, both of which are extremely variable in desert regions. During favorable years seed production is very high and heteromyid rodent populations increase dramati cally and decrease rapidly as the seed supply is exhausted by predation (Cockrum 1974). In years of high seed production survival rates are high and heteromyids occupy all available habitat in large numbers. Periods of favorable conditions for the growth of annuals are generally short and separated by long periods in which very few or no seeds are produced. During these inter vals only those individuals that are efficient at gathering seeds and avoiding predation are able to survive. The net result is isolation of small demes, of a formerly dense population, in the more favorable habitats or in areas where conditions were ideal for the accumulation of. seeds and the animals were able to store adequate amounts. 66 An efficient locomotor mechanism (saltation) and effective method for gathering the largest number of seeds in the shortest amount of time (cheek pouches) becomes ex tremely important both in terms of energetics and predator avoidance. Situations such as that described above provide ideal conditions for the formation of well marked geographic races and species. Deserts are not the only habitat in which these adaptations would be an advantage, although at the present time their value is most clearly seen there. In North- Central Texas, accumulations of sandy soil along the Red River provide suitable habitat for Dipodomys ordii. Here meandering of the river maintains extremely large areas of open sandy soils that support only annual vegetation. Unlike the desert, however, seeds are generally produced yearly. No other kangaroo rat occurs in this area and Dipodomys ordii is restricted to this habitat. In the adjacent mesquite grassland several species of cricetid rodents abound, but they are rare in the open sandy areas nearby. Perhaps an cestral heteromyids evolved in habitat similar to this and were able to successfully fill a niche for which cricetids or other rodents were unfit. Ancestral heteromyids invading desert regions for the first time would encounter favorable habitat and evolve and diversify under changing climatic conditions and 67 increased climatic variability. Cricetids, lacking hetero- myid adaptations (saltatorial locomotion, cheek pouches, etc.) would be at a disadvantage under new climatic regimes and resultant habitat changes. The result would be an in crease in the number of heteromyid species and a decrease in the number of cricetids. The modern fauna is a reflection of this change. Pre-adaptation, and not in situ evolution, appears to have been the source of the morphological adaptations in the heteromyid genera Dipodomys and Perognathus that in the past have been attributed to increased desertification. These features, auditory bullae inflation, enlarged hind feet and elongate tail, certainly do not correlate in any meaningful way with the degree of openness of the wide diversity of habitat in Baja California. Note in Table 13 the great variability in relative specialization exhibited in the various species present. Only in one region, the southern, non-desert Giganta-Laguna, are any significant differences in relative specialization observed. This does not mean the environmental parameters of temperature, aridity and openness have not influenced the evolution of these or ganisms. These factors may have played an important role in the evolution of the group and their proliferation in North American deserts. Their effect on the locomotor functions and auditory apparatus is not clear, however, and the Table 13. Relative specialization of Perognathus and Dipodomys from Baja California, Mexico. -- Only those species tound in two or more phytogeographic provinces are included. Numbers represent ranking of the index of specialization from Tables 2 to 9. Specialization decreases from 1 to 6. Lower Central Magda- San Giganta- Species Colorado Gulf • Viz Pedro Valley Coast lena caino Martir Laguna % Plant Cover Perognathus arenarius 4 2 1 3 3 5 Perognathus baileyi 1 3 5 4 2 5 Perognathus spinatus 3 2 1 4 - 5 Perognathus formosus 1 2 - 3 - - Perognathus fallax - - - 1 1 - Dipodomys merriami 5 2 1 4 3 6 Dipodomys peninsularis - 3 2 1 - - Dipodomys agilis - - - 2 1 - 69 practice of basing phylogenetic relationships solely on these features seems unwise until they are better understood. The fossil record as it is now known indicates that heteromyids with well developed "desert adaptations" lived during the Miocene, before desert communities had evolved from the Madrotertiary Geoflora (Axelrod 1950). It is gen erally accepted that these communities evolved during the Pliocene. Most Miocene and Pliocene heteromyids seem to have existed under mesic or semi-arid conditions. Grassland and open savanah or mesic forests seem to have been the domi nant habitat of ancestral heteromyids. CONCLUSIONS The dipodomyine features generally assumed in the past to have evolved in response to openness and increased desertification show no correlation with these environmental parameters. Rather it appears that heteromyids acquired those morphological features that so well suit them to a desert existence in some other habitat and only later invaded deserts. A greater development of the morphological spe cialization of the auditory bullae, hind feet, and tail is seen in heteromyid populations in Baja California where there is an increase in openness of 20 to 30 percent. However, further increases in openness (to 88 percent or more) does not necessarily result in additional speciali zation. Since the various dipodomyines are known to occupy limited ecological niches, it may well be that the micro- habitat actually occupied by a given species in an extremely barren desert (e.g., the 12 percent plant cover of the Lower Colorado Valley) does not differ significantly from the microhabitat actually occupied in a less open region (e.g., the 60-80 percent cover of the Giganta-Laguna). 70 71 The basic dipodomyine adaptive features are of sur vival advantage in any open habitat, be it sandy regions along streams in woodlands (as indicated by the fossil record) or in the desert proper. Further increases in open habitat does not necessarily mean increases in populations. Being primarily seed feeders, these rodents must be in the region of at least some plant cover. Thus in the more barren regions they occur primarily in favorable microhabitats wherein some plant cover is present. The result is a "spotty" distribution of the species. Further, mean annual rainfall, and associated seed production, varies greatly in the more arid regions. The result is a multi-annual variation in the numbers of Dipodomys and Perognathus present. Conditions such as these provide ideal situations for the isolation of demes and the subsequent development of well marked geographic races and species. APPENDIX A SKIN AND SKULL MEASUREMENTS FOR SPECIMENS USED IN THIS STUDY Sample size (N), mean, standard deviation, and range (minimum and maximum) are given for each measurement for specimens examined from each of the phytogeographic provinces in which a species occurs. 72 ?nfl .DO w Body Tail Hind Cranial ^ * Length Length Foot Skull Breadth Perognathus arenarius Lower Colorado 78 72. 3± 4. 5 89.6± 7.6 21.8± 1.1 23 .6± 0 .9 12. 3± 0 .4 Valley 54-82 67-107 20-25 21.7-26 .2 11.5-13 .2 Central Gulf 42 69.7± 4.0 88.6± 5. 7 21.9± 0.8 23 .6± 0.8 12. 3± 0 .5 Coast 60-78 76-105 20-24 22.0-25 .12 11.4-13 .7 Magdalena 91 68.6± 4.4 87. 3± 7. 3 21.4± 0.9 23 .1± 0 .9 12.4± 0 .5 60-82 71-101 20-23 21.0-25 .4 11.5-13 .4 Vizcaino 89 69. 2± 3.6 87. 5± 4. 7 20.9± 0.5 23 .0± 0 .6 12. 2± 0 .3 58-77 72-97 20-22 21.1-24 .3 11.4-13 .0 San Pedro 67 67.9± 4. 2 84.6± 4.8 20. 5± 0.8 22. 6± 0.5 11.9± 0 .3 Martir 57-86 71.97 18-23 21.5-24 .6 10 .9-12.6 Giganta-Laguna 33 69. 2± 6.8 84.0± 8. 8 21.1± 1.6 23 .4± 1.4 12.1± 0.7 58-81 68-102 19-25 21.8-26 .1 11.0-13 .6 Perognathus baileyi Lower Colorado 50 86.0± 5. 2 112. 5± 8. 6 24.9± 1.1 28. 0± 0.8 14.9± 0 .5 Valley 74-99 93-130 21. 26 26 .4-30.0 14.0-16 .2 . .oe M Body Tail Hind Cranial ^ Length Length Foot Skull Breadth Central Gulf 40 88. 1± 5.9 109.7± 7. 7 25.0± 1.0 28. 3± 1.2 15. 2± 0 .5 Coast 80-98 95-122 23-27 25.9-29 .9 14.3-16 .2 Magdalena 21 93.0± 7.9 115.4±10. 3 25.1± 1.1 28. 7± 1 .0 15. 2+ 0 .5 85-107 100-128 24-27 . 27.7-30 .2 14.6-16 .0 Vizcaino 53 88.9± 5. 1 110.2± 6.1 25.0± 1.2 28. 5± 0 .9 15. 3± 0 .5 80-102 100-127 23-28 26.7-31 .2 14.4-16 .8 San Pedro 7 87.0± 9.9 113.0± 2. 8 23. 5± 0 .7 28. 3± 0.4 15. 2± 0 .8 Martir 80-96 111-118 23-26 27.9-29 .8 14.6-16 .0 Giganta-Laguna 42 87. 8± 5.0 108.2± 6.4 25. 2± 1.3 28. 7± 0 .8 14. 8± 0.4 81-92 99-116 24-27 28.0-29 .6 14. 3-15 .1 Perognathus spinatus Lower Colorado 17 75. 1± 5. 3 103.4±10.4 21. 7± 1.0 24. 5± 0 .8 12.6± 0.4 Valley 61-81 83-116 20-23 23. 2-25 .9 12. 1-13 .5 Central Gulf 60 79. 5± 4,7 111.3± 5. 2 24. 2± 0.8 24.9± 0.9 12.6± 0.5 Coast 71-85 104-119 23-25 23.7-25 .6 11.9-13 .0 Magdalena 20 74.9± 4.8 106.6± 5. 2 22. 3± 0.5 24.9± 0 .5 12.6± 0.3 68-83 98-116 22-23 24.4-25 -6 12. 2-12.8 5nan. 0 *j Body Tail Hind Cranial P Length Length Foot Skufl Breadth Vizcaino 8 72.8± 9.5 99.4± 6.5 20.4± 1.1 24.5± 0.6 12.6± 0.3 64-85 92-108 19-22 23.7-25.0 12.2-13.0 Giganta-Laguna 78 80.6± 7.6 104.4± 7.8 23.6± 1.6 25.6± 1.3 13.1± 0.6 69-98 82-118 20-26 23.7-28.0 12.2-14.1 Perognathus formosuis Lower Colorado 12 76.5± 5.3 104.9± 7.5 23.1± 0.8 25.6± 0.7 13.8+ 0.3 Valley 62-88 90-120 22-25 24.2-27.0 13.1-14.5 Central Gulf 17 86.8± 5.9 109.7± 3.2 23.5± 1.0 27.7± 0.7 14.7± 0.4 Coast 1 80-94 106-112 23-25 26.8-28.5 14.1-15.1 Vizcaino 8 86.4± 5.2 106.0± 5.7 23.9± 0.6 27.1± 0.8 14.4± 0.5 80-96 100-116 23-25 26.0-28.5 13.8-15.2 Perognathus fallax Vizcaino 23 81.0± 5.6 103.4± 7.8 23.4± 1.4 26.4± 1.0 13.8± 0.5 73-88 87-116 22-26 25.1-28.3 12.8-14.6 San Pedro . 10 83.0± 5.9 106.1± 6.7 24.0± 1.7 26.3± 1.4 13.7± 0.7 Martir 70-94 88-117 20-28 22.4-28.0 11.8-14.6 n . *j Body Tail Hind Cranial P Length Length Foot Skull Breadth Dipodomys merriami Lower Colorado 26 96.4+ 5. 2 144.9± 6.7 36.6± 2. 1 35. 2± 1. 0 22. 7± 0.7 Valley 82-108 130-163 30-40 33. 2-38. 2 21.5-24 .2 Central Gulf 30 95.9± 6.3 154.4± 8.9 38.4± 1. 5 36.6± 0.7 23. 2± 0.6 Coast 88-111 145-173 35-40 35.0-38. 0 22.4-24 .0 Magdalena 25 90.0± 4.8 150.0± 6.9 36.0± 1.8 36.3± 0.8 23. 2± 0 .7 87-101 141-166 34-38 35. 3-38.1 22.5-24 .1 Vizcaino 36 97.0± 5.6 147.4±12. 1 37. 8± 1.6 36.0± 1. 1 22. 8± 0.7 81-106 121-172 34-40 32.9-38. 3 21. 5-24.4 San Pedro 26 94.7± 4. 5 148.3± 7. 4 37. 3± 1. 3 34.8± 0. 8 22. 5± 0 .9 Martir 86-106 136-165 35-40 33.8-36. 7 19.9-23 .6 Giganta-Laguna 48 98.5± 6.8 144.3± 9.4 36.4± 1.9 36. 2± 1. 1 23. 1± 0.7 85-115 115-165 30-39 34.5-40. 5 21.9-25 .7 Dipodomys peninsuiaris Central Gulf 18 115.3± 4. 3 167.9± 8. 3 42.5± 0.7 39. 2± 1.0 25.1± 0.8 Coast 108-122 156-182 42-•44 37.5-41. 5 24. 2-26 .5 n . vr Body Tail Hind Cranial ^ Length Length Foot Skull Breadth Magdalena 25 113.7± 5.9 170.8± 9.6 42.0± 1.0 40.0± 1.0 25.7± 0.9 98-125 145-185 40-44 37.6-41.5 23.9-27.0 Vizcaino 27 106.4± 5.9 172.6±11.4 41.9± 1.3 38.6+ 1.5 24.7± 1.0 94-118 148-203 40-45 34.7-40.6 22.3-26.6 Dipodomys agilis Vizcaino 22 113.6± 2.8 164.8± 3.8 40.7± 0.5 39.7± 1.0 24.5+ 0.3 109-122 160-171 40-42 37.8-40.7 24.1-25.1 San Pedro 35 111.2± 2.1 168.7± 2.7 42.1± 1.0 38.9± 0.8 24.3± 0.2 Martir 100-119 163-173 40-44 37.6-40.1 23.9-25.3 REFERENCES CITED Anderson, D. 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