Proc. Nat. Acad. Sci. USA Vol. 72, No. 5, pp. 1702-1706, May 1975

South American Zoogeography: Evidence from Convergent Evolution in Desert (physiological ecology/multivariate analyses) MICHAEL A. MARES Department of Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Communicated by George Gaylord Simpson, February 6, 1975

ABSTRACT Current theories regarding colonization of uplifted. Much of the debate centers about the degree of South America by are divided between those diversification of South American cricetine rodents, which supported by fossil evidence, which suggest the original mammal fauna of the isolated continent was augmented represent 40 genera, and from 150 to 200 species. Hershkovitz by early immigrants (primates, caviomorph rodents, and suggests that such great differentiation of taxa, and adapta- later, procyonids) with a final large influx of northern tion for very specialized habitats, could not have occurred in mammals occurring with the formation of the Panama the relatively short time since the mid-Pliocene. Nevertheless, land bridge, and an opposing view which states that the purported "recent invaders" are too taxonomically and fossil evidence indicates that caviomorph rodents and pri- ecologically differentiated to have colonized since the land mates were in South America in early Oligocene, and that bridge arose. The second theory suggests that most extant procyonids entered in earliest Pliocene (1-3). No cricetines mammals entered before the Plio-Pleistocene land con- appear as fossils before mid-Pliocene. Caviomorph rodents in nection. An analysis of degree of physiological adaptation, today represent 11 families, 40 natural history, distribution patterns, and a multivariate continental South America assessment of convergent evolution of Monte Desert genera, and 120 species; primates are grouped into 2 families, rodents indicate that South American cricetine rodents 15 genera, and 62 species (7). Many of the various taxa are are not highly specialized for desert life. Their degree of quite distinct, one from another, within each higher category, adaptation could be accounted for, in large part, by adapta- indicating an old fauna with intermediate extinction. tions for arid or semiarid Andean habitats. No Monte Desert has developed the specialized desert traits The arguments on both sides of this problem are logical, that have evolved in most desert rodent faunas of the conflicting, and stimulating. They have led Savage (8), largely world, although extinct marsupials similar to living bi- on a reinterpretation of the same data, to propose yet a third pedal desert rodents were present in- the Monte as recently hypothesis, which seeks a middle ground between the previous as late Pliocene. Evidence suggests that Monte cavio- theories. He felt that the Central American bridge formed in morphs have been associated with the desert for a longer period than cricetines, and that the latter represent a early Pliocene, that cricetine rodents entered over water in fairly recent invasion of the Monte Desert. The data thus Miocene times, and that the bulk of mammal exchange be- support the first hypothesis of South American mammal tween the continents took place after the land bridge was colonization. complete. Mammal colonization patterns Desert mammal convergence There are at present two major conflicting views about the I have been studying temperate South American mammal time of arrival of most mammals into South America. One ecology for the last few years, and some of my findings are hypothesis states that the arrival has been primarily since the germane to the above hypotheses. My work has largely middle of the Pliocene (1-3). The other argues for a much centered in the Monte Desert of northwestern Argentina earlier migration into the continent (4-6). In the absence of (particularly near Andalgala, Catamarca Province), where I recent new fossil evidence, progress in solving this problem have attempted to assess the degree of desert adaptation of has seemed to reach an impasse. While working on a project to mammals found there with that found in faunas from other determine the degree of convergence of the mammals of two desert regions, particularly the Sonoran Desert of the south- disjunct desert scrub ecosystems, one in North America and western United States near Tucson, Ariz. Both deserts are one in South America, I have been able to devise an inde- remarkably similar in physiognomy, geomorphology, and pendent test of these two theories and find that this multi- climate (9-11). Deserts are harsh environments, and the faceted approach supports the hypothesis of a late arrival number of ways in which small mammals may exploit re- into South America by murid (cricetine) rodents. sources and maintain thermal and osmotic balance are prob- Since the Paleocene, the original mammal fauna of South ably limited. I suggest that a number of characteristics that America (which included marsupials, edentates, a number of are obvious adaptations to desert life have a high probability ungulate groups, and condylarths) has been augmented by of appearing among some representatives of any assemblage of primates, caviomorph rodents, procyonids, and a large group small mammals exposed for sufficient time to selective pres- of placental mammals with close taxonomic affinities to extant sures in a desert. For example, the propensity for bipedalism North American taxa. This last assemblage included seven to develop among desert rodents throughout the world (7) orders and 16 families. G. G. Simpson and others (1-3) argue implies that selective forces (for predator avoidance or more that this great influx of mammals occurred after the Central efficient seed gathering, perhaps) are particularly strong for American land bridge formed, while Hershkovitz (4-6) feels the evolution of such traits. Since the Monte and Sonoran that most, if not all, major groups had island-hopped, or deserts are about the same age, and as old as some other crossed over water into the continent before the bridge was deserts (9-11), I expect that a number of adaptations for 1702 Downloaded by guest on October 1, 2021 Proc. Nat. Acad. Sci. USA 72 (1975) Zoogeography of South American Mammals 1703

Perognathus tallax 100.

901 \\ Dipodomys agilis

I- 80 C!, Phyllotis gnseoflavus l-

70 z

00r 60 \hyllotis darwini

50~ *ma * \" Peromyscus maniculatus Nteotomafs cfucies"es*

0 8 16 24 32 40 48 52 DAYS WITHOUT WATER FIG. 1. Rates of water loss during complete water deprivation (i.e., no free water available, air-dry seed diet) for three species of Monte rodents and selected North American rodents (27).

desert life will be common to some species of both faunas, species and those from more mesic areas in their rates of water assuming that the mammal groups involved had been asso- loss. Monte species probably obtain water from succulent ciated with the desert for a similar (or at least sufficient) vegetation and/or insects. Although no other Monte mammal period of time. Thus a high degree of adaptation for xeric life, was examined physiologically, observations on natural history such as water conserving mechanisms and the entire array of of a number of species suggest that all include moist vegetation physiological characteristics this implies, bipedalism, graniv- as part of their diet. Although there may be genetic restraints ory, and so forth, would be expected to be represented in each on the ability of Monte rodents to evolve a high degree of fauna. This is not to suggest that each small mammal fauna of physiological adaptation to aridity, the presence of Phyllotis each desert of the world will possess exactly the same types of gerbillus (14) of the Peruvian Sechura Desert (which is adaptations in exactly the same combinations, but that similar probably water-independent), and a water-independent non- adaptations will commonly be found among some members of desert species from bordering areas of the Monte suggest that the various faunas. Monte cricetines possess the necessary genetic plasticity. Physiological evidence suggests that Monte cricetines are Physiological evidence not as highly evolved for desert life as are many rodents from Within the northern Monte Desert, a number of rodent other deserts. The caviomorphs that inhabit the Monte may species can be found in various arid and semiarid habitats also be unable to exist without moist food. No Monte species (Table 1). Most, however, are limited to riparian situations where water flow occurs at least intermittently, and where TABLE 1. The small rodent fauna of the Monte Desert and their dense- woodlands (Prosopis, Acacia, Cercidium, Bulnesia) habitats in the northern Monte may be found. Rodents living in these areas climb readily into trees to forage on green plant material. Only a few species Species Habitats occur in more arid localities, and these are generally uncom- mon (but regular) inhabitants of creosote bush flats (Larrea) or Caviomorphs presaline areas with halophytic plants. The rodents living in Tympanoctomys Lowland desert flats, low scrub areas, hal- vegetation, sandy flats the most arid microhabitats should be among the most desert- barrerae ophytic Octomys mimax Lowland desert areas with rocks, rocky are not highly adapted of the southern species. Small rodents desert foothills with large cacti, low mobile, and most spend their entire life within a relatively scrub areas small area. Thus they are highly subjected to selective forces Ctenomys fulvus Desert flats with Larrea or other low favoring particular adaptations for existence in such habitats. shrubs, riparian forests along dry gul- Traits characteristic of evolution in a desert should therefore lies, upper bajadas be particularly marked in such species, and I expect rodents to Microcavia australis Desert washes with riparian forests, low show much evidence of convergent evolution. scrub areas in high altitude northern- I thus examined physiological adaptations of South most Monte, permanent water forests American Monte mammals in order to compare their degree of Cricetines desert adaptation with that of North American desert species. typus Larrea flats, sandy.lowland scrub areas, I studied a number of aspects of the water balance of Elig- scrub areas interspersed with taller mondontia typus, Phyllotis griseoflavus and P. daruwini desert trees, sandy flats with halophytic (12, 13), inhabitants of Monte flats, or, for the latter two plants species, its arid hillsides, and found that none of them was Phyllotis griseoflavus Desert washes with riparian forests, rocky hillsides and upper bajadas, per- independent of free water. The water loss curves of these desert manent water forests are compared with those of rodents from other desert species Phyllotis darwini Rocky desert hillsides, permanent water and non-desert areas in North America in Fig. 1. Monte forests species appear to be intermediate to many northern desert Downloaded by guest on October 1, 2021 1704 Zoolo : Mares Proc. Nat. Acad. Sci. USA 72 (1976)

0 .5 I TABLE 2. Distributional patterns (latitude and longitude) of PEROGNATHUS Monte Desert rodents Longitude 50°- 550- 60°- 70°- DIPODOMYS Latitude 550 600 650 650-700 750 <200 S 6 6 5,7 OCTOMYS 200-250 S 6 6 6 4,5,6,7 3,7 PEROMYSC US 250-300S 6 4, 6 2, 3,4, 5, 6, 7 7 P GERBILLUS 300-350 S 4 1,2, 3, 4, 5, 6, 7 7 PEROMYSCU S 350400 S 4,5, 6 4, 5, 6, 7 400-450 S 4,5 6, 7 450-500 S 4,5 6, 7 7 ELIGMODONTIA >500 S 4,5 7 7 RE ITHRODONTOMYS is 1 = T. barrerae, 2 = 0. mimax, 3 = C. fulvus, 4 = M. australis, 1. 5 = E. typus, 6 = P. griseoflavus, 7 = P. darwini. ONYCHOMYS NEOTOM A ,1 ments into southern South America would be somewhat P GRISEOFLAVUS preadapted to the aridity of the lowland desert. The cricetines SIGMODON do not appear to have evolved in the Monte Desert at either M ICROT US the generic or specific level, although the new phyllotine could

AMMOSPERMOPH I LU S be an exception. A number of caviomorph species are restricted EUTAMIAS to the Monte, including two genera (Octomys and Tympanoct- SC IURUS omys); these are also inhabitants of the most xeric habitats. They have probably been associated with the desert for a MICROCAVIA long period. Distribution patterns can be interpreted as THOMOMYS supporting the suggestion that caviomorph rodents have evolved for a longer period in the Monte, with some groups CCTENOMYS being autochthonous, while cricetines generally have more FIG. 2. Correlation phenogram of North and South American catholic habitat choices and seem to include the Monte as a desert rodents, and the a rodent fauna from New Mexico conif- small part of much broader distribution patterns. erous community. Twenty-eight morphoecological characters used; cophenetic correlation coefficient = 0.852. With few excep- Multivariate analyses tions, ecological equivalents are clustered together. I utilized multivariate analyses to attempt to arrive at a more is bipedal, although E. typus has elongate hind legs and may rigorous determination of convergence and degree of desert hop at times. adaptation. Cluster analysis (16) groups organisms (or popula- tions, species, etc.) together on the basis of number of shared Distribution patterns characteristics over many measurements. I used morpholog- When distribution patterns of Monte rodents are examined ical measurements that strongly implied ecological function (Table 2), it is evident that only a few species are wholly rather than any random assortment of traits. Characteristics restricted to the geographic limits of the desert. Interestingly, associated with desert life (e.g., degree of inflation of auditory all but one of these species are caviomorphs, elements of the bullae, degree of bipedality, color, countershading, and so old fauna. (The exception is a new species of phyllotine which forth), as well as more general ecological characters (e.g., I collected near AndalgahU. Only one specimen was taken, so body size, relative ear and tail lengths, vibrissae density, it is presumptuous to speak of distribution pattern, although numerous dental traits which may reflect diet, and so on) were it may well be restricted to the Andalgall area. This species used in analyses. Thus, instead of clusters of with possesses some degree of desert adaptation and somewhat shared phenotypes, I expect organisms with similar niches resembles E. typus externally. It is not bipedal and does not (i.e., ecological equivalents) to be grouped together. Since possess greatly inflated auditory bullae or have other traits desertic habitats have never been contiguous between the correlated with a very high degree of adaptation to a xeric continents, possession of desert traits by species that have existence.) Eligrodontia typus, P. griseoflavus, and P. darwini evolved from forest or montane ancestors suggests conver- all extend well beyond the limits of the Monte. Eligmodontia gence. Those species that are closely equivalent would be is found from Tierra del Fuego to Peru, and from sea level to tightly clustered, while species pairs only slightly similar, over 4500 m in the Andes. Phyllotis griseofiavus occurs in ecologically, should be more loosely grouped. For compara- thorn scrub habitats to the east of the Monte and in high tive purposes, the rodent fauna of a non-desert site (the mountain scrub areas in Argentina, Paraguay, Bolivia, and Ponderosa pine community of the Sandia Mountains of possibly Brazil, from sea level to 3000 m, while P. darwini central New Mexico) was included in analyses. The New may be found in various forest habitats or high altitude scrub Mexico fauna is phylogenetically closely related to that of the areas from the Straits of Magellan to Peru, over a grea4 Arizona Desert (some genera and species are shared), while the altitudinal range (15). The high puna, thorn scrub, and rocky two desert faunas are more distantly related (17-20). Thus, if Andean hillsides are dry habitats, and it would be expected that a broad suite of characteristics were being used in analyses, a animals evolving in, and dispersing through, such environ- clustering of taxonomically closely related species might be Downloaded by guest on October 1, 2021 Proc. Nat. Acad. Sci. USA 72 (1975) Zoogeography of South American Mammals 1705

O MONTE

04 *.6

I- C4 z a" 4 0

V 0 0

O .2 1 OREST V SONORA

-6 -11 .2 9 CANONICAL VARIATE 1 -1.2

-1.2 -.6 0 *.6 +1.2 FIG. 4. Distribution of the Sonoran Desert, Monte Desert and

PRINCIPAL COMPONENT 1 New Mexico coniferous forest faunas along the first two canonical FIG. 3. Distribution of small North American cricetines axes. Faunal convergence is suggested by the positioning of the (Reithrodontomys, Peromyscus, Onychomys), South American three faunal assemblages along canonical variate 1, which ac- phyllotines (Phyllotis gerbillus, P. griseoflavus, Eligmodontia counts for 99% of the dispersion. Thus the two desert rodent typus), kangaroo rats (Dipodomys) and pocket mice (Perognathus), faunas are grouped close together while the Forest rodent group and the Monte Desert caviomorph Octomys mimax along the is plotted a great distance from either desert fauna. The Sonoran first two principal components. Principal component 1 is essen- and Forest groups are more closely related, taxonomically, than are the tially positively correlated with body size, while principal com- desert faunas. Group means are denoted by a solid circle. ponent 2 is positively associated with the degree and number of desert adaptations. Positioning of groups is based on a 44-trait small rodent faunas. If community convergence has occurred, analysis. then when each fauna is plotted in n-dimensional space (n = number of traits analyzed), the distance between the two des- expected, rather than a grouping of unrelated taxa. A correla- ert faunas should be less than that found between the related tion phenogram of species with similar ecologies (using 28 northern faunas. Traits that maximize the variance between unweighted traits) is given in Fig. 2. A number of Sonoran the various assigned groups are utilized in forming canonical Desert species have an ecological equivalent in the Monte. variates. The first canonical axis (which accounts for 99% Thus field mice, peromyscines, are clustered with the two of the total group dispersion) is shown in Fig. 4. The desert small desert phyllotines (E. typus and Peruvian P. gerbillus), faunas are closely converged when either is compared to the while North American pack rats (Neotoma) are grouped with forest fauna. In the second analysis, I assigned two groups, the P. griseoflavus. Squirrels are clustered with cavies (Microcavia Sonoran Desert and Forest assemblages, and listed each australis), and the closest cross-continental cluster occurs South American species as being of unknown affinity. On the between fossorial rodents (tuco-tucos and gophers). All of basis of shared characteristics, each southern species, except these groupings are logical when the natural history of each the cavy (Microcavia australis), was given 100% probability species is considered (12). The caviomorph Octomys mimax, of pertaining to the Sonoran Desert fauna. Microcavia, which which is similar in overall ecology to pack rats, was loosely is most common in thorn scrub habitats to the east of the clustered with heteromyids (kangaroo rats and pocket mice), a Monte and generally limited to forested areas within the group which is practically the apotheosis of desert-adapted northern lowland Monte Desert, was assigned to the Forest species. Octomys shares a number of desert traits with hetero- fauna. myids. Distance measurements (rather than correlation) and a 44-trait analysis clustered Octomys with pack rats. Zoogeographic implications Principal Component Analysis (PCA) indicated that tuco- The multivariate analyses essentially support the physiolog- tucos (Ctenomys) possessed more desert characteristics than ical, distributional, and natural historical observations of North American pocket gophers (Thomomys), and that Monte rodents. Overall faunal convergence of desert rodents is Octomys mimax was one of the most desert-adapted Monte indicated when those communities are compared with a species. Basically, however, there were no ecomorphological coniferous forest fauna. Also, a number of northern desert equivalents of heteromyids in the Monte, although PCA species find their analogue (albeit seldom an "exact" analogue) ranked South American E. typus and P. gerbillus as inter- in the southern desert. Nevertheless, the most highly desert- mediate between northern pocket mice (heteromyids) at an adapted rodents in the United States, the heteromyids, have upper level, and field mice at a lower level, in their degree of no close ecological equivalents in the Monte, although desert adaptation (Fig. 3). Octomys mimax, a caviomorph, approaches them in some des- Ecological equivalents of various degrees of similarity exist ert traits. Fossorial tuco-tucos are extremely similar to pocket in both deserts, but if one wishes to examine all species of a gophers, and are probably more desert-adapted than gophers. community at one time, other methods such as canonical and At this point the situation is somewhat puzzling. The theory discriminant function analyses must be used (21). I used the presented here would predict that a "classic" desert rodent same morphological measurements and compared three should have evolved in the Monte, and yet no such species communities: the Monte, Sonoran, and New Mexico Forest lives there today. Eligmodontia appears to be evolving in that Downloaded by guest on October 1, 2021 1706 Zoology: Mares Proc. Nat. Acad. Sci. USA 72 (1975)

direction, but it has not yet attained a very marked degree of suggest that diversity of living cricetines is not so great, nor desert adaptation. Why has this type of failed to adaptations to very specialized habitats so pronounced, to evolve in the Monte? This problem was clarified for me when I warrant the supposition of an early invasion by this group encountered the paper by G. G. Simpson on fossil marsupials into South America. If the cricetines do not support ideas of (family Argyrolagidae) which inhabited parts of the present- early colonization, then it may not be necessary to invoke a day Monte and bordering regions in the Pliocene (22). These pre-Pliocene invasion by other mammal groups which are less marsupials were strikingly similar to living kangaroo rats or diverse than the rodents, and which also have left no fossils. Old World desert jerboas. Apparently the entire morpho- Occam's razor leads me to discount dispersal over a fairly logical mosaic associated with a desert existence (and similar- extensive water barrier by diverse mammal taxa and their ities presumably would extend to other areas of their biology) subsequent failure to leave fossil traces. While data of living evolved in or near the Monte, and the group later became species can be interpreted in various ways, only subsequent extinct. The existence of this group is important, for it demon- fossil discoveries will clarify the very interesting patterns of strates that suitable habitat existed at that time for such a colonization of South America by mammals. strategy to have evolved and that such individuals lived in the Monte as recently as the Pliocene. This could explain why This work was supported as a part of the International Biologi- bipedal caviomorphs never evolved. Such a niche was filled by cal Program Integrated Research Program on Origin and Struc- marsupials and the niche only became "empty" fairly recently ture of Ecosystems through a National Science Foundation Grant GB27152 to the University of Texas at Austin. I thank Dr. in the evolutionary time scale. I speculate that it was probably Beryl Simpson for commenting on this manuscript. not competition which caused the extinction of the argyro- lagids, since no potential competitor is known. The Monte was Pleistocene periods of advancing and 1. Simpson, G. G. (1950) Amer. Sci. 38, 361-389. greatly affected by 2. Simpson, G. G. (1951) in Science in Progress (Yale Univ. retreating ice. Some of the northern valleys where argyrolagids Press, New Haven, Conn.), Chap. 11. occurred were possibly occupied by glaciers. The southern 3. Patterson, B. & Pascual, R. (1972) in Evolution, Mammals, Monte and adjacent habitats were also modified by Pleisto- and Southern Continents (State Univ. New York Press, Al- cene glaciers and associated climatic effects (ref. 23, and bany, N.Y.) pp. 247-309. 4. Hershkovitz, P. (1966) in Ectoparasites of Panama (Field B. Simpson, personal communication). Argyrolagids could Mus. Nat. Hist. Press, Chicago, Ill.). have been restricted to a relictual area of desert where they 5. Hershkovitz, P. (1969) Quart. Rev. Biol. 44, 1-70. faced new predators with varied hunting methods (the 6. Hershkovitz, P. (1972) in Evolution, Mammals, and Southern Pliocene-Pleistocene influx of northern carnivores), as well as Continents (State Univ. New York Press, Albany, N.Y.), pp. 311-431. some competition from invading cricetines and from cavio- 7. Walker, E. P. (1964) Mammals of the World (Johns Hopkins morphs. Such a relict desert area may have been located in Press, Baltimore, Md.). what is currently the hottest and driest part of the Monte, and 8. Savage, J. M. (1974) Contr. Sci. Nat. Hist. Mus., Los An- where autochthonous caviomorphs are found today. The geles, Calif. no. 260, 1-51. Pleistocene Monte could have been greatly reduced in area 9. Morello, J. (1958) Opera Lilloana 2, 1-155. 10. Solbrig, 0. T. (1972) Anns. Missouri Bot. Garden 59, 218- and could have been an ideal setting for the subsequent 223. desert marsupial extinction. 11. Lowe, C., Morello, J., Goldstein, G., Cross, J. & Neuman, Cricetines are not known as fossils in South America before R. (1973) Ecologia 1, 35-43. late Pliocene. My data suggest that they have not been asso- 12. Mares, M. A. (1973) Ph.D. Dissertation, University of Texas, Austin; see Diss. Abstr. Internat. 34(8). ciated with the Monte for an extended time period (i.e., pre- 13. Mares, M. A. (1975) in Rodents in Desert Environments (Dr. Pliocene). Desert adaptation evidenced by Monte cricetines W. Junk N.V., The Hague, The Netherlands), pp. 155-175. could well have evolved in response to selective pressures in 14. Koford, C. B. (1968) Science 160, 552-553. Andean habitats which would have been preadaptations for 15. Hershkovitz, P. (1962) Fieldiana Zool. 46, 1-524. life in 16. Sneath, P. H. A. & Sokal, R. R. (1973) Numerical the lowland Monte. Highly desert-adapted marsupials (W. H. Freeman and Co., San Francisco, Calif.). did exist in the Monte, however, and living caviomorphs also 17. Simpson, G. G. (1945) Bull. Amer. Mus. Nat. Hist. 85, 1- appear to have been present in the desert for a long time, as do 350. armadillos (28). Interestingly, Australian murids have also 18. Cabrera, A. (1957) Rev. Mus. Argent. Cienc. Natur. "Bernar- diversified greatly since invading that continent and it was dino Rivadavia," Zool. 4(1), 1-307. 19. Cabrera, A. (1960) Rev. Mus. Argent. Cienc. Natur. "Bernar- originally thought that a Miocene invasion was required in dino Rivadavia," Zool. 4(2), 309-732. order to allow time for such marked speciation (24), but recent 20. Hall, E. R. & Kelson, K. R. (1959) The Mammals of North fossil evidence indicates that they did not enter before the America (Ronald Press, New York). Pliocene (25). The Australian continent is much less diverse in 21. Dixon, W. J., ed. (1970) BMD biomedical computer programs is (Univ. California Publ. Automatic Computations, no. 2). numbers of habitats and topographic relief than South 22. Simpson, G. G. (1970) Bull. Mus. Comp. Zool. 139(1), 1-86. America, so it should not be too surprising to find an even 23. Simpson Vuilleumier, B. S. (1971) Science 173, 771-780. greater diversification of cricetine rodents in the Neotropics. 24. Simpson, G. G. (1961) Evolution 15, 431-446. We are learning that population isolation and subsequent 25. Turnbull, W. D. & Lundelius, E. L., Jr. (1970) Fieldiana genetic diversification can occur much more rapidly than was Geol. 19, 1-163. 26. McCluskey, J., Olivier, T. J., Freedman, L. & Hunt, E. previously supposed (26). Had cricetines been associated with (1974) Nature 249, 278-279. the Monte for a long period, we would expect either fossils, or a 27. MacMillen, R. E. (1964) Univ. Calif. Publ. Zool. 71, 1-59. high degree of desert adaptation, or both. We have neither. I 28. Gregor, D. (1974) Ph.D. Dissertation, University of Arizona. Downloaded by guest on October 1, 2021