Great Basin Naturalist Memoirs

Volume 7 Biology of Article 3

8-1-1983

Desert adaptation and community structure

Michael A. Mares Stovall Museum, University of Oklahoma, Norman, Oklahoma 73019

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Recommended Citation Mares, Michael A. (1983) "Desert rodent adaptation and community structure," Great Basin Naturalist Memoirs: Vol. 7 , Article 3. Available at: https://scholarsarchive.byu.edu/gbnm/vol7/iss1/3

This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist Memoirs by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. DESERT RODENT ADAPTATION AND COMMUNITY STRUCTURE'

Michael A. Mares-

Abstrac:t.— Desert rodent communities are compared for evidence of convergent evolution at various levels of or- ganization, including the systemic (physiological, anatomical, etc.), autecological, and synecological. Convergence is quite pronounced at the systemic level, less pronounced at the autecological level, and even less detectable at the svnecological level. This is not to imply that community convergence does not occur, but rather that our current abilities to quantify and detect convergence at the community level are nidimentary— and our data base is still far from adequate to the task of rigorously comparing community attributes. Most research on the ecology, behavior, physiology, and community structure of desert rodents has been conducted on North American inhabiting of the . The patterns of species coexistence that have been elucidated in these deserts are often presumed to apply in other deserts of the world. It has become apparent in recent years, however, that the complex

North .American desert system is unique in many ways, perhaps especially in the biogeographic history of its habitats and faunas, from most of the other deserts of the world. The North American deserts offer an unusually diverse fauna of desert rodents (both alpha and beta diversity are high) which evidences patterns of distribution and coexistence that excite biologists working with the mechanisms of competitive interactions. Similar studies carried out in other deserts might very well lead to a different set of ideas concerning the ways in which desert rodents manage to coex- ist and how desert communities develop over time. The present paper is an attempt to compare community struc- ture and development as well as patterns of coexistence among the various faunas of desert rodents of the world. Al- though data are sketchy for many areas, sufficient information is available to allow a preliminary comparison of methods of adaptation and coexistence to be made.

Research on desert rodents began over a dents to arid environments; this research was century ago in the United States. The earHest greatly stimulated by the studies of the studies examining desert rodents were those Schmidt-Nielsens (see Schmidt-Nielsen 1964, of Coues (e.g., 1868), Coues and Allen (1877), for a review), who showed convincingly that and C. Hart Merriani and his team of in- some small were well adapted vestigators from the old Biological Survey. In physiologically to pronounced aridity. Later addition to the taxonomic investigations of research has allowed a finer resolution of the Merriam himself (e.g., Merriam 1889) and mechanisms of physiological adaptation to those of his subordinates (e.g., Osgood 1900, deserts (e.g., McNab and Morrison 1963, Goldman 1911, Howell 1938), there were MacMillen 1964a, 1964b, 1972, Hudson other .studies by contemporaries of the survey 1964a, Chew 1965, Carpenter 1966, Brown scientists (e.g., Grinnell 1932, Benson 1933, 1968, Brown and Bartholomew 1969, Mullen Blos.som 1933, Hall and Dale 1939). After the 1971, Abbott 1971, Whitford and Conley initial work had formed a rather firm tax- 1971, Maxson and Morton 1974, Baudinette onomic foundation, field research entered the 1974). stage of natural historical, ecological, and Within the last 15 years, desert research in biogeographical .studies (e.g., Taylor and Vor- the United States has centered on problems hies 1923, Bailey 1931, Benson 1935, Dice dealing with species coexistence. It has long and Blos.som 1937, Blair 1943, Monson and been remarked that the deserts of the United Ke.s-sler 1940, Tappe 1941, Fitch 1948). Al- States support a broad diversity of species, though ecological and taxonomic in- but only since the mid-1960s have research- vestigations continued during the mid- ers attempted both to understand the causa- twentieth century, much research was cen- tive agents of this diversity as well as the tered on the physiological adaptations of ro- mechanisms of species coexistence. Earlier

'From Ihc symposium "Biology of Desert Rodents," presented at the annual meeting of the American Society of Mammalogists. hosted In Brigham Young • » . University, 20-24 June 1982. at Snowbird. Utah. 'Stovall Museum, University of Oklahoma, Norman, Oklahoma 73019.

30 1983 Biology of Desert Rodents 31

studies of coexistence had examined the pos- Since ecologists tend to extrapolate the re- sible roles of abiotic factors on species distri- sults of research carried on in one biome to bution patterns (e.g., Hardy 1945), but later other areas supporting apparently similar research has focused on the role of inter- ecosystems, it is tempting to believe that as specific competition as a possible determi- we explain patterns of coexistence or adapta- nant of distributional patterns (see Brown et tion within the deserts of the United States

al. 1979, for a review). Research emphasis we will have described these patterns for over the last decade has centered on the body deserts around the world. As MacArthur

sizes of coexisting rodent species (e.g., Brown (1972:1) noted, "To do science is to search 1973, Brown 1975, Bowers and Brown 1982), for repeated patterns." In this brief essay I the sizes of seeds taken by granivorous ro- will characterize the patterns of adaptation dents (e.g.. Brown and Lieberman 1973, of desert rodents that have been described Mares and Williams 1977), the distribution of largely within the conterminous United the seed resource in the desert and whether States. Realizing full well that "natural selec or not clumped seeds are favored by bipedal tion depends for its effectiveness on a series of chances" (Leigh 1971:221), I believe it is species (e.g., Reichman and Oberstein 1977, important to local Wondolleck 1978, Price 1978, Hutto 1978, distinguish between pat- terns and those of a global nature. Perhaps all Trombulack and Kenagy 1980), and on the important questions regarding life in deserts importance of microhabitat selection in can be answered by studying intensively one maintaining coexistence (e.g., Rosenzweig particular geographic unit—then again, per- 1973, 1977, 1979, Rosenzweig et al. 1975, haps not. If all deserts are not equal, a very Schroder and Rosenzweig 1975, Lemen and real problem develops in discovering which Rosenzweig 1978). patterns are truly generalizable. Each of these areas of research is con- troversial. For example, Lemen (1978) has strongly criticized the proposed seed The Patterns size-body size relationship, and support for The first problem that presents itself is that his position can be garnered from Stamp and of scale—does one seek patterns at the level Ohmart (1978), M'Closkey (1978), and others. of biochemical reactions, organ systems, or Ekrly indications that bipedal rodents are communities? The second problem is that of able to travel greater distances more rapidly confounding causation. Does bipedality de- and at lower energetic costs than quad- velop, for example, because of intrinsic prob- rupedal species (e.g., Dawson 1976) have lems related to integrated locomotor design been shown to be in error (Thompson et al. (e.g., Alexander 1975), or do such seemingly 1980), thus casting doubt on the validity of a unrelated factors as seed distributions, gran- linchpin in the theory relating locomotor ivory, predator avoidance, and substrate all mode (bipedality) to the habit of foraging on play a part in the selection of a particular widely dispersed seed clumps (see also Frye type of movement? Although it is easy to be- and Rosenzweig 1980). Evidence for body come overwhelmed by the complexity of size differences among coexisting competitors desert rodent adaptations, I will limit my has been challenged by Conner and Sim- analysis to characteristics above the purely berloff (1979) and Rebar and Conley (in biochemical level. This broad brush approach press). Even the basic premise that com- will give an overview of adaptations of desert petition has helped mold desert rodent com- rodents from the United States and will com- munities (Brown Munger and Brown 1976, pare these with rodents from other parts of has hypothesis that 1981) been shown to be a the world that have also successfully made is testable only the greatest difficulty, if with the transition to desert life. I will in essence it can be unambiguously tested at all (e.g., be assessing the available literature on desert Rosenzweig 1981). rodent biology for examples of convergence, The many basic studies done in the arid "the strongest sort of evidence for the effi- portions of the United States have made this cacy of selection and for its adaptive orienta- region one of the best studied areas on earth. tion of evolution" (Simpson 1953:171). 32 Great Basin Naturalist Memoirs No. 7

Physiological Adaptations Not all rodents inhabiting North American arid areas are desert specialists (e.g., Lee Water Balance—North America 1963, Andersen 1973, MacMillen and Chris- topher 1975). Although it is clear that the Perhaps one of the most widely known ability to withstand water deprivation has a traits of small mammals in desert regions is strong phylogenetic component (e.g., Hudson the ability to withstand water deprivation. and Rummel 1966, Fleming 1977), it can de- Schmidt-Nielsen (1964) has provided the velop readily in species inhabiting non- most complete summary of the complex ad- desertic habitats where water is scarce (e.g., aptations associated with this ability in North Fisler 1963, MacMillen 1964b). American rodents (see also Schmidt-Nielsen 1975, for a discussion of the mechanisms of water conservation in desert rodents). It is Water Balance—Other Deserts clear that withstanding either low free envi- ronmental water or high solute loads de- Because of the widespread nature of vari- mands numerous physiological and anatomi- ous physiological adaptations among species cal specializations. Certainly, the North of the North American fauna, one might ex- American Heteromyidae, kangaroo rats and pect that similar types of adaptations would pocket mice, are the most specialized rodents develop in other deserts. Despite the com- in this regard in the deserts of the United plexity of the suite of traits associated with States. Their adaptations include specialized water independence, this does not appear to kidneys, elongated renal papillae, long nasal be a particularly difficult path for evolution passages for countercurrent heat exchange, to follow. Indeed, water independence has and numerous other characteristics that mini- developed among one or more species of ro- mize water loss or increase their ability to dents from deserts in (e.g., MacMil- obtain vegetational water (e.g., Schmidt- len and Lee 1969, Baudinette 1972), Asia Nielsen 1964, Mullen 1971, Kenagy 1973a, (Winkelman and Getz 1962), India (e.g., Soholt 1975). Similar adaptations, although Ghosh 1975), North Africa (e.g., Burns 1956, perhaps not as pronounced, are known to oc- Kirmiz 1962 for Jactiliis, but see Ghobrial cur in North American cricetines (e.g., Ab- and Nour 1975), southern Africa (e.g., Chris- bott 1971, Andersen 1973), and sciurids (e.g., tian 1978, 1979), and Peru (Koford 1968). Hudson 1962, Maxson and Morton 1974). In The extensive Monte Desert of Argentina all these higher taxa, some species are ca- lacks water-independent species, although pable of producing fairly concentrated urine, EUgmodontia typus, a cricetine, is well reducing fecal and respiratory water loss, and adapted to process high concentrations of so- existing on minimal inputs of free or vegeta- dium chloride (Mares 1977a). Curiously, al- tional water. There is little doubt that the though Mares (1977b) did encounter a water physiological and anatomical adaptations of independent rodent in Argentina {Calomijs desert rodents that minimize water loss en- musculinus), it was an inhabitant of the mes- compass all the major systems of the organ- ic fringes of the desert. ism. For example, Hatton et al. (1972) Only a relatively small percentage of the showed that in desert rodents the cells of that desert rodents of the world has been exam- portion of the brain responsible for produc- ined physiologically. Similar adaptations may ing vasopressin (ADH) are multinucleate, a have developed repeatedly in all deserts of trait that is uncommon in rodents from moist the world. There is some question as to how habitats; this trait is very likely related to wa- physiologically specialized the dipodids are ter retention ability. They examined several (Ghobrial and Nour 1975), but there is little species from both New and Old World doubt that pronounced adaptations toward deserts. aridity have occurred in such disparate fami- As physiological studies are extended to lies as the , Dipodidae, Hetero- the arid portions of Mexico, numerous other myidae, and Sciuridae. Similar adaptations species will probably be foimd to be highly will probably be found in other families of adapted for existing in an environment hav- desert rodents (e.g., Octodontidae, ing minimal moisture available for ingestion. Ctenodactvlidae). 1983 Biology of Desert Rodents 33

The apparent regularity with which phys- these species did not produce exceptionally iological adaptations develop is illustrated by concentrated milk. A later study to examine their being characteristic not only of gra- whether or not these rodents actually re- nivorovis or herbivorous rodents, but of duced the amount of milk produced during insectivorous-carnivorous rodents (e.g., Whit- lactation (and thereby reduced water loss) ford and Conley 1971) and small marsupials was inconclusive (Baverstock and Elhay (e.g., Schmidt-Nielsen and Newsome 1962, 1979). What is really needed is a broadscale MacFarlane 1975, Morton 1980). study designed to examine all avenues of wa- Mares (1975a, b, 1976, 1977c) found that ter loss and to compare these across taxa. Emphasis not all rodents inhabiting the Monte Desert should be placed initially on gen- era of Argentina showed pronounced levels of that are known desert specialists (e.g., Dipodomys, physiological adaptation (see also Meserve Microdipodops, Perognathus, , Gerbillurus, 1978). Many species inhabit that region by Desmodillus, Me- riones, Dipus, Jaculus, Allactaga, etc.), rather limiting their activities to relatively mesic than on species that inhabit only the climatic microhabitats. In view of the widespread na- peripheries of deserts. Extreme adaptations ture of physiological adaptation toward a will be more easily detected than will the xeric existence. Mares (1975a, 1976) hypoth- fine shadings of "average" adaptations that esized that most of the rodents of the Monte have been modified to allow persistence only Desert had not reached the region until latest at the environmental peripheries of deserts. Pliocene, or even Pleistocene, times. Thus, there had not been sufficient time to evolve the complex group of physiological, ana- Other Physiological Adaptations tomical, behavioral, and ecological attributes characteristic of desert life. Various secretory glands are known in Although much work remains to be done desert rodents (e.g., from India, on the comparative physiology of desert ro- Wallace et al. 1973; Notomys from Australia, dents, pronounced convergence and paral- Watts 1975), but their function is not clear. lelism have occurred in all deserts as the re- The products of sebaceous glands in Di- sult of similar regimens of natural selection podomys may function as other than secre- acting on the colonizing stocks of rodents, re- tions to aid in the care of the pelage (Quay gardless of their phylogenetic affinities. This 1953). Whether or not such glands are wide- convergence (or parallelism, in some cases) spread among other taxa of desert rodents is extends to many aspects of the behavioral- unknown, but a comparative assessment of physiological-anatomical complex involved these structures could prove useful toward in osmotic balance. Similarities are seen in understanding their function. Eisenberg the stRicture of kidneys (e.g., Hudson 1962, (1963, 1975) discusses possible olfactory com- Schmidt-Nielsen 1964, MacMillen and Lee munication in desert rodents, an area of re- 1969, Abdallah and Tawfik 1969, Fleming search essentially unexplored in mammals, 1977), in their urine concentrating abilities, particularly desert rodents. in the ability of the to withstand des- Several species of desert rodents in the sication or elevated solute loads, in the elon- United States are known to undergo facul- gated nasal passages for heat exchange (this tative torpor: these species include cricetine characteristic is in need of comparative stud- rodents, heteromyids, and sciurids (e.g., Hud- ies), and in reduced fecal water loss. Only a son 1964, 1967, Tucker 1966, Chew et al. few studies have been done examining other 1967, Brown and Bartholomew 1969, Kenagy avenues of water loss in desert rodents and 1973b, Reichman and Van De Graff 1973, the adaptations that have evolved to mini- Reichman and Brown 1979). Presumably such mize these losses. For example, Kooyman a strategy allows a rodent to remain inactive (1963) shows that Dipodomys merriami pro- during periods of resource scarcity; however, duces a very concentrated milk (thus mini- periodic torpor is not limited to rodents from mizing lactational water loss). Working with xeric habitats (e.g., Hill 1977). It has been hy- native Australian rodents (Notomys, Pseud- pothesized that desert rodents have a lower omys), Baverstock et al. (1976) found that metabolic rate (irrespective of torpor) than 34 Great Basin Naturalist Memoirs No. 7 species from mesic habitats (e.g., McNab and to gross modifications in brain tissue. These Morrison 1963). Hayward (1965) questioned structures play a role in ADH secretion and this idea, suggesting that stored fat reserves thereby affect osmotic balance. Nevertheless, of laboratory animals had led to artificially from the viewpoint of convergent evolution, low metabolic rates. McNab (1968), however, it is interesting to know whether similar showed that lower metabolic rates for species structures have developed and whether or from xeric habitats (i.e., a North American not they function in similar ways. It is also cricetine, Peromyscus crinitus, and the naked instructive to learn that similar functions are mole rat of Africa [{Heterocephalus glaher), a performed by dissimilar structural bathyergid]) characterized individuals whose adaptations. body fat levels were well within normal lim- its. Yousef and Johnson (1975) found a corre- lation between the lower metabolic rate of Bipedality various North American desert rodent species Quite often, the term "desert rodent" con- (representing three families) and reduced notes the Dipodomys. Much research thyroxine secretion rate, suggesting a rela- has centered on species of Dipodomys, and tionship between thyroid activity and meta- kangaroo rats are almost synonymous with bolic rate; species from xeric areas had signif- "desert adaptation." Nevertheless, kangaroo icantly lower rates of thyroid activity than rats are but one of many genera inhabiting species from mesic habitats. North American deserts. It is probably be- Energy metabolism in North American cause of the familiarity of many scientists desert rodents has been examined in both the with Dipodomys that most desert rodents are laboratory (e.g., Dawson 1955, Yousef et al. assumed to mirror the adaptations character- 1970) and in the field (e.g., Mullen 1971, So- istic of that single genus. holt 1973, Kenagy 1973b). There are very Dipodomys are saltatorial and bipedal; few comparative studies available on rodents they are also granivorous. Because of the as- from other deserts (e.g., Dawson 1976, sociation between bipedality and granivory Thompson et al. 1980). in Dipodomys, a causal link between these The fact that many similar adaptations are characteristics has been suggested (e.g., common among species of the three families Reichman and Oberstein 1977). It is instruc- of rodents inhabiting North American deserts tive therefore to examine bipedality in some would lead one to speculate that similar traits detail. might be expected in other faunas. All infor- Several anatomical studies have examined mation to date supports the idea that similar bipedality in desert rodents (e.g., Hatt 1932, physiological strategies toward aridity have Howell 1932, Klingener 1964, Pinkham 1971, evolved independently and repeatedly Kaup 1976, Berman 1979). The most exten- throughout the world. sive study was that of Berman (1979), who compared hind limb osteology and myology Anatomical Adaptations in a broad spectrum of desert rodents of the world. She noted that bipedal saltation has North America arisen independently in five families of ro- dents: four of these (Heteromyidae, Di- Like physiological adaptations, anatomical podidae, Pedetidae, and Muridae) have their specializations for desert life are essentially bipedal species essentially restricted to xeric limitless—depending on one's scale, anatomy habitats, whereas the Zapodidae are forest can be viewed from the cell to the whole or- species. Small bipedal saltators have also aris- ganism. (3i)viously, an organism evolves as an en among extant and extinct marsupials. Ber- integrated luiit. Thus, viewing any structural man's analyses led her to conclude that there specialization without regard to its associ- has been a striking convergence in major ation with function lends a certain arti- musculoskeletal modifications of the hind ficiality to the analysis. For example, the limb of desert rodents. Similarities in struc- supraoptic nuclei described above (Hatton et ture are so pronounced tiiat unrelated bipe- al. 1972) are cellular specializations leading dal species were generally grouped more 1983 Biology of Desert Rodents 35 closely in multivariate space than were bi- 1954). In view of the large number of bipedal pedal and quadrupedal members of the same rodents that lack cheek pouches (including families. Her analyses also showed that there all pedetids, dipodids, and zapodids), the were nimierous significant differences among many quadrupedal species that have internal desert rodents in the ways in which biped- cheek pouches (e.g., cricetids, sciurids, etc.), ality had been achieved—different muscles and the presence of cheek pouches in fosso- were elongated or shortened, different me- rial geomyids and quadrupedal Perognathus, chanical advantages had evolved, and differ- Liomys, and Heteromys, there is little com- ent modifications characterized the feet. pelling support for this hypothesis. Mares (1980) examined the majority of One hypothesis that has been invoked to desert rodent genera in a multivariate analy- explain bipedality (although it has been tied sis of morphoecological characteristics. He to the pattern of seed distribution) is differen- noted that bipedality in North American tial microhabitat utilization. There is some deserts is restricted to granivores (although evidence that bipedal species forage in open many obligate granivores in North America areas more frequently than they do under are quadrupedal), but when all desert rodents shrubs (e.g., Rosenzweig and Winakur 1969, are examined, the supposed link between bi- Brown and Lieberman 1973, Rosenzweig pedality and seed eating is not found. There 1973, Brown 1975, Price 1978, Wondolleck are bipedal granivores (e.g., Dipodomys, Car- 1978); this observation appears to hold for bipedal diocranius, Stylodiptis, some Jacidus), Old World desert species as well (e.g., Nau- plant herbivores feeding on above-ground mov and Lobachev 1975), although rigorous parts (e.g., Pedetes, which also feed in below- quantification of this pattern is needed for all ground plant parts, Pygeretmus, Alactagulus, deserts, particularly those of the Old World. some AUactaga); bipedal herbivores feeding Nevertheless, if foraging in open areas is cor- on below-ground plant parts (some AUactaga, related with bipedality, then it is inferential some Jacidus), bipedal herbivores eating all evidence that predator avoidance is a pri- plant parts (e.g., some AUactaga, some Ja- mary selective factor of locomotor mode. cidus, Dipus, Paradipus); bipedal omnivores This is an old idea (e.g., Howell 1932) that (some AUactaga, Notomijs); and bipedal in- has been restated repeatedly (e.g., Eisenberg sectivores (Salpingotiis, the marsupial An- 1975, Berman 1979, Mares 1980), but ap- techinomys). In Old World deserts, most obli- pears to have merit. There is little doubt that gate granivores are quadrupedal (e.g., is an important factor in sparse Meriones, Gerbdlus, Tatera, Phodopus, Bra- desert habitats— evolutionarily opting to for- chiones, Sekeetamys, etc.). [Information on age in open microhabitats very likely forces the diets of the various genera can be found rodents into an entirely new adaptive mode, in Lobachev and Khamdamova (1972), Nau- that of bipedality. mov and Lobachev (1975), Happold (1975), Bipedality is also associated with other Prakash (1975), Watts (1977), and Wassif and anatomical adaptations for predator avoid- Soliman (1979).] (although some of these occur in quad- Thus, bipedality, when viewed on a global ance rupedal desert species as well). Enlarged scale, appears to have little relation to diet; bullae (e.g., Howell 1932, Webster 1962, Lay bipedal species fill all major trophic cate- pinnae (e.g., Howell 1932, gories. Although research limited to North 1972) or elongated adaptations for American desert species might be interpreted Eisenberg 1975) are probably Legiouix and Wisner as supporting a link between diet and loco- predator detection (e.g., it might be supposed motion, I find no evidence to support this hy- 1955, Lay 1974). While thermoregulation, pothesis in other deserts. that the pinnae function in In addition to elongated hind limbs, biped- as is the case in Lepus (Hill and Veghte al rodents have shortened forelimbs, prompt- 1976), in fact, the large pinnae of AUactaga ing suggestions that the freeing of the fore- are not well vascularized and do not function limbs for stuffing food into the cheek in heat loss (Hill et al. 1974). Bullar hyper- pouches was the primary selective force lead- trophy is common in desert rodents through- ing to bipedality (Bartholomew and Carey out the world and in other mammals as well 36 Great Basin Naturalist Memoirs No. 7

(e.g., Roig 1969, 1972). Fitzwater and Pra- 1975, Lockard 1978). Data from the Old kash (1969) described Meriones in India re- World are in accord with these observations sponding to the wingbeats of avian predators (Naumov and Lobachev 1975). Generally, by escaping into . most desert species are nocturnal (especially Finally, desert rodents are generally very bipedal species), although each desert has one pale colored, usually matching the desert or more species of diurnal rodents (usually soils (e.g., Harrison 1975, Mares 1976, these are herbivores, Mares 1980). Cloudsley-Thompson 1979). Most authors

is response concur that cryptic coloration a Autecology to visual predators (cf., Kaufman 1974). Some bipedal species possess a conspicuous black Smith and Jorgensen (1975) and Conley et and white tuft on the tip of the tail (almost al. (1977) review reproductive patterns in all bipeds have long tails with a terminal North American desert rodents, and French tuft). Tail tufts often regenerate if the tail has et al. (1975) and Wagner (1981) review de- been injured (Howell 1932), and it is likely mographic patterns of desert species through- that the tuft itself fimctions as a rudder that out the world. Heteromyids generally have allows the to turn abruptly in midair, small litters, relatively long life spans, low particularly since the wind resistance of the densities, and reproduce during moist and tuft acts at the end of a long lever arm. The warm times of the year. A complete review white tail tuft may well act as a flag to con- of desert rodent reproduction that includes fuse or distract predators during their pursuit species from each desert has not been pro- and/ or as a target for predator attack, thus duced. In addition to the above reviews, limiting an attack to a tail that may break there is some general information available quite easily and allow the rodent to escape. on reproduction for the following areas: Aus- An examination of the morphology of tralia (Smith et al. 1972, Crichton 1974, desert rodents leads to the conclusion that Watts 1979, Aslin and Watts 1980); USSR convergent evolution of structures that re- (Naumov and Lobachev 1975); North Africa duce the probability of predation is a major (Poulet 1972, 1978, Khammar et al. 1975, evolutionary force. Happold 1975, Ghobrial and Nour 1975, Amirat et al. 1977); southern Africa (Nel Behavioral and Autecological 1978, Christian 1979, 1980, Butynski 1979); Adaptations (Lay 1967, Misonne 1975); India (Pra- kash 1975); (Beg et al. 1977); Chile Behavior (Fulk 1975). Although demography has been studied in Eisenberg (1975) has done the most com- some detail in North American desert rodents prehensive comparative behavioral work (see above citations), there have been few ex- with desert rodents. Most are nocturnal; most tensive demographic studies in either South live in burrows that are plugged during the American deserts or in the Old World. Most day. There are many differences among spe- of these can be located using those citations cies in aspects of social behavior, but many referring to reproductive patterns (see also species in disjunct deserts have remarkably Pearson and Ralph, 1978, for Peru). similar behavioral patterns. Unfortunately, little quantitative behavioral research has Synecology been done on other than North American species, and even these have been studied Perhaps the most exciting area of desert primarily in the laboratory. Studies on Old ecology today is that dealing with species in- World species include Nel (1975), Daly and teractions and community organization.

Daly ( 1975a, b), and Agren ( 1979). Brown et al. (1979) and Mares (1980) review Some workers have examined activity pat- much of this literature. Research done in terns of desert rodents (e.g., Schwab 1966, Ja- North America would suggest that deserts hoda 1973, Kenagy 1973b, 1976, Lockard support elevated levels of both species rich- and Owings 1974, Rosenzweig 1974, French ness and abundance. However, Mares (1979) 1983 Biology of Desert Rodents 37 has argued that the deserts of the United cused on manipulative field experiments States support an unusually high diversity of (particularly the work of Rosenzweig, Brown, species due to their unique Pleistocene his- Reichman, and their associates, see above ci- tory of refugial formation wherein allopatric tations). Unfortunately, there has been no speciation processes were amplified. High parallel movement in experimental research relative abundance of rodents in U.S. deserts in deserts outside of the United States (or is probably related to the elevated rainfall even outside the Sonoran Desert). Theory has characterizing much of the North American far outstripped our empirical data base in desert system (e.g.. Brown et al. 1979). Much desert ecology and experimental data are U.S. desert research has been conducted in only beginning to be applied to the many hy- the Sonoran Desert of southern Arizona, a re- potheses that currently abound in the gion that some consider a semidesert due to literature. its relatively high precipitation (e.g., Eisen- Recent studies dealing with competition berg 1975). This preponderance of research between distantly related taxa promise excit- in an extremely productive area may have ing results if they can be replicated in other led to a fairly common belief that deserts of- deserts (e.g., Brown 1976, Brown and David- ten support many small mammals. Actually, son 1977, Davidson et al. 1980). Mares and most deserts seem to support few species of Rosenzweig (1978) have done comparative desert rodents at fairly low levels of abun- work on this topic and found different pat- dance (e.g., Mares 1976, 1980, Pearson and terns in North and South American deserts— Ralph 1978, Morton 1979, Brown 1980, they offer an evolutionary explanation for Christian 1980), although some areas seem to different strategies of granivory in distantly be equally as rich in species as portions of the related taxa. U.S. desert system (e.g., Nel 1978). Perhaps the area of research that has been Just how desert species manage to coexist most neglected is that of comparative faunal is the major area of research at the moment, studies. Mares (1975, 1976, 1980), MacMahon with competition assumed to be a primary (1976), Mares et al. (1977a, b). Mares and selective force leading to observed patterns Hulse (1977), Pearson and Ralph (1978), and of microhabitat selection (Rosenzweig 1979), Morton (1979) have attempted to compare body size differences (Bowers and Brown quantitatively diverse desert rodent assem- 1982), or differential utilization of the seed blages. Unfortunately, such studies are ham resource (e.g., Reichman and Oberstein pered by a paucity of data for deserts outside 1977). Little comparative work that might of the United States. As data accrue from shed light on current controversial points has current desert research, and as statistical and been done in deserts outside the United computational techniques are refined, there States, but certainly habitat specificity is a should be a great deal of information forth- well-known factor characterizing small mam- coming on the ways in which desert rodent mal communities (e.g., Hubert et al. 1977). communities assemble over time. Nevertheless, Pearson and Ralph (1978:75) found that small species richness in Closing Comments several desert habitats in Peru could be ex- plained by "evolutionary and zoogeographic- If one were to go into an unknown desert al accident," rather than habitat selection region, there are many predictions that could differences. be made concerning the small mammal fauna One reason that controversy surrounds co- (particularly the rodent fauna) of the area. existence studies in deserts is that most re- Beginning at the most basic levels (anatomy search to date has been descriptive and infer- and physiology), we could say that at least ential. Studies dealing with seed selectivity some rodents inhabiting the area would ex- by rodents have had to contend with the hibit the following adaptations: specialized enormous variability in background seed lev- kidneys (with elongated renal papillae and els and the methodological difficulties of micro- and macroscopic morphological adap- samphng the seed resource (e.g., Brown et al. tations) able to concentrate the urine and 1979). Nevertheless, recent trends have fo- perhaps process high electrolyte loads; a 38 Great Basin Naturalist Memoirs No. 7

counter-current heat exchange system in the similarities in adaptive strategies become nasal region; modified brain cells responsible evident. for ADH secretion; lowered metabolic rate; At the community level, however, our pre- facultative torpor; ability to exist without dictions become more tenuous. Our hypo- free water; minimization of water loss thetical desert would probably possess a bi- through respiratory, excretory, and defeca- pedal and/or a quadrupedal granivore; a tory pathways; inflated tympanic bullae or micro-omnivore; a medium (squirrel)-sized elongated pinnae; bipedality (some species)— diurnal omnivore; a small insectivore; a bi- or a fossorial medium-sized herbivore with foreshortened forelimbs, long tails, con- pedal eating below-ground plant parts; and a larger centrations of muscle mass in proximal limb herbivore (rabbit size). Species richness regions, smaller mechanical advantages for would be low (although high species richness hind limb muscles, elongated distal limb seg- would not be surprising, particularly if the ments, toe reduction, terminal tuft of hair on biogeographic history indicated a multiple- the tail (often colored black and white); se- refugial system). Bipedality could occur in all baceous glands would be present—sand bath- trophic categories except the completely fos- ing would be common; dorsal coloration sorial niche. Coexisting species might exhibit would match the background (pale colors regular patterns of body size differences, and predominating) and countershading would be microhabitat selection might be the primary pronoimced; species living on sand would mechanism maintaining coexistence. Gra- hirsute feet; eyes have extremely hind would nivorous rodents might show inverse relation- be placed dorsally; vibrissae would be abun- ships in abundance and diversity to the abun- dant and long; white flank markings would dance and diversity of other granivores, such be common in bipedal species. There are as ants or birds. Ants and rodents might be many other physiological and anatomical mutualistic over evolutionary time; thus, a traits that would very likely characterize the lack of mammalian seed predators could rodents of this unexplored desert. prove detrimental to ant seed predators.

Above the systemic level, we could predict There is some controversy as to whether or

the possession of numerous autecological not there is convergence at the community traits: noctumality would predominate (par- level (Schall and Pianka 1978). Certainly ticularly in bipedal species); both diurnal and community studies based in morphometries nocturnal species would inhabit burrows— will have a proportion of their overall sim- these would be plugged during hot periods; ilarity explained by morphological con- bipedal species would differentially forage in vergence. However, since morphology often

open microhabitats, and quadnipedal species reflects function, there is strong evidence that exists would favor , closed microhabitats; bipedal pronounced convergence above fonns would occur in flat areas having few the systemic level of organization. It is equal- rocks; reproduction would be associated with ly clear, however, that strong commmiity the rainy season, with birth taking place after convergence is yet to be demonstrated when only utilized in the rains—populations would peak at this ecological parameters are the faunal comparisons. This is that time; territoriality would be pronounced; not to say such convergent evolution does not exist, but home ranges would be relatively large; survi- rather that the influence of history on faunal vorship would be high and fecunditv low development and our inability to quantify (e.g., French et al. 1975); population levels rigorously the many ecological attributes of a would generally be low (although they are fauna (and to produce highly predictive and often quite high in North American deserts). quantitative theories) have not yet allowed us Clearly, at the levels of organization from to assess the presence or absence of commu- population down to cell, there are niunerous nity convergence. Our best work is yet to be predictions that could be made regarding the done. The complexity of the seemingly suite of desert adaptations that would charac- simple desert ecosystem has not yielded to in- terize our unknown species, and the lists ferential science—the ability of experimental presented are far from exhaustive. As our lev- science to clarify the many remaining el of understanding is refined, more and more enigmas is yet to be tested. 1983 Biology of Desert Rodents 39

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