Dipodomys Deserti Research Articles

Boundary processes between a desert sand dune community and an encroaching suburban landscape

In contrast to the body of work in more mesic habitats, few studies have examined boundary processes between natural and anthropogenic desert landscapes. Our research examined processes occurring at boundaries between a desert sand dune community and an encroaching suburban habitat. We measured responses to an anthropogenic boundary by species from multiple trophic levels, and incorporated measures of habitat suitability, and temporal variation, at multiple spatial scales. At an edge versus core habitat scale the only aeolian sand species that demonstrated an unambiguous negative response to the anthropogenic habitat edges was the flat-tailed horned lizard ( Phrynosoma mcallii). Conversely loggerhead shrikes ( Lanius ludovicianus) demonstrated a positive response to that edge. At a finer scale, species that exhibited a response to a habitat edge within the first 250 m included the horned lizards along with desert kangaroo rats ( Dipodomys deserti). The latter species’ response was confined to 25 m from the edge. For the flat-tailed horned lizard, edge effects were measured up to 150 m from the habitat boundary. Three potential causal hypotheses were explored to explain the edge effect on horned lizards: (1) invasions of exotic ant species reducing potential prey for the lizards; (2) road avoidance and road associated mortalities; and (3) predation from a suite of avian predators whose occurrence and abundance may be augmented by resources available in the suburban habitat. We rejected the exotic ant hypothesis due to the absence of exotic ants within the boundary region, and because native ant species (prey for horned lizards) did not show an edge effect. Our data supported the predation and road mortality hypotheses. Mechanisms for regulating population dynamics of desert species are often “bottom-up,” stochastic processes driven by precipitation. The juxtaposition of an anthropogenic edge appears to have created a shift to a “top-down,” predator-mediated dynamic for these lizards.

Short interspersed elements (SINEs) of the Geomyoidea superfamily rodents

A new short interspersed element (SINE) was isolated from the genome of desert kangaroo rat ( Dipodomys deserti) using single-primer PCR. This SINE consists of two monomers: the left monomer (IDL) resembles rodent ID element and other tRNA Ala(CGC)-derived SINEs, whereas the right one (Geo) shows no similarity with known SINE sequences. PCR and hybridization analyses demonstrated that IDL-Geo SINE is restricted to the rodent superfamily Geomyoidea (families Geomyidea and Heteromyidea). Isolation and analysis of IDL-Geo from California pocket mouse ( Chaetodipus californicus) and Botta's pocket gopher ( Thomomys bottae) revealed some species-specific features of this SINE family. The structure and evolution of known dimeric SINEs are discussed.

Water economy of free-living desert animals

Deserts are relatively dry and dehydrating places, by definition, but desert animals are composed of 65–75% water, just like non-desert animals. Do desert animals maintain water balance and body hydration level by conserving water better (decreasing output, as compared with non-desert species) or by getting more water each day (increasing input), or both? These questions can be evaluated with measurements of daily water flux rates (in ml/day) as determined with isotopically labeled water through free-ranging animals that are maintaining constant body masses. Inspection of the many field measurements now available reveals great variation (10 million-fold among terrestrial vertebrates alone), due largely to differences in body mass, but also due to taxonomic affinity (endothermy vs. ectothermy), season, diet and age, as well as to variation in habitat aridity. Fortunately, water flux rates can be normalized for much of this variation by dividing them by field metabolic rates, to yield units of ml water used per kJ energy metabolized (termed Water Economy Index, WEI). The WEI values of desert-dwelling eutherian mammals are significantly lower than in non-desert eutherians, and (surprisingly) desert endotherms have lower WEIs than desert ectotherms. Year-round WEI measurements on desert kangaroo rats, which have the lowest index yet measured, revealed that they often obtained more water than was available just from dry seeds, not by drinking, but by eating young green vegetation when available in spring, and especially by hydrating dry seeds before consumption by storing them in humid burrows for a while.

How does the ecological foraging behavior of desert kangaroo rats (Dipodomys deserti) relate to their behavior on radial mazes?

Experiment 1 showed that laboratory-reared desert kangaroo rats, like domestic Norway rats, efficiently search for food on a radial arm maze (RAM) by avoiding revisiting arms within a trial. By placing an RAM on the floor so the animals could approach food from any direction, Experiment 2 tested whether efficient search by kangaroo rats was based on tactics of distance minimizing, central-place foraging, trail following, or meandering. In contrast to the dominant trail-following tactic of domestic Norway rats (Hoffman, Timberlake, Leffel, & Gont, 1999), kangaroo rats tended to distance minimize, whether maze arms were present or not. Experiment 3 indicated that kangaroo rats treated a floor configuration of eight food cups as two patches of four, based on beeline travel between patches and meandering within them. We conclude that similar performance in an elevated RAM by different species can be based on different tactics, and we suggest that a laboratory apparatus can be used to cast light on niche-related mechanisms.

Absence of aquaporin-4 water channels from kidneys of the desert rodent Dipodomys merriami merriami.

Recently, we found that aquaporin-4 (AQP4) is expressed in the S3 segment of renal proximal tubules of mice but not in rat proximal tubules. Because mice have relatively larger papillae than rats, it was proposed that the renal distribution of AQP4 in various species could be related to their maximum urinary concentrating ability. Therefore, kidneys and other tissues of Merriam's desert kangaroo rat, Dipodomys merriami merriami, which produce extremely concentrated urine (up to 5,000 mosmol/kgH(2)O), were examined for AQP4 expression and localization. Contrary to our expectation, AQP4 immunostaining was undetectable in any region of the kidney, and the absence of AQP4 protein was confirmed by Western blotting. By freeze fracture electron microscopy, orthogonal arrays of intramembraneous particles (OAPs) were not detectable in plasma membranes of principal cells and proximal tubules. However, AQP4 protein was readily detectable in gastric parietal and brain astroglial cells. Northern blotting failed to detect AQP4 mRNA in kangaroo rat kidneys, whereas both in situ hybridization and RT-PCR experiments did reveal AQP4 mRNA in collecting ducts and proximal tubules of the S3 segment. These results suggest that renal expression of AQP4 in the kangaroo rat kidney is regulated at the transcriptional or translational level, and the absence of AQP4 may be critical for the extreme urinary concentration that occurs in this species.

Assessment and defence of solitary kangaroo rats under risk of predation by snakes

Prey that spend a large proportion of their time foraging must trade-off between predator defence and feeding. Because predation risk can be a cost in desert granivores, we predicted that kangaroo rats (Dipodomys) should weigh the risk of predation when deciding which behavioural option to pursue. We manipulated apparent predation risk of the giant kangaroo rat, D.ingens, and the desert kangaroo rat, D.deserti, and predicted they should be more vigilant and show greater defensive behaviour to the more dangerous stimulus of a real snake than to an artificial snake decoy. Results showed that kangaroo rats decreased foraging with increased vigilance when the snake and decoy were present. The largest decrease in foraging was in response to the live snake, and both species oriented towards, approached and footdrummed more in the presence of the live snake than the decoy. The species also differed in their antipredator behaviour: D.ingens footdrummed more and spent more time within striking distance of the snake than the decoy. Dipodomys deserti spent more time within 1m of the snake than the decoy and kicked sand at the snake. Sand kicking was never observed inD.ingens . We conclude that kangaroo rats are able to discriminate predation risk. They decrease foraging and increase vigilance in the presence of live snakes to assess risk and may approach and footdrum as a pursuit deterrent to communicate to the snake its chances of ambush are no longer available to cause the snake to leave. Sand kicking may function to harass the snake to cause it to leave.

Paleobiogeographic Changes at the Pleistocene–Holocene Boundary near Pintwater Cave, Southern Nevada

In 1996, approximately 70,000 mammal and lizard bones were recovered from Pintwater Cave in the northern Mojave Desert of southern Nevada. These bones date between 32,000 and 7350 14C yr B.P. Between 32,000 and 10,100 14C yr B.P. the local fauna consisted of a mix of xeric- and cool/mesic-adapted species. Ochotona princeps and Thomomys talpoides then occupied the region, although these animals were extirpated by the onset of the middle Holocene. Sauromalus obesus and Dipodomys deserti probably migrated to the region during the latest Pleistocene. Dipsosaurus dorsalis entered the Pintwater Cave record after 8000 14C yr B.P. Consistent with climatic interpretations for the northern Great Basin, these data suggest a cool and moist latest Pleistocene climate for the northern Mojave Desert. In contrast to the northern Great Basin, however, this region experienced predictable summer precipitation coupled with increasingly warmer winters by 10,100 14C yr B.P. In both regions, the warm middle Holocene began ca. 8300 14C yr B.P. However, whereas the northern Great Basin probably experienced warm and dry conditions at that time, the northern Mojave Desert remained warm with relatively predictable summer precipitation. The modern northern Mojave Desert biota probably was not established until after 8300 14C yr B.P.

Species-specific footdrumming in kangaroo rats:Dipodomys ingens,D. deserti,D. spectabilis

Footdrumming was compared in three allopatric species of kangaroo rat,Dipodomysfrom three habitats. Analysis of footdrumming recordings revealed species-specific patterns of drumming ranging from single thumps to individual footdrumming signatures. The desert kangaroo rat,D.desertidrums single thumps spaced 0.25–0.30s apart that are sometimes introduced with a short footroll. The giant kangaroo rat,D.ingensdrums long footrolls that can average over 100 drums at 18drums/s. The banner-tailed kangaroo rat,D.spectabilisdrums three to 38 footdrums in a footroll combined into sequences of two to 12 footrolls at a rate of 17drums/s. In playback tests, all three species stood in alert postures and entered the burrow in response to footdrumming of their own and the other species. The rats also responded in species-specific ways.Dipodomys spectabilisdrummed to its own species’ footdrumming, but not to playbacks of the single drums ofD.desertiInstead of footdrumming to playbacks of its own species,D.desertiapproached the speaker more frequently than did either of the other two species.Dipodomys ingensfootdrummed equally to all footdrumming playbacks. The species’ differences reflect differences in social tolerance and spacing.Dipodomys desertirarely engages in footdrumming exchanges and chases visitors from the burrow.Dipodomys spectabilisengages in frequent footdrumming exchanges and some chases, andD.ingensseems to tolerate close neighbours and footdrums periodically.

Comparison of low-frequency communication by footdrumming in three species of solitary, desert rodent, kangaroo rats (Dipodomys)

The ears of desert rodents are well adapted for receiving low-frequency vibrations created during footdrumming. Kangaroo rats drum species-specific patterns, ranging from single thumps to individual footdrumming signatures. The desert kangaroo rat, D. deserti, drums single thumps spaced 0.25 to 0.30 s apart. The giant kangaroo rat, D. ingens, drums long footrolls that can average over 100 drums at 18 drum/s. The banner-tailed kangaroo rat, D. spectabilis, drums footdrumming signatures consisting of 3–38 footdrums in a footroll combined into sequences of 2–13 footrolls. In playback tests, all three species stood alert and entered the burrow in response to footdrumming of their own and other species. The rats also responded in species-specific ways. D. spectabilis drummed to its own species’ footdrumming, but not to playbacks of single drums of D. deserti. D. deserti did not footdrum in response to the playbacks, but approached the speaker more frequently than either of the other two species. D. ingens footdrummed equally to all footdrumming playbacks. The species’ differences reflect differences in social tolerance and spacing. D. deserti chases visitors from the burrow, D. spectabilis engages in frequent footdrumming exchanges, and D. ingens tolerates close neighbors and footdrums periodically.

Kangaroo rats exhibit spongiform degeneration of the central auditory system similar to that found in gerbils

Kangaroo rats develop spongiform degeneration of the central auditory system similar to that seen in the gerbil. Light microscopic and transmission electron microscopic study of the cochlear nucleus and auditory nerve root (ANR) of Dipodomys deserti and D. merriami show that spongiform lesions develop in dendrites and oligodendrocytes of the cochlear nucleus and in oligodendrocytes of the ANR that are morphologically indistinguishable from those extensively described in the Mongolian gerbil, Meriones unguiculatus. As in Mongolian gerbils, the spongiform degeneration in Dipodomys were much more numerous in animals continually exposed to modest levels of low-frequency noise ( < 75 dB SPL). The kangaroo rats with extensive spongiform degeneration also show slightly, but significantly, elevated auditory brainstem evoked response (ABR) thresholds to low-frequency stimuli, a result also found in Mongolian gerbils. These results suggest that the elevated ABR thresholds may be the result of spongiform degeneration. Because low-frequency noise-induced spongiform degeneration has now been shown in the cochlear nucleus of animals from separate families of Rodentia (Heteromyidae and Muridae), the possibility should be investigated that similar noise-induced degenerative changes occur in the central auditory system of other mammals with good low-frequency hearing.

CAMBIOS EN LA COMPOSICION DE LAS COMUNIDADES DE ROEDORES EN RELACION A LOS TIPOS DE VEGETACIÓN Y GEOMORFOLOGÍA EN EL PINACATE, SONORA, MEXICO

Durante la primavera de 1981, se realizaron muestreos de roedores en las nueve comunidades vegetales principales del Gran Desierto de Sonora. Se registraron 16 especies de roedores: tres de sciúridos, una de geómido, siete de heterómidos y cinco de múridos. La subfamilia Perognathinae fue el grupo más diverso, con cinco especies. Se determinó su distribución respecto a las asociaciones vegetales y al sustrato. Se encontró que los suelos firmes fueron los más utilizados por los roedores. En ellos se localizó al 93.75% de las especies; el ardillón [SpermophHus variegatus) y la rata nopalera {Neotoma albigula) caracterizaron a los sustratos rocosos. Solamente la rata canguro (Dipodomys deserti) fue exclusiva de las dunas más arenosas. Esta última especie separó claramente dentro de la clasificación que se hizo de los sitios con base en las especies de roedores, a los sitios arenosos inestables de los arenosos consolidados. El ratón de bolsas {Perognathus longimembris) y las ratas {Neotoma albigula y N. lepida) separaron dentro de la clasificación a los habitats más diversos de los menos diversos. La vegetación riparia fue importante en cuanto a la diversidad y abundancia de los roedores

Interactions Between Predation Risk and Competition: A Field Study of Kangaroo Rats and Snakes

The effects of predation risk from snakes on microhabitat use of kangaroo rats (Dipodomys deserti and D. merriami) was studied in the Mojave Desert. I concentrated on the effects of the predator on the foraging behavior of the rodents and eliminated effects of prey capture on prey population size. Foraging stations contained three seed trays, one in each of three microhabitats (Open, Bush, Grass). The amount of seed left in each tray after one night of foraging was used to assess the responses to predation risk and other foraging costs; additional data were collected during direct observations and by trapping. To investigate the effect of snakes on foraging and microhabitat use of kangaroo rate I manipulated snake presence at the stations. I studied the interactions between predation risk from snakes and moonlight by conducting experiments near full and new moon nights. Both species of kangaroo rats preferred to forage in the Open and avoided the Bush. The preference is opposite to the preference of the main rodent—eating snake at the study site, the sidewinder (Crotalus cerastes), which prefers the Bush over the Open. At stations with snakes, D. deserti reduced its foraging and avoided the Bush more than in control plots. However, D. merriami foraged more at stations with snakes. D. merriami thus foraged at the trays that were avoided by D. deserti and it reduced the risk of interference from the dominant D. deserti. In contrast to studies by other investigators, moonlight (which is associated with increased risk from owls) did not reduce the foraging or affect the micro habitat use of kangaroo rates in summer; moonlight effect was seen only in the fall, when snakes were not active. Unlike risk from owls, risk from snakes is high under bushes and during dark nights. The activity or rodents in summer is a combined reaction to the different predation risks posed by snakes and by owls.

Spatial memory in the desert kangaroo rat (Dipodomys deserti).

Desert kangaroo rats (Dipodomys deserti) forage for seed distributed in patches in the desert environment and may remember patch locations. In Experiment 1, 7 desert kangaroo rats that had discovered the location of a plastic token in 1 box accurately dug for a token hidden in the same location in a 2nd identical box. Results of Experiment 2 indicated that the rats primarily remembered the spatial location of the token within the box in relation to extramaze objects and the walls of the experimental box. Female rats also remembered the chip's location in relation to objects inside the box, but males did not. Experiment 3 demonstrated that the rats' ability to locate the buried token did not depend on detection of the odor of the token. In the discussion I propose that spatial memory in kangaroo rats may have evolved as a result of an overall change in the ontogeny of the species rather than as a specialized adaptation for foraging efficiency.

Patch use by Dipodomys deserti (Rodentia: Heteromyidae): profitability, preference, and depletion dynamics.

Granivorous desert rodents of the family Heteromyidae forage nonrandomly among "microhabitats" that vary in substrate, seed densities, and seed species composition. To explore the hypothesis that microhabitat use is sensitive to seed patch profitability, we quantified effects of seed size (1.96 vs. 5.21 mg/seed) and density (0.4-10.6 seeds/cm2) on Dipodomys deserti harvest rates, which is a measure of profitability when expressed as mg of seed taken per min. By manipulating seed density, we created large-seed and small-seed patches of known relative profitability and exposed D. deserti individuals to pairwise choices in the laboratory and field. We used three treatment classes: 1) large-seed patches that were more profitable than small-seed patches (equal seed densities); 2) large-seed and small-seed patches that were equally profitable (small-seed densities somewhat higher): and 3) large-seed patches that were less profitable than small-seed patches (small-seed densities much higher). Harvest rate increased nearly linearly with seed density, and profitability of large-seed patches was greater than small-seed patches of the same density. Cumulative harvest from a patch increased linearly with residence time up to a plateau; this "gain curve" indicates that animals move systematically within patches and hence avoid resampling already depleted areas. In the laboratory, animals visited small-seed patches first more often and visited them more frequently when they were more profitable than large-seed patches. When large-seed patches were of greater or equal profitability, large-seed patches were preferred by both measures. The expressed preference for large-seed patches, when animals were presented with equally profitable patches, suggests an underlying preference for large seeds. In the field, animals depleted all patches to a constant low profitability, in accord with qualitative predictions of optimal patch use models. These results suggest that patch preferences by D. deserti are affected by the economics of seed harvest.

New observations on the genus Geomylichus Fain, 1970 (Acari, Listrophoridae) with description of four new species and a new subgenus

Abstract Four new species of Geomylichus Fain, 1970 (Acari, Listrophoridae) are described from North American heteromyid rodents: Geomylichus (G.) microdipodops sp.n. from Microdipodops megacephala, G. (G.) formosus sp.n. from Perognathus formosus, G. (G.) utahensis sp.n. from Dipodomys microps and G. (Whitakerobius) deserti sp.n. from Dipodomys deserti. The female of G. (G.) dipodomius (Radford) is redepicted and figures of G. (Ageomylichus) mexicanus Fain and G. (Neogeomylichus subg.n.) postscutatus Fain are given for the first time. A key to the known species of the genus is provided.

Owl Predation on Desert Rodents Which Differ in Morphology and Behavior

Many characteristics of desert rodents are beneficial in avoiding predation. For example, hyperinflated auditory bullae found in the genera Dipodomys and Microdipodops improve auditory acuity and allow the detection of approaching predators (Webster, 1962; Webster and Webster, 1971); elongated hind legs and bipedal locomotion aid in escape (Bartholomew and Caswell, 1951; Eisenberg, 1963). In addition, microhabitat utilization in Dipodomys shifts in response to increased predatory risk due to increased illumination (e.g., Kotler, 1984) or the presence of predators (J. S. Brown, unpubl. data), and even species coexistence may be influenced by predation (Kotler, 1984; Thompson, 1982). However, species lacking in conspicuous anti-predator morphology (e.g., Perognathus, Peromyscus) restrict their foraging to the safety of cover (e.g., Kotler, 1984). Furthermore, Kotler (1984) has suggested that predation has had great influence on community organization of desert rodents through its effect on microhabitat utilization. If this is true, the relative rates of predation on coexisting desert rodent species should be influenced by particular morphological adaptations of rodents, such as bullar volume or types of locomotion (Clarke, 1983; Webster and Strother, 1972), and behavior (microhabitat preference). Animals with anti-predator morphology should suffer lower rates of predation despite using open microhabitats more than do animals lacking such morphology. To assess this, I examined predation by long-eared owls (Asio otus) on a community of desert rodents whose species differ in abundance, morphology, and use of microhabitat while foraging. The study was conducted on a 6 km2 area of semi-stabilized sand dunes in the Great Basin Desert located at Tonopah Junction, 12 km south of Mina, Mineral Co., Nevada (elevation 1,343 m). The dominant vegetation on the dunes is greasewood (Sarcobatus vermiculatus), a stiff, spiny shrub which covers approximately 30% of available space. The rodent species present on or near the dunes included: Merriam's kangaroo rat (Dipodomys merriami), Ord's kangaroo rat (D. ordii), Great Basin kangaroo rat (D. microps), desert kangaroo rat (D. deserti), pallid kangaroo mouse (Microdipodops pallidus), little pocket mouse (Perognathus longimembris), Great Basin pocket mouse (P. parvus), Southern grasshopper mouse (Onychomys torridus), desert woodrat (Neotoma lepida), white-footed deer mouse (Peromyscus maniculatus), and Western harvest mouse (Reithrodontomys megalotis). Dipodomys and Microdipodops are bipedal and have hyperinflated auditory bullae. Perognathus is quadrupedal and has only modestly inflated bullae. The species in the remaining genera are all quadrupedal and lack inflated bullae. Owl pellets were collected from a permanent natural roost of long-eared owls located in the middle of the study site. All pellets were collected prior to the start of the study and then in July and November, 1979; March, July, and November, 1980; and May and July of 1981. All pellets in the roost were collected each time. Pellets were gently picked apart, and all large bones and fragments were removed and saved. Individual prey items were identified to species from entire skulls or large skull fragments. This yielded a count of the minimum number of each species appearing in the pellets each period (Pearson and Pearson, 1947). Rodent populations were censused prior to each pellet collection (except July 1981) by live trapping using parallel census lines and assessment lines (O'Farrell et al., 1977). Assessment line techniques yield a more accurate estimate of the area being sampled by the trap lines, but do not allow error bounds to be placed on the density estimates. Two parallel census lines were spaced 40 m apart; each consisted of 23 stations with stations 20 m apart. At each station, one live trap was placed in each of four microhabitats (Brown and Lieberman, 1973): I. under a bush; II. at the edge of a bush; III. 1 m from the edge of a bush; IV. 2 to 8 m from the edge of a bush. Three assessment lines, 120 m apart, cut the census lines at 45? angles. Each line contained 15 trap stations at 20 m intervals, with 4 traps placed at each station, as described above. During each census, the census lines were trapped first for 2 to 4 nights, until new animals comprised 824 Vol. 66, No. 4

Bipedal Hopping and Seed‐dispersion Selection by Heteromyid Rodents: The Role of Locomotion Energetics

Selection of seeds with different dispersion patterns, facilitated by the energetic differences between bipedal—hopping and quadrupedal heteromyid rodents, has been proposed as a mechanism of resource allocation for microsympatric species. To test several aspects of this hypothesis, field observations of foraging behavior and laboratory measurements of locomotion costs were made for several species of heteromyids. Neither the field observations nor the laboratory measurements supported predictions generated by the seed—dispersion hypothesis. Locomotion costs of the bipedal—hopping Dipodomys deserti and D. merriami were similar to those predicted for quadrupeds of equal mass. At low speeds, the cost of quadrupedal locomotion was not greater for a small, hopping biped than would be expected for a similar—sized quadruped. Free—living Dipodomys deserti, D. merriami, and Perognathus longimembris spent most of their foraging time either stopped or moving at slow speeds. High—speed locomotion, at which energetic "savings" by bipeds should be greatest, was rare. These results led to the conclusion that the energetic costs of different locomotory modes do not play a major role in resource allocation for Mojave Desert heteromyids.

Changes in the thermal characteristics of hypothalamic neurons during sleep and wakefulness

The characteristics of the mammalian thermoregulatory system are dependent upon arousal state. During NREM sleep thermoregulatory mechanisms are intact but body temperature is regulated at a lower level than during wakefulness. In REM sleep thermoregulatory effector mechanisms are inhibited and thermal homeostasis is severely disrupted. Thermosensitivity of neurons in the preoptic/anterior hypothalamus (POAH) was determined for behaving kangaroo rats ( Dipodomys deserti) during electrophysiologically defined wakefulness, NREM sleep and REM sleep to elucidate possible neural mechanisms for previous findings of state-dependent changes in thermoregulation. Thirty cells were tested during at least two arousal states. During wakefulness, 70% of the recorded cells were sensitive to changes in local temperature, with the number of warm-sensitive (W) cells outnumbering cold-sensitive (C) cells by 1.6:1. In NREM sleep, 43% of the cells were thermally sensitive, with the ratio of W:C remaining the same as in wakefulness. In REM sleep only two cells were thermosensitive (both W). The decrease in neuronal thermosensitivity of POAH cells during REM sleep parallels findings of inhibition of thermoregulatory effector responses during REM, although further work is necessary to determine the source and nature of the inhibition.

Risk of Predation and the Structure of Desert Rodent Communities

Communities of granivorous desert rodents may be influenced by either predation risk or resources. To examine the influence of these factors, I manipulated illumination, using lanterns, and resources, using seeds. Foraging behavior is responsive to changes in predation risk; increased illumination reduces foraging in open areas without cover. Foraging behavior is also affected by resource enrichments. Differences among species in habitat selection are correlated with specific abilities to detect and avoid predators. The last vulnerable species, Dipodomys deserti, foraged heavily in the open and was largely unaffected by treatments; the other species of kangaroo rats and kangaroo mice (Dipodmys merriami, Dipodomys microps, and Microdipodops pallidus) also prefer the open, but responded to both risk and resource manipulations; highly vulnerable Peromyscus maniculatus was restricted to bushes, even under the best of circumstances; Perognathus longimembris was displaced from preferred microhabitat by the presence of kangaroo rats. A correlation between auditory bullar volume and use of open habitat by the various species in this community suggests that predation risk provides an axis along which habitat segregation occurs. Predation can structure communities of mobile prey when risk differs among habitats. Animals specializing in predator avoidance and in exploitation of risky environments have reduced interactions with superior competitors; this promotes coexistence.

Microhabitat Utilization and Foraging Behavior of Bipedal and Quadrupedal Hetermoyid Rodents

Direct observations of free—living individuals were employed to quantify foraging behavior in Dipodomys deserti, D. merriami, and Perognathus longimembris in two Mojave Desert communities. D. merriami and P. longimembris do not partition shrub and open microhabitats, and all three species concentrate their foraging efforts (&gt;75%) near or beneath the canopies of desert shrubs. Results of livetrapping assessments of microhabitat usage (conducted concurrently with foraging observations) resemble those of earlier studies but differ greatly from observational data; trapping may not produce accurate representations of spatial utilization. P. longimembris avoids open areas and utilizes shrubs in a relatively coarse—grained fashion. D. merriami and D. deserti frequently traverse open areas at high speeds and hence utilize shrubs in a relatively fine—grained manner. Beneath shrubs, D. merriami and D. deserti are more active and discrete in their foraging than is P. longimembris; both Dipodomys apparently search for large clumps of seeds, while P. longimembris forages slowly and continuously, taking individual seeds from scattered dispersions. The distinctive morphology of Dipodomys is interpreted as an adaptation that allows fine—grained use of desert shrubs in the face of high predation risk during intershrub transits. Differences in heteromyid body sizes may also reflect two foraging strategies: small size would reduce energy demands, and thus lower the need to move between shrubs and reduce the need for predator avoidance; larger size would permit year—round activity (avoidance of torpor), reduce susceptibility to starvation (B. K. McNab, personal communication), increase the range of thermal tolerance, and generally increase abilities to cope with variations in environmental parameters (Boyce 1978). It is concluded that the morphological and behavioral differences between Dipodomys and Perognathus reflect divergent foraging strategies which may reflect equivalent but alternative strategies for exploitation of desert seeds.