Species Of Heteromyid Rodents Research Articles

Caching Propensities and Effectiveness of Five Coexisting Heteromyid Rodent Species as Dispersers of Indian Ricegrass (Achnatherum hymenoides) Seeds

Recruitment of Indian ricegrass (Achnatherum hymenoides) seedlings occurs primarily from surface seed caches made by scatter-hoarding desert rodents of the family Heteromyidae. We used radiolabeled Indian ricegrass seeds in field seed-caching experiments with 5 coexisting heteromyid species (Dipodomys deserti, D. merriami, D. microps, Microdipodops pallidus, and Perognathus longimembris) to compare their potential effectiveness as dispersers. Dipodomys microps individuals deposited seeds almost exclusively in larders rather than in scatter hoards, implying that this species is unlikely to be an important seed disperser. Among the other species, individual caches made by larger species had more seeds than those made by smaller species, but smaller species made more caches. At the level of individual animals, M. pallidus was the most effective disperser in a quantitative sense; they made more caches than other species tested and placed fewer excess seeds in caches relative to optimal cache sizes for Indian ricegrass seedling establishment. However, because D. merriami individuals were considerably more abundant at the study site than other species and were also avid scatter hoarders, D. merriami is likely to be the most quantitatively effective disperser of Indian ricegrass seeds at the species level. Ranking species according to qualitative effectiveness (e.g., by considering effects such as the caching microsite on seedling establishment) was more ambiguous. For example, P. longimembris made relatively shallow caches that most closely match optimal planting depths for Indian ricegrass seedling emergence, but such shallow caches are probably more likely than deeper caches to be discovered by foraging rodents before they can germinate. With the possible exception of D. microps, any of the species we tested may be effective dispersers of Indian ricegrass seeds.

Small‐mammal isotope ecology tracks climate and vegetation gradients across western North America

Stable carbon, nitrogen, hydrogen and oxygen isotopes have been used to infer aspects of species ecology and environment in both modern ecosystems and the fossil record. Compared to large mammals, stable isotopic studies of small‐mammal ecology are limited; however, high species and ecological diversity within small mammals presents several advantages for quantifying resource use and organism–environment interactions using stable isotopes over various spatial and temporal scales. We analyzed the isotopic composition of hair from two heteromyid rodent species, Dipodomys ordii and Perognathus parvus, from localities across western North America in order to characterize dietary variation in relation to vegetation and climatic gradients. Significant correlations between the carbon isotopic composition (δ13C) of these species and several climatic variables imply that seasonal temperature and precipitation control the composition and distribution of dietary resources (grass seeds). Our results also suggest a moisture influence on the nitrogen isotopic composition (δ15N) of heteromyid diets. Population‐ and species‐level variation in δ13C and δ15N values record fine‐scale habitat heterogeneity and significant differences in resource use between species. Using classification and regression‐tree techniques, we modeled the geographic variation in heteromyid δ13Cdiet values based on 10 climatic variables and generated an isotope landscape model (‘isoscape’). The isoscape predictions for δ13Cdiet differ from expectations based on observed C4 distributions and instead indicate that D. ordii and P. parvus record seasonally abundant grass resources, with additional model deviations potentially attributed to geographic variation in dietary selection. The oxygen and hydrogen isotopic composition of D. ordii is enriched relative to local meteoric water and suggests that individuals rely on highly evaporated water sources, such as seed moisture. Based on the climatic influences on vegetation and diet documented in this study, the isotopic composition of small mammals has high potential for recording ecological responses to environmental changes over short and long time scales.

Comparative analysis of active retrotransposons in the transcriptomes of three species of heteromyid rodents

Virtually every eukaryotic genome is replete with transposable elements (TEs). TE activity, or lack thereof, is of considerable evolutionary interest as TE insertions actively drive genome evolution by altering gene expression and/or function through different mechanisms. Herein, we take a comparative approach to better understand the variation in TE transcriptional activity. Our goals were to identify transcriptionally active TEs and to evaluate their relative expression levels in an effort to identify key determinants of TE activity. We do so in three related non-model rodent species (Dipodomys spectabilis, Chaetodipus baileyi, and Heteromys desmarestianus). We used these species (a) because rodents have long been productive models for the study of TEs; (b) because the known and dated relationships among these species permit strong phylogenetic inference (e.g., ancestral character states); and (c) because we have previously characterized key genes that underlie evolutionary adaptations in these species (e.g., osmoregulation). We used RNA-seq to characterize the transcriptomes of two different tissues – kidney and spleen – in each of these three species. Our data revealed a diversity of retrotransposons that were actively transcribed in these rodents (including LINEs, retroviruses/ERVs, and SINEs). We also identified tissue-specific differences in retrotransposon-activity in about half of the retrotransposons detected. We interpret these differences in TE activity in light of overall gene expression, with the transcribed SINEs enriched in differentially expressed sequences in all three species. These transcribed TEs may simply represent transpositionally active TEs. Alternatively, they may be TEs that have been co-opted to represent functionally important components of the host transcriptome (i.e., exaptations). Overall, our results contribute to a growing appreciation for the evolutionary interplay between TEs and their hosts.

The foraging behavior of granivorous rodents and short-term apparent competition among seeds

The foraging behavior of a predator species is thought to be the cause of short-term apparent competition among those prey species that share the predator. Short-term apparent competition is the negative indirect effect that one prey species has on another prey species via its effects on predator foraging behavior. In theory, the density-dependent foraging behavior of granivorous rodents and their preference for certain seeds are capable of inducing short-term apparent competition among seed species. In this study, I examined the foraging behavior of two heteromyid rodent species (family Heteromyidae), Merriam’s kangaroo rats (Dipodomys merriami) and little pocket mice (Perognathus longimembris). In one experiment I tested the preferences of both rodent species for the seeds of eight plant species. Both rodent species exhibited distinct but variable preferences for some seeds and avoidance of others. However, the differences in preference appeared to have only an occasional effect on the strength of the short-term apparent competition detected in a field experiment. In another experiment, I found that captive individuals of both rodent species had approximately equal foraging effort (i.e., time spent foraging) in patches that contained a highly preferred seed type (Oryzopsis hymenoides) regardless of seed density and the presence of a less preferred seed type (Astragalus cicer) in the patches. The rodents also harvested a large proportion of O. hymenoides seeds regardless of initial seed density; this precluded a negative indirect effect of A. cicer on O. hymenoides. But there was a negative indirect effect of O. hymenoides on A. cicer caused by rodents having a lower foraging effort in patches that only contained A. cicer seeds than in patches that contained A. cicer and O. hymenoides seeds. The indirect interaction between O. hymenoides and A. cicer thus represented a case of short-term apparent competition that was non-reciprocal. Most importantly, it was caused by the foraging behavior of the rodents.

Differences in Food Hoarding among Six Species of Heteromyid Rodents

Storing food is an important adaptation of heteromyid rodents to desert environments of southwestern North America. These rodents store seeds in burrows (larder hoarding) and in small, widely scattered caches buried at shallow depths in soil (scatter hoarding). Laboratory trials were done with five species from one site and two species from two other sites to test the hypothesis that larger species would larder hoard relatively more seeds than smaller species. Subjects were tested in arenas consisting of a sand-filled box for scatter hoarding and a nest box for larder hoarding. Chisel-toothed and Merriam's kangaroo rats ( Dipodomys microps and D. merriami ) from one site larder hoarded more than sympatric dark kangaroo mice ( Microdipodops megacephalus ) and long-tailed and little pocket mice ( Chaetodipus formosus and Perognathus longimembris ). Chisel-toothed kangaroo rats larder hoarded more than Merriam's kangaroo rats and little pocket mice larder hoarded more than dark kangaroo mice. These results were consistent with reported information on the relative aggressive behaviors of the species. For subjects from the other two sites, Ord's kangaroo rats ( D. ordii ) larder hoarded more than the smaller Merriam's kangaroo rats. Pocket mice made larger scatter hoards than kangaroo rats and placed these scatter hoards closer to edges and corners of arenas than kangaroo rats. Differences among species in food-hoarding behavior should be considered in future studies of the community ecology of desert rodents.

Cheek pouch capacities and loading rates of heteromyid rodents.

Rodents of the family Heteromyidae are proficient gatherers and hoarders of seeds. A major component of their adaptive specialization for harvesting and transporting seeds is their spacious, fur-lined cheek pouches. Precise measurements of cheek pouch capacities are essential if ecologists are to understand the foraging ecology, possible constraints on locomotion patterns, and competitive relationships of heteromyid rodents. To measure the size of these cheek pouches and the rate at which animals load seeds into their pouches during seed harvest, we attracted 56 individuals representing ten species of heteromyid rodents to bait stations in the field and allowed them to fill their cheek pouches with seeds several times while we observed and timed the events with the aid of night-vision equipment. The largest load taken by each individual was used as an estimate of its cheek pouch capacity. At the end of observations, each subject was captured and its mass and other data gathered. The allometric relationship between cheek pouch capacity and body mass for ten species of heteromyids was significant [pouch capacity (ml) = 0.148 body mass (g)0.992, r 2=0.91, P<0.0001]. The regression coefficient is ≈1.0, which indicates that the volume of the cheek pouches scales in direct proportion to body size. When the data were subdivided into quadrupeds (Perognathus and Chaetodipus) and bipeds (Dipodomys) (n=5 for each), the relationships between pouch capacity and body mass were significant, but the two regressions were not significantly different from each other. When all loads (full and partial) were considered, subjects filled their cheek pouches an average of 93 ± 10% of pouch capacity (n=185). Cheek pouch capacities from published studies of artificially filled pouches of heteromyids in the laboratory averaged about 40% below the field measurements obtained here. The allometric relationship between mean loading rate and body mass was also significant [seeds/s=1.067 bodymass (g)0.830, r 2=0.85,P=0.0011), but when quadrupeds and bipeds were considered separately, the relationships were not significant. Seed densities and bulk densities were used to calculate packing coefficients for seed species, which, when used in conjunction with the allometric relationship between cheek pouch capacity and body size, can be used to estimate the maximum load carried by a heteromyid. Except for the very largest kangaroo rat species, a full pouch load of Indian ricegrass seeds represents less than the daily energy requirements of an active heteromyid.

Direct Observations of Owls and Heteromyid Rodents: Can Predation Risk Explain Microhabitat Use?

Coexisting heteromyid rodent species of North American deserts differ in habitat use and in locomotory morphology. Quadrupedal species forage primarily in structurally complex microhabitats, such as under bush canopies, while bipedal species forage in open spaces. A common explanation for this morphology—microhabitat association is that species differing in morphology also differ in vulnerability to predators, that microhabitat structure affects predation risk, and that animals preferentially forage in the safest microhabitats. We tested this for two bipedal and two quadrupedal heteromyid species (matched by body size), and one cricetid species, by quantifying effects of habitat and illumination on activity and on risk of predation by Great Horned Owls. Capture frequencies were lower for all heteromyid species than for the cricetid species, Peromyscus maniculatus. Heteromyid activity was lower in open habitat and under bright illumination. Illumination had no significant effect on risk, perhaps because rodents changed activity patterns under full moon to compensate for a potential increment in risk. Habitat, however, did affect risk: all species were attacked and captured more frequently in the open. Bipedal species were attacked relatively more in the open than were quadrupeds. If these results apply to all predators, they indicate that predation alone cannot account for the divergent microhabitat associations of bipedal and quadrupedal species. Bipedal heteromyids, however, escaped owl attacks more frequently than did quadrupeds of equivalent size. It is therefore conceivable that they experience lower overall risk in nature, where owls may preferentially attack more easily captured prey species when given a choice. Under these circumstances, owl predation could reinforce divergent microhabitat specializations based on some other factor, such as foraging economics, by restricting quadrupeds more strongly than bipeds to the safety of bushes.

Niche Relationships of Dipodomys agilis and D. stephensi: Two Sympatric Kangaroo Rats of Similar Size

-Congeneric heteromyid rodent species of similar size rarely occur in sympatry and use distinct habitats when they do. We characterized habitat use by two such species, Dipodomys stephensi and D. agilis, at two sites in western Riverside Co., California, and compared their selection of seeds and seed patches in the laboratory. At both localities, D. agilis.and D. stephensi were segregated spatially; D. agilis captured in areas with significantly greater cover of shrubs at both sites. In the laboratory, the species were very similar in choice of seed sizes and native seed species. Substrate use also was similar, but D. agilis avoided heavy substrates more than D. stephensi, and discriminant analysis correctly classified all individuals to species. The species differed significantly in response to light and artificial shrubs; D. agilis foraged under shrub canopies and avoided lighted open areas more than D. stephensi. Dipodomys stephensi consumed significantly more commercial seed with high oil and protein content. These differences suggest that the basis for divergent habitat selection in this case could involve a suite of factors related to effects of habitat on predation risk, water balance and, perhaps, foraging efficiency.

Dipodomys populations as energy-processing systems: Regulation, competition, and hierarchical organization

The role of extrinsic and intrinsic factors in the regulation of population dynamics and community structure has been poorly understood. To address this question, an energetic model of population dynamics is developed for a system of two species of heteromyid rodents ( Dipodomys spp.) using a single homogeneous seed resource. The model suggests that large-scale, extrinsic factors, such as climate, and intrinsic properties of individual organisms and populations interact to determine the observed community dynamics. Previous modelling approaches have confused organismic level processes, such as foraging, with population and community level processes, such as reproduction and competition. Systems approaches incorporating energetics and hierarchy theory should provide a more realistic basis for dealing with complex ecological systems such as populations and communities.

Relationships between Local Distribution and Geographic Range of Desert Heteromyid Rodents

The extent to which desert heteromyid rodent species are distributed geographically is negatively correlated with their variability in abundance across sites within local regions. In pairwise comparisons, those species with larger ranges generally had lower coefficients of variation in local density across sites than species with smaller ranges. Computer simulations showed this pattern to be significantly non-random in three of the eight data sets examined (data analyzed from three other studies showed the same pattern at 0.1 < p < 0.2). These results are consistent with the abundance/ distribution theories proposed by Hanski and Brown. The results support a large body of literature suggesting that wide-ranging species may have greater ecological success and more evolutionary inertia than species with more restricted distributions. The results also suggest that the ability of species to occur consistently over space may be only indirectly related to the ability to occur stably over time.

The effects of owl predation on the foraging behavior of heteromyid rodents.

Researchers have documented microhabitat partitioning among the heteromyid rodents of the deserts of North America that may result from microhabitat specific predation rates; large/bipedal species predominate in the open/risky microhabitat and small/quadrupedal species predominate in the bush/safer microhabitat. Here, we provide direct experimental evidence on the role of predatory risk in affecting the foraging behavior of three species of heteromyid rodents: Arizona pocket mouse (Perognathus amplus; small/quadrupedal), Bailey's pocket mouse (P. baileyi; large/quadrupedal), and Merriam's kangaroo rat (Dipodomys merriami; large/bipedal). Both kangaroo rats and pocket mice are behaviorally flexible and able to adjust their foraging behavior to nightly changes in predatory risk. Under low levels of perceived predatory risk the kangaroo rat foraged relatively more in the open microhabitat than the two pocket mouse species. In response to the presence of barn owls, however, all three species shifted their habitat use towards the bush microhabitat. In response to direct measures of predatory risk, i.e. the actual presence of owls, all three species reduced foraging and left resource patches at higher giving up densities of seeds. In response to indirect indicators of predatory risk, i.e. illumination, there was a tendency for all three species to reduce foraging. The differences in morphology between pocket mice and kangaroo rats do appear to influence their behavioral responses to predatory risk.

The influence of ambient temperature, seed composition and body size on water balance and seed selection in coexisting heteromyid rodents.

The water balance of three different sized coexisting species of heteromyid rodents (Dipodomys merriami ca. 39 g;Perognathus fallax ca. 23 g;Perognathus longimembris ca. 9 g) was assessed while consuming two different diets (either wheat or hulled sunflower) at ambient temperatures of 15-30°C. The metabolism of wheat as the sole food source was calculated to provide a greater metabolic water production (MWP) than the consumption of sunflower seed because of their different composition. The state of water balance was assessed by measuring urine concentrations and body weight maintenance on each diet at each temperature. Both measures showed that (i) all species were able to maintain a more positive water on the higher MWP seed, (ii) for all species there was an ambient temperature above which water balance could no longer be maintained, (iii) that this temperature was higher with the higher MWP food source and (iv) water regulatory efficiency was negatively correlated with body mass.Dipodomys showed a reduced digestive efficiency compared toPerognathus. When presented with both seedsDipodomys showed no preference for either seed irrespective of the state of water balance whilst thePerognathus species showed a tendency for an increased preference for the high MWP food source at the higher ambient temperatures. The ecological implications of these findings are discussed.

Distribution of Seeds in Sonoran Desert Soils: Implications for Heteromyid Rodent Foraging.

We used a hierarchical sampling design to investigate temporal and spatial patterns of variation in the topsoil seedbank at a site on the Santa Rita Experimental Range, 50 km south of Tucson, Arizona. Replicate sets of contiguous 4 ° 4 cm samples to 2 cm depth ("transects") were collected three times per year, from 1980 to 1983, from four microhabitats that are differentiated by desert heteromyid rodents: large and small open spaces, under canopies of large shrubs, and under canopies of small trees. We also analyzed texture and organic content of soils from the four microhabitats. Seeds from 54 plant taxa were encountered, most of them rare. Average seed mass across species was 1.28 mg/seed, but because tiny seeds were most numerous, the abundance-weighted average was 0.16 mg/seed. Total seed abundance was highly variable, ranging from 0 to 256 seeds and from 0 to 146 mg per sample. The fourfold temporal variation in mean mass or number of seeds per sample was largely due to between-year rather than between-season effects. Seed standing crops were positively correlated with rainfall during the previous 6-mo rainy period, and were lowest in fall and winter of 1982 after 2 yr of poor winter rains. Mean seed number per sample, which reflects primarily abundance of tiny seeds, did not vary among microhabitats. Mean per-sample seed mass, which reflects abundance of the large seeds eaten by rodents, did vary among microhabitats, however, with values being higher under shrubs and trees than in open spaces. The low mean abundance in open spaces was due more to a high frequency of seedless samples than to low maximum per-sample abundance. Hence, profitable seed patches do occur in open spaces, but are rarer than under shrubs or trees. Within microhabitats there was significant variation among replicate transects and among samples within transects. On a local scale, seeds occurred in patches 12 cm in diameter in all microhabitats and sampling periods. Seed species composition varied seasonally and among microhabitats, and there was an overall correlation between seed and adult microhabitat distribution for spring ephemerals. Soil from open spaces was finer and had lower organic content than that under shrub or tree canopies. Our results suggest that the resource upon which desert granivores depend is highly variable in time and space. Microhabitat is one of the most conspicuous sources of variation, and heteromyid rodent species differ in use of those microhabitats (open spaces vs. areas under canopies of perennial plants) that differ most noticeably in seed abundances, species composition, and soil characteristics.

Interference Competition in a Heteromyid Community in the Great Basin of Nevada, USA

Removal experiments with three species of heteromyid rodents were performed during two summers in the Great Basin of Nevada, USA. These experiments were designed to determine the importance of interference competition in these species by quantifying the short-term responses to the removal of one or more of the species. Our results indicate that the removal of a large species (Dipodomys merriami) does have a positive effect on a smaller species with a similar diet (Perognathus longimembris). These results and others presented are consistent with a hypothesis of interference competition. However, while there is short-term increase in number of rodents in response to removals, the increase is not commensurate with the number of animals removed. We conclude that interference competition was present but weak in the two summers we manipulated this community.

Microhabitat Use by Heteromyid Rodents: Effects of Artificial Seed Patches

Coexisting heteromyid rodent species differ in their affinities for foraging microhabitats. Bipedal forms (Dipodoyms, Microdipodops) use open spaces preferentially, while quadrupedal forms (Perognathus) use spaces under or near vegetation. Despite consistent reports that these preferences occur in heteromyid assemblages throughout North American deserts, and despite frequent speculation about the importance of foraging economics and predation risk in producing them, few experimental analysis of microhabitat use exist. Here we describe studies showing that (1) microhabitats distinguished by heteromyids in nature differ in seed abundance, soil particle size, and soil density, all of which are features that have been shown to influence heteromyid foraging efficiency; (2) four coexisting species differ in their preferences for artificial seed patches in a large laboratory foraging arena, and differences in the properties of preferred and nonpreferred patches correspond qualitatively to differences in the attributes of preferred and nonpreferred microhabitats in nature; and (3) microhabitat use in nature, as measured by livetrapping, can undergo rapid shifts that track the location of preferred artificial seed patches placed either in open spaces or under vegetation. These results suggest that divergent microhabitat specializations of coexisting heteromyids are in part functions of divergent preferences for the particular combinations of seeds and soils found in various microhabitats. Further experimental studies are needed, however, to determine whether these preferences can be predicted accurately from knowledge of the economics of foraging, and to what extent other factors may also influence microhabitat choice.

Effects of body size, seed density, and soil characteristics on rates of seed harvest by heteromyid rodents.

Although it is well established that coexisting heteromyid rodent species forage in different microhabitats, we do not yet know the basis for divergent microhabitat choice. One possibility is that seed harvest rates differ among microhabitats, and each species forages where it can extract seeds most efficiently. Microhabitats vary in several factors that could affect heteromyid foraging efficiency, including seed density, soil organic content and particle size distribution. We have explored the effect of each of these variables on harvest rates of several species feeding from petri dishes containing known densities of millet seeds embedded in soil of known particle size and density. Results indicate that the number of seeds harvested per second increases uniformly with seed density and soil density and decreases with soil particle size. Body size affects these relationships: larger animals have higher harvest rates for a given set of conditions and experience a greater relative change in harvest rate for a give change in conditions. This implies that heteromyids can be expected to exhibit species-specific microhabitat preferences while foraging in nature.

Laboratory studies of seed size and seed species selection by heteromyid rodents.

There have been conflicting reports in the last decade on the extent to which coexisting heteromyid rodent species select different sizes or species of seeds. Here I report results of laboratory choice experiments that show heteromyids of different size and genus to have similar preferences for wheat particles of various sizes, and for commercially available seed species that differ considerably in nutritional and morphological characteristics. I also discuss the significance of this finding for mechanisms of coexistence in natural communities.

Quantification of Competition among Coexisting Heteromyids in the Southwest

Three trapping grids, 225 m on a side, where established in the desert of southern New Mexico. On grid I all Dipodomys (Heteromyidae, Rodentia) were removed; on grid 2 all Perognathus (Heteromyidae, Rodentia) were removed; and on grid 3 no rodents were removed. All grids were censused at two-week intervals for six weeks. When the larger Dipodomys (40g) were removed, the Perognathus (15g) population increased 2.56-fold (from 27 to 69 individuals) in two weeks. Removal of Perognathus had no effect on numbers of Dipodomys. Our data allow us to estimate the magnitude of the competition between Dipodomys and Perognathus. About 45 species of heteromyid rodents inhabit the southwestern deserts of North America. Most of these taxa belong to the genera Dipodomys (kanga- roo rats) and Perognathus (pocket mice). All are nocturnal, burrowing, lar- gely granivorous rodents with cheek pouches. At localities of greatest diver- sity, four or five species live syntopically (Brown, 1975). Coexistence in these ecologically similar rodents offers the opportunity to test a basic tenet of competition theory: to what extent can competing spe- cies coexist (Volterra, 1928; Gause, 1934; Levins, 1968; MacArthur and Lev- ins, 1968; May, 1973)? In response to this question, much of the research on heteromyids has centered on quantification of methods of niche separation among syntopic species (Brown, 1975; Lemen and Rosenzweig, 1978; Won- dolleck, 1978; Price, 1978). Relatively few studies (Munger and Brown, 1981; Schroder and Rosenz- wig, 1975) actually try to measure competition directly. Schroder and Rosenzweig (1975) removed rodents from large study plots with the surpris- ing result that little competition exists between two coexisting species of similar size. They concluded that competition had been important in the past, but is not presently affecting interspecific interactions. Munger and Brown (1981) demonstrated the existence of competition in coexisting desert rodents by documenting an increase in the density of small rodents, Perog- nathus (7-17g), Reithrodontomys (1 g) and Peromyscus (17g), in response to the removal of Dipodomys (40g). However, this increase in the density of Perognathus took place only after Dipodomys has been kept off experimen- tal plots for 8 months. We have concentrated on short-term effects of density perturbations. Schroder and Rosenzweig (1975) found it difficult to lower a species' density because after removing most individuals from an experimental grid other individuals of that same species took their places, often within two weeks. This might indicate an intraspecific mechanism for adjusting local popula- tion density that worked very quickly. Our goal was to determine if there is a corresponding short-term mechanism acting intergenerically as well. Here we report an additional experiment to determine the effects of removal on population densities of coexisting heteromyids.

Microhabitat use in coexisting desert rodents-The role of population density.

Among some species of Sonoran Desert rodents microhabitat differences are density dependent. I studied the differences in microhabitat use among four species of heteromyid rodents (Dipodomys merriami, Perognathus amplus, P. baileyi, and P. penicillatus) at low and at high population densities. Microhabitats are defined by the abundance and size distribution of desert shrubs. During a period of low population density the rodent species showed substantial microhabitat differentiation. Following a large increase in pocket mouse (Perognathus spp.) numbers differences in microhabitat use between species disappeared. The lack of microhabitat differentiation at high density is due to microhabitat shifts rather than an expansion in the number of microhabitats used. The shifts lead to increased similarity among species in microhabitat use. Microhabitat overlap is not constant but it is highly variable and sensitive to changes in rodent abundance.

Tactile discriminatory ability and foraging strategies in Kangaroo rats and pocket mice (Rodentia: Heteromyidae).

A comparative study of seasonal food hoarding activity and tactile discriminatory ability in four species of heteromyid rodents (Dipodomys panamintinus, D. merriami, Perognathus longimembris, and P. formosus) was conducted in laboratory test arenas. Animals were tested individually to determine their treatment of seed (white millet) and seed mimics (glass beads and gravel) offered as food. In general, all animals showed low levels of millet hoarding activity during winter months with higher levels in fall and spring. Observations revealed that all species manipulated (with the forepaws) each potential food item prior to eating, pouching, or rejecting it. These tactile cues appear to surpass visual and olfactory cues as critical factors in distinguishing between food and food mimics. Pocket mice (Perognathus) showed high levels of tactile discriminatory ability which may serve as the mechanism by which they achieve high foraging efficiency in nature when "filter-feeding" for widely dispersed seed resources. Kangaroo rats (Dipodomys), on the other hand, are less adept at distinguishing between food and very similar non-food items. The fact that, in nature, kangaroo rats depend heavily on clumped food resources may obviate the need for highly efficient tactile discriminatory abilities.