Perognathus Amplus Research Articles

A New Species of Heteromyoxyuris (Nematoda: Oxyuridae), Parasite of Perognathus flavus (Rodentia: Heteromyidae) From Mexico

Heteromyoxyuris otomii n. sp., which inhabits the intestinal caecum of Perognathus flavus (Heteromyidae), in Zaragoza, Hidalgo, Mexico, is described. This new species differs from the 2 other congeneric species in the morphology and length of lateral alae in males. Heteromyoxyuris deserti has simple lateral alae located at both sides of the body, whereas in the new species, these structures are double at both sides; in contrast, lateral alae of Heteromyoxyuris longejector begin at the posterior half of the body, whereas they arise in the first third in the new species. Heteromyoxyuris longejector was found in 2 new host species, i.e., Perognathus amplus and Chaetodipus hispidus. This record represents the first record for the species in Mexico, increasing its geographic distribution.

Morphological, chromosomal, and molecular evolution are uncoupled in pocket mice

C-, and G-banded chromosomes are presented for Perognathus amplus and Perognathus longimembris from Arizona, USA and Chaetodipus nelsoni from Coahuila, Mexico. The two species of Perognathus reveal similar C-band patterns, and extensive autosomal and X chromosome G-band identity with only pericentric inversions distinguishing pairs 4 and 6 and a difference in the morphology of pair 20. Three pairs of autosomal secondary constrictions were found in P. amplus and only one in P. longimembris. Only 50% of the amplus/longimembris G-banded karyotype could be aligned with that of C. nelsoni indicating extensive chromosomal restructuring has taken place since these genera last shared a common ancestor. A review of the literature suggests variable rates of morphological, chromosomal and molecular evolution in these animals.

MITOCHONDRIAL DNA PHYLOGEOGRAPHY OF PEROGNATHUS AMPLUS AND PEROGNATHUS LONGIMEMBRIS (RODENTIA: HETEROMYIDAE): A POSSIBLE MAMMALIAN RING SPECIES.

Substantiated cases of ring species in mammals are rare. I examined the variation in mitochondrial DNA (mtDNA) of Perognathus amplus and P. longimembris in and around Arizona to test the hypothesis proposed by Hoffmeister (1986) that these two taxa are members of a single ring species demonstrating circular overlap. Through digestion of purified mtDNA from 45 P. amplus and 35 P. longimembris with 16 type II restriction enzymes, I identified 38 distinct haplotypes that belong to eight different evolutionary lineages. I then amplified and directly sequenced a portion of the mitochondrial cytochrome-b region from individuals representative of the lineages identified by restriction fragments, and used these data for phylogeny reconstruction in both a parsimony and neighbor-joining setting. The resulting phylogeny was consistent with the ring hypothesis, but, based on the incompleteness of the ring of subspecies and the apparent timing of evolutionary events in this group, I conclude that P. amplus and P. longimembris are distinct lineages that have completed the speciation process.

Karyotypic Variation in the Pocket Mice Perognathus amplus and P. longimembris

We studied the karyotypic variation of Perognathus amplus and P. longimembris in Arizona and adjacent Nevada and Utah. We extend the distribution of previously known fundamental-number (FN) variants. C-banded karyotypes showed that the FN variants do not result from addition or deletion of heterochromatic arms. We hypothesize that karyotypic evolution in these pocket mice proceeded in three directions from a presumed ancestral FN = 88 karyotype shared by both species. In both P. longimembris and P. amplus , parallel decreases in FN led to low-FN populations. In P. amplus , a separate lineage is characterized by increasing FN. The short-term cell-culture technique we used was superior to in vivo-colchicine, hypotonic methods for the production of chromosome spreads from pocket mice.

Rodent Dynamics in Relation to Fire in the Sonoran Desert

I trapped rodents on and near adjacent desert areas for 13 months before and after fire burned one area. Dipodomys merriami colonized the burned area after fire, but abundance of Neotoma albigula, Perognathus amplus, and Chaetodipus baileyi declined on the burned area after fire. Neotoma albigula showed immediate and long-term changes to fire, whereas burrowing species showed mostly long-term changes. Much about fire can be inferred through the elimination of competing hypotheses, despite statistical limitations of studying single fires.

Desert Rodent Community Structure: A Test of Four Mechanisms of Coexistence

Four mechanisms of coexistence are considered that may contribute to the diversity of desert granivorous rodent communities. In the first, bush/open microhabitat selection, coexistence is possible if there is a trade—off between foraging efficiency in the bush and open microhabitats. In the second, temporal variation in resource abundances, coexistence is possible if there is a trade—off between foraging efficiency and maintenance efficiency. The first species can forage profitably on low resource abundances while the second uses dormancy to travel inexpensively in time between periods of high resource abundances. In the third, spatial variation in resource abundance, coexistence is possible if there is a trade—off between foraging efficiency and the cost of travel. The first species forages patches to a lower giving—up density, (the density of resource at which a forager ceases foraging), while the second can inexpensively travel between patches with high resource abundances. In the fourth, seasonal rotation in foraging efficiencies, coexistence is possible if there is a trade—off between the costs of foraging during different seasons. The species that is the more efficient forager changes seasonally. The first mechanism of coexistence has received much empirical attention and support. The other three have not previously been considered with desert rodents. In a community of four granivorous rodent species, I used artificial seed patches to measure species— and habitat—specific foraging efficiencies and live—trapping to measure population sizes and mean distances between recaptures. Of the four, the fourth mechanism of coexistence best explained the presence of Perognathus amplus, Dipodomys merriami, and Spermophilus tereticaudus in the community. Each species enjoyed a period of the year during which it was the most efficient forager. Furthermore, the annual population densities of these three species fluctuated out of phase. Seasonal changes in species—specific predation risks and body size—dependent metabolic costs may have accounted for these results. The third mechanism of coexistence best explained the presence of Ammospermophilus harrisii in the community. This species preferred to forage a large number of widely spaced patches to a high giving—up density rather than forage a few patches to a low giving—up density.

The Effects of Morphology and Body Size on Rates of Owl Predation on Desert Rodents

Predation has been implicated in promoting microhabitat partitioning among desert rodents, but little direct evidence exists on how microhabitat or rodent morphology affect predation rates. We measured rates of predation by barn owls (Tyto alba) on heteromyid rodents which differ in body size (Perognathus amplus versus P. baileyi) and morphology (P. baileyi versus Dipodomys merriami). The experiments were conducted in a large, semi-natural enclosure. Owls captured the larger species at a significantly higher rate than the smaller species. We were unable to detect a consistent effect of rodent morphology on rates of predation. Also, rates of predation were higher for full moon levels of illumination than for starlight levels. In a separate experiment, we demonstrated that rates of predation are higher on rodents foraging in the open than on rodents foraging with cover. The results support the hypothesis that desert rodents represent a community where predation and competition interact directly to determine rodent foraging behavior and species interactions.

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.

Patch use as an indicator of habitat preference, predation risk, and competition

A technique for using patch giving up densities to investigate habitat preferences, predation risk, and interspecific competitive relationships is theoretically analyzed and empirically investigated. Giving up densities, the density of resources within a patch at which an individual ceases foraging, provide considerably more information than simply the amount of resources harvested. The giving up density of a forager, which is behaving optimally, should correspond to a harvest rate that just balances the metabolic costs of foraging, the predation cost of foraging, and the missed opportunity cost of not engaging in alternative activities. In addition, changes in giving up densities in response to climatic factors, predation risk, and missed opportunities can be used to test the model and to examine the consistency of the foragers' behavior. The technique was applied to a community of four Arizonan granivorous rodents (Perognathus amplus, Dipodomys merriami, Ammospermophilus harrisii, and Spermophilus tereticaudus). Aluminum trays filled with 3 grams of millet seeds mixed into 3 liters of sifted soil provided resource patches. The seeds remaining following a night or day of foraging were used to determine the giving up density, and footprints in the sifted sand indicated the identity of the forager. Giving up densities consistently differed in response to forager species, microhabitat (bush versus open), data, and station. The data also provide useful information regarding the relative foraging efficiencies and microhabitat preferences of the coexisting rodent species.

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.

Ability of Desert Rodents to Find Buried Seeds

Ability of Desert Rodents to Find Buried Seeds

Spatial Patterns in Sizes of Seeds Collected by Four Species of Heteromyid Rodents

This research was designed to provide information on the spatial variation in size of seeds collected by four species of heteromyid rodents. During summer 1975, rodents (Dipodomys merriami, Perognathus amplus, P. baileyi, and P. penicillatus) were live—trapped on several Sonoran Desert habitats at the Saguaro National Monument in southern Arizona, USA. In 1976 and 1977 a single study area containing all four rodent species was live—trapped to provide information on microhabitat and seed use among coexisting species. Any seeds contained in the cheek pouches of the rodents were identified and their linear dimensions were measured. Microhabitats were quantified by enumeration of shrubs and measurement of their size. Much of the variation in collection of different—sized seeds is attributable to habitat and microhabitat differences between rodent species. Not only do rodent species collect seeds of different size among different habitats, but within a habitat, locally coexisting rodent species collect seeds of different size. As a test of the concept of independent use of two resources (seed size and microhabitat) by coexisting species, seed size differences were compared to microhabitat differences between all pairs of heteromyid rodents. The results show that the difference between rodent species in the size of seeds they collect depends upon the difference in microhabitat selection of the species. Hence, these two components of the niche (seed size and microhabitat) of coexisting heteromyid rodents reduce to a single dimension.

The Use of Torpor by Perognathus amplus in Relation to Resource Distribution

Small (11 g) pocket mice ( Perognathus amplus ) were tested in a laboratory study for their use of torpor in relation to the distribution of food resources (seeds). Generally, the rodents spent more time in torpor the deeper the seeds were buried, but there was essentially no relationship between the dispersion of the seeds and the time spent in torpor. Regardless of the depth or spatial distribution of seeds, the pocket mice maintained similar weight loss patterns, indicating that they can adjust the amount of time spent in torpor to the environmental conditions under which they find themselves.

Forage-Area Separation and Overlap in Heteromyid Rodents

Forage-area separation and overlap of four species of seed-eating desert rodents were measured by placing seed dyed four colors in four microhabitats and allowing the rodents to forage and ingest the seed. The four microhabitats delineated were as follows: (1) areas lacking vegetation; (2) areas containing small scattered bushes .5 m tall and .5 m across; (3) areas containing clumped bushes .1 m across and 1 m tall; (4) areas containing large bushes greater than 1 m tall and 1 m across. Rodents were live-trapped and their feces collected and analyzed for color, which would indicate the forage area of each species. Results of these experiments demonstrated that Dipodomys merriami foraged in areas lacking vegetation, Perognathus amplus among scattered and clumped vegetation, P. baileyi among clumped vegetation and large bushes, and P. penicillatus under large bushes. A population of D. merriami was perturbed and P. amplus significantly shifted its forage pattern to the open microhabitat. Thus, Dipodomys affected habitat selected by P. amplus. Preliminary analysis of captures of heteromyids on sandy or gravelly substrate indicated that P. baileyi prefers gravelly soils and may exclude D. merriami from this substrate. Habitat selection was found to be an important factor in the coexistence of these species, with interspecific interactions partially influencing the habitat these species selected.

Selection of Seed Distribution Types by Dipodomys Merriami and Perognathus Amplus

Laboratory experiments were conducted to investigate the foraging responses of kangaroo rats (Dipodomys merriami) and pocket mice (Perognathus amplus) to aggregates of seeds which differed in size, depth, and degree of clumping. Kangaroo rats effectively used clumps of seeds, whereas the pocket mice did not differentially select clumps of seeds over a scattered distribution of seeds. In addition to distribution, clump size and depth were important criteria for both species in determining foraging efforts. These data, coupled with recent information which suggests wide arrays of seed distribution types in the desert, lead us to propose that rodents select seeds on the basis of spatial distribution of the resource. This is an attractive hypothesis because it can accommodate both of the primary mechanism previously suggested for desert rodent coexistence, seed size selection and habitat selection.

Optimization of Diets through Food Preferences by Heteromyid Rodents

The number of seeds requiring per day by four species of Sonoran Desert rodents (Dipodomys merriami, Perognathus amplus, Perognathus baileyi, and Perognathus intermedius), and the average energy content/seeds of the species the rodents ingested, were determined from information on the rodent daily energy requirement, proportions of seeds in the diets and cheek pouches of the rodents and the soil near their capture sites, and the energy content per seed of the important seed species. All rodent species except P. amplus gather seeds which are energetically superior to those randomly available in the soil, and all species choose to ingest, from the seeds they have gathered, those seeds which are richest in energy. The data indicate that the rodents primarily base their dietary preferences on energy considerations. The rodents do not, however, maximize energy intake, but rather optimize their diets by also ingesting a number of seed species which are relatively low in energy content, but which apparently provide some undefined value (e.g . nutrition, moisture).

Seasonal Activity and Reproductive Patterns of Five Species of Sonoran Desert Rodents

Activity patterns of five species of Sonoran Desert rodents (Dipodomys merriami, Perognathus amplus, P. intermedius, P. baileyi and Peromyscus eremicus) were closely related to temperature fluctua- tions, the heaviest species being most affected by high temperatures. There was a direct correlation between low ambient temperature, body weight and inactivity, with the lightest rodent, P. amplus, being the least active in the winter. D. merriami had two reproductive peaks (June and October) during the year. The three species of Perognathus reproduced only in the early summer. Young Peromyscus eremicus were consistently trapped through the summer and autumn.