Species Of Heteromyid Rodents Research Articles

Clump size selection: A field test with two species of Dipodomys.

Seed distribution (clump size) selection has been proposed as a possible mechanism of resource subdivision for competing heteromyid rodent species. To test this hypothesis, field experiments were conducted over two years during both richer and sparser seasons of the year. None of the predictions derived from the hypothesis were supported by our results. Though some selectivity was displayed by both Dipodomys spectabilis and D. merriami, the patterns of selectivity did not match the expected patterns. Our results further indicate that clump selection may be influenced by variables other than the density of seeds within a clump. These results have led us to conclude that clump size selection is unlikely to play a role in the coexistence of different species of the genus Dipodomys.

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.

Effects of seed distribution and competitors on seed harvesting efficiency in heteromyid rodents.

The foraging strategies of four naturally co-existing heteromyid rodent species were investigated: Dipodomys deserti (≈100 g), D. merriami (≈38 g), Microdipodops pallidus (≈13 g), and Perognathus longimembris (≈7 g). In 208 over-night laboratory foraging trials animals were provided with millet seed distributed in clumped and scattered patterns. Net removal of seeds from the foraging arena and amounts of seeds in cheek pouches and in caches were determined. When alone in an areana none of these species specialized extensively on either clumped or scattered seeds, although each tended to take more clumped than scattered seeds. When placed together with other individuals, animals once again tended to cache more clumped than scattered seeds in all but one paired combination of species: P. longimembris cached more scattered than clumped seeds when opposed by D. deserti. This suggests that the smaller species obtained a less preferred distribution of seeds in the face of competition. The two smaller species showed a great reduction in general foraging success in the presence of either of the two larger species. In general, a species cached less seeds when faced by larger opponent species.

Resource Utilization by Desert Rodents in the Lower Sonoran Desert

Four species of heteromyid rodents were studied in 5 communities in the Lower Sonoran Desert to examine differences in resource utilization by heteromyids in terms of: (1) collection of seed species, seed size and seed type (shrub—tree, forb and grass) and (2) association with amount of perennial cover of habitat. These rodents were not allocating resources by seed species or seed size. These rodents were collecting seeds to some degree by seed type, however, this was related to foraging in particular microhabitats. We found no correlation between numbers of each species of seed in cheek pouch contents and energy content per seed species. These heteromyids were opportunistic in collecting seeds as shown by considerable variation in seed species collected by populations and species among habitatas and seasonal variation in seed species collected. These heteromyids appear to partition seed resources by specializing on particular microhabitats. If these heteromyids use microhabitats optimally as granivores, they cannot allocate resources by seed species, size or type without risking local extinction due to the high variable production of specific seed species and sizes in the desert. The regular distribution of body sizes among heteromyids in a community appears to be a consequence of exploiting particular microhabitats in the desert community rather than allocating resources by seed—size selection.

Microhabitat selection in two species of heteromyid rodents.

An experiment was conducted to determine the microhabitat preferences of two heteromyid rodents, Dipodomys ordi and Perognathus flavus. This experiment used marked seeds and the atomic absorption spectrophotometer in order to study the environment as a mosiac of microhabitats. The results of our analysis indicate that these two heteromyids are microhabitat selectors. The preferences of the rodents are D. ordi: grass habitat 0.0%, near grass habitat 22.5%, open habitat 77.4% and P. flavus: grass habitat 46.2%, near grass habitat 32.2%, open habitat 21.4%. The overlap between the two species is only 0.43.

A mechanism for resource allocation among sympatric heteromyid rodent species.

Laboratory feeding experiments were conducted with Dipodomys ordii and Perognathus flavus in an attempt to discover a mechanism which might result in seed size selection. There was no marked difference in the proportions of four seed types collected whether the rodents foraged in the presence or absence of one another. However, analysis of the variability in weight of each of the seed types collected by the two species showed that when alone, the larger kangaroo rat was less effective at harvesting all of a uniformly distributed mixture of seeds. When in the presence of one another both species could harvest enough of the mixed, uniformly distributed seed to coexist indefinitely, but when the food source was presented as four large clumps the kangaroo rat's foraging effectiveness increased tremendously so that the pocked mouse was almost entirely unable to harvest any seed. These data, in light of mobility differences between large and small heteromyids, suggest a mechanism whereby the larger, more mobile kangaroo rats forage for the most readily available (large or clumped) seeds over a relatively large area. The smaller pocket mice, by virtue of their relative efficiency in harvesting seeds can utilize the less detectable seeds which are energetically too demanding for the larger kangaroo rats to harvest. Behavioral dominance of the larger animals may help prevent the smaller from utilizing the most readily available seeds. The patterns of seed size and foraging site selection described in the literature may be easily accounted for by this difference in foraging strategy.

Diversity of middle-ear transformer mechanisms in kangaroo rats, kangaroo mice, and pocket mice

The tympano-ossicular system has been anatomically analyzed in 26 species of heteromyid rodents. Based on anatomical measurements, 24 of the 26 species should have a transmission efficiency of 94%–100% from tympanic membrane to cochlear, at the resonant frequency. The areal ratio of stapes footplate to 2/3 tympanic membrane is remarkably constant among the species, varying only from 0.04 to 0.07: in Dipodomys and Microdipodops this small ratio is due to the very large tympanic membrane; in Perognathus and Liomys it is due to the extremely small stapes footplate. The lever ratio of incus to malleus varies from 0.28 to 0.33 in Dipodomys and Microdipodops, from 0.37 to 0.46 in Perognathus, and from 0.55 to 0.60 in Liomys. In addition, the middle-ear volumes and the morphology of tympanic membrane, ossicles, ligaments, and muscles all combine to minimize both mass and stiffness. All these data suggest middle-ear mechanisms which are very efficient over a broad frequency range. We have cochlear microphonic data consistent with this conclusion. [Supported by NIH Grant 11459.]

A Population Study of Irradiated Desert Rodents

An experiment was begun in 1963 to evaluate effects of long—term low—level radiation exposure on desert populations of plants and animals. Monthly censuses of four species of heteromyid rodents in three 9—ha enclosures and one non—enclosed area indicate great year—to—year changes in densities of the nonirradiated populations and even greater fluctuations in the irradiated populations. Although magnitude of density changes varied, timing of fluctuations was similar. In a good growing season a population may increase by a factor of 5 or more. In a poor growing season there may be no reproduction. Chronic radiation exposure to Perognathus formosus was measured to be 211—360 r/year. This reduced survival, particularly before the age of 6 mo. The instantaneous rate of death for the irratiated population during this period was 0.219, and for the control populations was .075 and 0.104. Future life expectancy at age 1 mo in the irradiated population was 9.2 mo, and for the control populations was 11.4 and 14.4 mo. The computed intrinsic rate of increase for the irradiated population was 0.314, compared to 0.493 and 0.498 for the two control populations. Demographic and histologic evidence indicate an increased birth rate in the irradiated population. A population consisting of animals 3 yr old and older, having been subjected to a lifetime of such radiation exposure, would likely be incapable of reproduction. Therefore, only in the most unfavorable periods, when reproduction is curtailed by natural factors, will the threshold levels of artificial radiation insult on the natural population be discernible. A rapid turnover of generations prevents a population from accumulating damaging effects of low—level exposure. The diets of rodents were analyzed, and the production of important foods was measured over 3 yr. Seeds and vegetative parts of plants made up approximately equal portions of the diet. All parts of the small annual plant Thelypodium were highly selected at all seasons. Total production of food shows a lag effect in relation to precipitation. Estimates of food requirements of the rodent population, based on laboratory feeding trials, suggest that food supply may be an important limiting factor. Populations respond strikingly to periods of favorable vegetative growth. Desert rodent populations are adapted to withstanding periods of unfavorable conditions. They have comparatively long life—spans and maintain fertility. These adaptations make them vulnerable to changed conditions, such as environmental contamination.

Population Ecology of Desert Rodent Communities: Body Size and Seed‐Husking as Bases for Heteromyid Coexistence

Seven species of heteromyid rodents varying in size from 8 to 114 g were studied in the laboratory. Starved rodents husked four kinds of large seeds (spinach, sunflower, squash and pumpkin) and were timed at the task. Geometric mean husking times varied from 3.1 to 70.1 seconds, depending on the seed and the rodent. Husking time was inversely proportional to an animal's body size. However, despite their slowness, smaller animals appear to be more efficient at dealing with these large seeds because they have a smaller metabolic drain. This conclusion is based on the assumption that gross caloric need increases linearly with (body weight).75. The authors defend the assumption and thus believe that smaller animals have a competitive advantage while husking seeds. These rodents might engage in a limited amount of habitat and resource selection based on their relative seed—husking abilities. However, the order in which the various species should prefer these seeds is quite similar, so resource allocation based on husking speed differences would seem of little potential importance in maintaining competing heteromyids in a state of coexistence. Previous investigators have shown that seed selection occurs in some cases, but many cases are also known in which little or no selection is practiced.

The Reproduction of Heteromyid Rodents in Captivity

By using large cages or a simultaneous encounter technique, several species of heteromyid rodents have been bred in captivity. The mating behavior of Dipodomys , although similar to Perognathus , is characterized by a prolonged mount. The data concerning gestation, parturition, maternal behavior, and the maturation of the young are described and discussed, comparing 7 species from the genera Liomys, Perognathus , and Dipodomys . It is concluded that in some species of Dipodomys there is a definite trend toward lengthening their gestation period, reducing their litter size and producing more precocial young.