Kangaroo rat

Abstract: We develop two criteria for measuring patch assessment ability. First, we examine the ability of foragers to equalize benefits and costs at manipulated resource patches. Second, we compare patch utilization patterns of four possible foraging strategies (prescient, fixed time, Bayesian, and rate assessor) with actual foraging patterns. Experi- ments with several desert rodent and avian species suggest that Merriam's kangaroo rat may obtain the best estimate of patch quality, followed by the round-tail ground squirrel and Arizona pocket mouse. Kangaroo rats exhibited both a prescient and Bayesian strategy. Pocket mice and ground squirrels exhibited both a fixed-time and Bayesian strategy. Gam- bel's Quail appeared to be the least sophisticated forager and exhibited only a fixed-time strategy. The fixed-time strategy was observed most frequently in the low variance environment where patch differences were relatively minor. In general, increased patch variation led to poorer patch estimates but allowed employment of sophisticated foraging strategies. Avian group foragers did not obtain better estimates of patch quality than solitary foragers.

Abstract: An experimental study of competition between kangaroo rats (Dipodomys spp.) and other sympatric desert rodents using exclosures with "semipermeable" fences has been continuously maintained at a site in the northern Chihuahuan Desert since 1977. A new set of experimental manipulations begun in 1988 at the same site repeated this study. As reported previously for this community, exclusion of three species of Dipodomys from both original and new experimental plots resulted in greater abundances of five species of small granivorous rodents (Chaetodipus penicillatus, Perognathus flavus, Peromyscus eremicus, P. maniculatus, Reithrodontomys megalotis) on these plots relative to controls. In contrast, there were no significant treatment effects on the abundances of insectivorous grasshopper mice (Onychomys spp.). The long time lag before the response by small grani- vores to Dipodomys removal observed in the original experiment was not repeated in the experiment begun in 1988. Long-term (10 yr) exclusion of kangaroo rats from experimental plots has resulted in changes in vegetative cover, particularly increased grassiness, on these plots relative to controls. We used the repetition of the Dipodomys exclusion experiment in 1988 to evaluate the importance of this potential indirect effect of kangaroo rats on other rodents in this community. By examining differences in rodent capture numbers on the original and new sets of Dipodomys exclusion plots, we could identify four species (C. penicillatus, Perog- nathus flavus, Peromyscus eremicus, P. maniculatus) whose responses to kangaroo rat removal reflected direct competition from kangaroo rats, one species (R. megalotis) whose response reflected both direct and indirect (via vegetation changes and habitat selection) effects, and two species (Sigmodon hispidus, S. fulviventer) whose responses reflected only vegetation-mediated effects. The continuous presence of competition between small granivores and kangaroo rats over the 13-yr study despite large, species-specific fluctuations in abundances suggests that competition is pervasive within this community.

Abstract: THE extremely large mastoid bullae of the kangaroo rat's skull are composed of paper-thin bone containing the largest part of their hypertrophied middle-ear cavities. Each cavity is partially divided into three sections. These are (1) the anterior and (2) posterior mastoid spaces, which are separated by a thin, bony plate; each of these communicates ventrally with the smaller (3) entotympanic portion of the middle ear (Howell, 1932; Webster, 1960, 1961b). The relative volumes of the parts of the middle-ear cavity are: anterior mastoid, 49 per cent; posterior mastoid, 33 per cent; entotympanic, 18 per cent. In Dipodomys merriami, with a body weight of 40 gm., each middle-ear cavity has a total volume of 0.49 cc., while in D. spectabilis, whose body weight is 110 gm., each middle-ear volume is 0.88 cc. (Webster, 1960). Thus, the middle-ear volume is approximately proportional to body weight in these two species. Although there has been no report of experiments to determine the functional significance of the kangaroo rat's inflated bullae, some theories have been advanced. Hatt (1932) suggested that the enlarged bullae may shift the skull's

Abstract: Long-term (1977-90) experimental exclusion of three species of kangaroo rats from study plots in the Chihuahuan Desert resulted in significant increases in abundance of a tall annual grass (Aristida adscensionis) and a perennial bunch grass (Eragrostis lehmanniana). This change in the vegetative cover affected use of these plots by several other rodent species and by foraging birds. The mechanism producing this change probably involves a combination of decreased soil disturbance and reduced predation on large-sized seeds when kangaroo rats are absent. Species diversity of summer annual dicots was greater on plots where kangaroo rats were present, as predicted by keystone predator models. However, it is not clear whether this was caused directly by activities of the kangaroo rats or indirectly as a consequence of the increase in grass cover. No experimental effect on species diversity of winter annual dicots was detected. Our study site was located in a natural transition between desert scrub and grassland, where abiotic conditions and the effects of organisms may be particularly influential in determining the structure and composition of vegetation. Under these conditions kangaroo rats have a dramatic effect on plant cover and species composition.