Michael A. Mares

An Experimental Analysis of Social Spacing in Tamias Striatus

We report on a series of experiments performed on a population of free-living eastern chipmunks, Tamias striatus, inhabiting a forest in northwestern Pennsylvania. The experiments were designed to examine, via perturbations of food supply and/or population density, the relationship between home range size, food availability, and population density. When food levels were increased within an area, a simultaneous effect was noted: mean home range size decreased and population density increased through recruitment from neighboring habitats. Whether the cause of the decreased patterns of movement was the increased food supply or the elevated population density was deter- mined in subsequent experiments. When population density was held constant and food supplies were experimentally increased, there was a significant reduction in mean home range sizes when compared to the normal (control) situation. When food supplies were undisturbed, but population density was greatly reduced, there was no change in the mean home range size over that determined at higher densities. Our results suggest that, in eastern chipmunks, food supply is a primary determinant of mean home range size, whereas population density, at least at the levels we examined experimentally, has no effect on movement patterns. These results are in accord with those theories relating movement patterns to resource abundance, but are not in accord with hypotheses suggesting that home range sizes and population density are inversely associated.

Home range perturbations in Tamias striatus

SummaryA 12-week experimental study on the responses of home range size and population density of eastern chipmunks, Tamias striatus, to perturbations in food resources was conducted at the Pymatuning Laboratory of Ecology in Pennsylvania. The study involved a total of 97 animals and 1,036 captures. Home ranges were determined for all animals marked and captured four or more times. Mean home ranges were calculated for three different experimental periods; a before-seeding period, a seeding period, during which an essentially unlimited supply of a preferred food (sunflower seeds) was available, and a post-seeding period when all seeds were withdrawn. Home ranges during the seeding period contracted in response to the food source supplied in seed trays distributed throughout the plot. The differences between the before and during mean home ranges was significant (P<0.05). Home ranges subsequently expanded after removal of the seeds. The population density also increased over 50% during the seeding period, both in response to the abundant food source and the contraction of resident home ranges. The density subsequently declined to its initial level in the post-seeding period. The replacement of home ranges of chipmunks which died during the study by the establishment of new, similar home ranges by immigrants, and the expansion of existing home ranges by residents into the vacated areas was also observed.

Home Range Size in Eastern Chipmunks, Tamias striatus, as a Function of Number of Captures: Statistical Biases of Inadequate Sampling

Four summers of trapping data for the eastern chipmunk ( Tamias striatus ) were examined quantitatively in order to identify factors accounting for variation in home range size. Home range is linearly dependent upon the number of captures utilized to calculate a home range, whereas the mean percent change in home range is an asymptotic function of capture interval. In order to measure home range accurately, a minimum of twenty capture points is suggested. In analyses where home range size was based upon a minimum of four captures, significant differences were found in home range sizes between sexes, age groups and years. When strong statistical data were used in home range analysis (i.e., a minimum of 20 captures determining home range), there were no statistical differences among any categories. These results demonstrate the importance of minimum capture number in home range studies.

Water economy and salt balance in a south american desert rodent, Eligmodontia typus

Abstract 1. 1. Water balance and utilization of sodium chloride solutions were examined for Eligmodon-tia typus of the Monte Desert of northwestern Argentina. 2. 2. Eligmodontia typus was generally unable to maintain body weight on a diet of air-dried seeds. 3. 3. Consumption of up to 2.0 M sodium chloride demonstrates that this species is particularly adapted to concentrate electrolytes and may be capable of using halophytic plants as water sources. 4. 4.Urine urea levels were highly correlated with total urine osmolarity ( r = 0.74). Urine sodium equivalents were correlated with the molarity of the drinking solution ( r = 0.84). 5. 5. E. typus is able to maintain body weight using cactus ( Trichocereus candicans ) as a water source. 6. 6. E. typus probably uses a multifaceted strategy of water procurement (cacti, halophytes, insects) to avoid desiccation.

Experimental Assessment of Several Population Estimation Techniques on an Introduced Population of Eastern Chipmunks

An experimental population of eastern chipmunks ( Tamias striatus ) composed of 85 individuals of known age and sex was released on a 9.4-ha island previously devoid of chipmunks. Three commonly used mark-recapture techniques (Lincoln-Petersen, Schnabel, and Schumacher-Eschmeyer methods) and the Least Squares Removal Method were used to estimate the size of the population. With the exception of equal catchability, the experiment was designed to satisfy the assumptions of the above methods. Point estimates always underestimated the true population size, and only the confidence intervals of the Lincoln-Petersen Method consistently included the actual population value. Least squares regression analyses suggest that the experimental population is composed of two groups of animals: those easily trapped and those hesitant to enter traps. As such, all population estimation methods estimate the easily captured portion of the population and underestimate the true population size. The mathematically simple Lincoln-Petersen Method was more robust than the other methods; when unequal trappability is suspected, it is more accurate than the other methods (although some precision is sacrificed). Caution is suggested in making inter- or intraspecific statistical comparisons of population size when close adherence to the assumptions of the models utilized is not demonstrable; otherwise, statistical differences may be spurious.

Water Balance and Other Ecological Observations on Three Species of Phyllotis in Northwestern Argentina

Water balance was examined in three species of leaf-eared mice ( Phyllotis griseoflavus , P. darwini , and P. osilae ) from northwestern Argentina. The degree of specialization for aridity in each species conforms with its general habitat requirements. P. griseoflavus , which exists in the driest habitats, is the species most capable of existing without access to free water in the laboratory, P. darwini is intermediate, whereas P. osilae is the least so adapted. Preliminary behavioral observations suggest that these three species differ greatly in their degree of aggressiveness. An interplay of behavior and physiology may explain the macro- and microdistributional patterns which have been described for these species.

Aspects of the water balance of Oryzomys longicaudatus from northwest argentina

Abstract 1. 1. Oryzomys longicaudatus , which occurs in mesic peripheral areas of the lowland Monte Desert of northwestern Argentina, does not possess any pronounced abilities to exist without free water while on a seed diet in the laboratory. 2. 2. Physiological limitations to aridity may keep this species from colonizing arid desert microhabitats.