The swimming behavior of Pseudomonas putida was analyzed with a tracking microscope to quantify its run time and turn angle distributions. Monte Carlo computer simulations illustrated that the bimodal turn angle distribution of P. putida reduced collisions with obstacles in porous media in comparison to the unimodal distribution of Escherichia coli.Soil bacteria of the species Pseudomonas putida propel themselves through their surrounding medium by rotating flagella that form tufts at one end of their bodies (10). A single cell traces a path that consists of a series of runs interrupted by changes in direction. As with Escherichia coli, the changes in direction are initiated by a reversal in the rotational direction of the flagellar motors of the bacteria (4,6,10,(12)(13)(14). In the absence of a chemical gradient this swimming pattern resembles a three-dimensional random walk similar to Brownian motion in molecular diffusion, except that changes in direction are due to the reversal of flagellar rotation and not molecular collisions.P. putida PRS2000 cells were originally obtained from Wayne Coco at the University of Illinois in Chicago (2). A small loopful of cells was inoculated into a solution consisting of a buffer, ammonium sulfate, and a mineral base, as described previously (2). Cultures were incubated for 20 to 30 h to a stationary cell density of approximately 10 9 cells/ml. One milliliter of this suspension was diluted 50-fold in motility buffer (1) with a pH of 7.0.The tracking microscope developed by Berg (3) tracks individual bacteria using a movable stage. The configuration of the microscope and its use are described comprehensively in Frymier et al. (9). The data were analyzed by using the algorithm developed by Berg and Brown and their empirically determined criteria for E. coli (6). Several alternatives for the angle criterion were considered for analyzing the behavior of P. putida. Angular speeds for flagging a tumble of from 360 to 600Њ/s were tried. However, the value of 420Њ/s previously used by Berg and Brown (6) appeared to give results most consistent with visual observations of three-dimensional images of the trajectories. In all, a total of 1,056 turn angles for 80 bacteria were measured.A three-dimensional visualization of the experimental traces for each of three bacteria is given in Fig. 1. Examples of various turning behavior are illustrated. A range of angles is possible (Fig. 1a). Bacteria sometimes continue with a bias in the forward direction (Fig. 1b) and sometimes reverse their direction (Fig. 1c).For P. putida there is a bimodal distribution for frequency of turn angles (Fig. 2). (Note that these results were presented previously in Duffy et al. (7) for a different data set.) If the bacteria chose new angles at random, then the frequency would be maximum at ϭ 90Њ. For P. putida there are peaks at approximately ϭ 40Њ and ϭ 160Њ. Angles which correspond to a bacterium continuing in a direction close to its original direction and angles for which the new direction is approximate...

The African elephant (Loxodonta africana) is a large-bodied, generalist herbivore that eats both browse and grass. The proportions of browse and grass consumed are largely expected to reflect the relative availability of these resources. We investigated variations in browse (C(3) biomass) and grass (C(4)) intake of the African elephant across seasons and habitats by stable carbon isotope analysis of elephant feces collected from Kruger National Park, South Africa. The results reflect a shift in diet from higher C(4) grass intake during wet season months to more C(3) browse-dominated diets in the dry season. Seasonal trends were correlated with changes in rainfall and with nitrogen (%N) content of available grasses, supporting predictions that grass is favored when its availability and nutritional value increase. However, switches to dry season browsing were significantly smaller in woodland and grassland habitats where tree communities are dominated by mopane (Colophospermum mopane), suggesting that grasses were favored here even in the dry season. Regional differences in diet did not reflect differences in grass biomass, tree density, or canopy cover. There was a consistent relationship between %C(4) intake and tree species diversity, implying that extensive browsing is avoided in habitats characterized by low tree species diversity and strong dominance patterns, i.e., mopane-dominated habitats. Although mopane is known to be a preferred species, maintaining dietary diversity appears to be a constraint to elephants, which they can overcome by supplementing their diets with less abundant resources (dry season grass). Such variations in feeding behavior likely influence the degree of impact on plant communities and can therefore provide key information for managing elephants over large, spatially diverse, areas.

Bacterial migration is important in understanding many practical problems ranging from disease pathogenesis to the bioremediation of hazardous waste in the environment. Our laboratory has been successful in quantifying bacterial migration in fluid media through experiment and the use of population balance equations and cellular level simulations that incorporate parameters based on a fundamental description of the microscopic motion of bacteria. The present work is part of an effort to extend these results to bacterial migration in porous media. Random walk algorithms have been used successfully to date in nonbiological contexts to obtain the diffusion coefficient for disordered continuum problems. This approach has been used here to describe bacterial motility. We have generated model porous media using molecular dynamics simulations applied to a fluid with equal sized spheres. The porosity is varied by allowing different degrees of sphere overlap. A random walk algorithm is applied to simulate bacterial migration, and the Einstein relation is used to calculate the effective bacterial diffusion coefficient. The tortuosity as a function of particle size is calculated and compared with available experimental results of migration of Pseudomonas putida in sand columns. Tortuosity increases with decreasing obstacle diameter, which is in agreement with the experimental results.

Elephants (Loxodonta africana) exhibit pronounced sexual dimorphism, and in this study we test the prediction that the differences in body size and sociality are significant enough to drive divergent foraging strategies and ultimately sexual segregation. Body size influences the foraging behaviour of herbivores through the differential scaling coefficients of metabolism and gut size, with larger bodied individuals being able to tolerate greater quantities of low-quality, fibrous vegetation, whilst having lower mass-specific energy requirements. We test two distinct theories: the scramble competition hypothesis (SCH) and the forage selection hypothesis (FSH). Comprehensive behavioural data were collected from the Pongola Game Reserve and the Phinda Private Game Reserve in South Africa over a 2.5-year period. The data were analysed using sex as the independent variable. Adult females targeted a wider range of species, adopted a more selective foraging approach and exhibited greater bite rates as predicted by the body size hypothesis and the increased demands of reproductive investment (lactation and pregnancy). Males had longer feeding bouts, displayed significantly more destructive behaviour (31% of observations, 11% for females) and ingested greater quantities of forage during each feeding bout. The independent ranging behaviour of adult males enables them to have longer foraging bouts as they experience fewer social constraints than females. The SCH was rejected as a cause of sexual segregation due to the relative abundance of low quality forage, and the fact that feeding heights were similar for both males and females. However, we conclude that the differences in the foraging strategies of the sexes are sufficient to cause spatial segregation as postulated by the FSH. Sexual dimorphism and the associated behavioural differences have important implications for the management and conservation of elephant and other dimorphic species, with the sexes effectively acting as distinct "ecological species".

Longitudinal studies have revealed how variation in resource use within consumer populations can impact their dynamics and functional significance in communities. Here, we investigate multi-decadal diet variations within individuals of a keystone megaherbivore species, the African elephant (Loxodonta africana), using serial stable isotope analysis of tusks from the Kruger National Park, South Africa. These records, representing the longest continuous diet histories documented for any extant species, reveal extensive seasonal and annual variations in isotopic-and hence dietary-niches of individuals, but little variation between them. Lack of niche distinction across individuals contrasts several recent studies, which found relatively high levels of individual niche specialization in various taxa. Our result is consistent with theory that individual mammal herbivores are nutritionally constrained to maintain broad diet niches. Individual diet specialization would also be a costly strategy for large-bodied taxa foraging over wide areas in spatio-temporally heterogeneous environments. High levels of within-individual diet variability occurred within and across seasons, and persisted despite an overall increase in inferred C 4 grass consumption through the twentieth century. We suggest that switching between C 3 browsing and C 4 grazing over extended time scales facilitates elephant survival through environmental change, and could even allow recovery of overused resources.