The Relationship Between Bite Rate and Local Forage Abundance in Wild Thomson's Gazelles

Abstract

The foraging of female Thomson's gazelles (Gazella thomsoni) on shortgrass plains was monitored over one annual cycle in southwestern Kenya. Sward dry green biomasses and protein densities were estimated regularly throughout the study site. Changes in protein densities with season and locale were strongly correlated with underlying changes in grass physiognomy: sward height and dry green bulk biomass density were particularly important and were found to vary inversely. The relationship between bite rates and underlying sward parameters varied with season: gazelle bite rates in the dry season were positively correlated with underlying dry green biomass and protein densities, as predicted by either the Process 1 or Process 2 foraging model of Spalinger and Hobbs. Nonlinear regressions of within—bout bite rates on these model equations significantly explained 21.8 and 23.7% of the dry season variance, respectively. In contrast, bite rates in the early wet season showed significant negative correlations with underlying protein densities: the fit of the within—bout bite rate data to Spalinger and Hobbs' Process 3 model explained 18.4% of the overall variation. The late wet season showed a flat (insignificant) relationship between bite rates and protein levels and was thus intermediate between early wet— and later dry—season patterns. Logistic regression of the type of correlation between bite rate and protein density (positive, flat, negative) on two principal components of grass physiognomy suggested that a component heavily weighting sward height was the major correlate of foraging process, whereas a second major component heavily weighting bulk density and other grass quality measures was less critical. At least during this single annual cycle, shorter swards were associated with Processes 1 or 2, whereas taller swards showed Process 3 foraging. One interpretation of these results is that sward height modulates bite mass, which in turn plays the major role in controlling foraging process. Whether the switching point remains the same in subsequent years or not, the results make it clear that the direction of the bite rate vs. foraging density relationship can change markedly with season, as predicted by the Spalinger and Hobbs models.

Predators, food and social context shape the types of vigilance exhibited by kangaroos

“…Eastern grey kangaroos preferentially select green leaves of grass, preferring short grass and avoiding long dry grass (Bell, 1973;Clarke, Jones, & Jarman, 1989). Similar preferences for patches of low to intermediate biomass have been observed in other short-grass grazers of similar body size to the eastern grey kangaroo, such as sheep, antelopes and other kangaroo species (Bradbury et al, 1996;Short, 1985;Wilmshurst et al, 1999). Such a preference would arise because an increase in biomass increases fibre content but decreases digestibility for short-grass grazers (Fryxell, 1991).…”

“…To measure patch quality, the observer put down a quadrat (90 Â 90 cm) with 81 grid crossings at the spot where the animal had spent most of its time feeding during the observation, and recorded a greenness index for all plants under each grid crossing. This index comprised two categories, brown (plants with low energetic value) and green (plants with high energetic value) (Bradbury et al, 1996), and allowed us to estimate the percentage of plant material that was green for each food patch. To measure the quantity of vegetation in patches, we determined grass biomass using a pasture meter (Herbo-LIS Arvalis, Paris, France), an instrument that uses the height of a plastic plate that has fallen onto the vegetation from a standard height to estimate the standing crop of the vegetation.…”

“…Pays et al, 2012). However, because rich patches should increase bite sizes, thereby increasing chewing time but decreasing searching time (Bradbury, Vehrencamp, Clifton, & Clifton, 1996;Spalinger & Hobbs, 1992), kangaroos should also have more spare time while chewing between bites and use it for vigilance with chewing (Fortin et al, 2004;Illius & FitzGibbon, 1994). Considering these two aspects, we predicted that kangaroos foraging on rich food patches should decrease their use of vigilance without chewing but increase their use of vigilance with chewing.…”

What Constrains Daily Intake in Thomson's Gazelles?

“…Nevertheless, this relationship must be resolved if we are to understand herbivore distribution and abundance in resource-limited environments. In the study area of Bradbury et al (1996), short swards consisted of widely spaced clumps of grass, whereas tall swards tended to have a more homogeneous spatial distribution. In general, the Spalinger and Hobbs model predicts that as forage density in patches increases, the cropping rate by grazers should shift from being constrained by searching for grass to being constrained by chewing and swallowing mouthfuls of grass.…”

“…Serengeti National Park in northern Tanzania lies south of the study area of Bradbury et al(1996) and is the principal wet-season range of the migratory Thomson's gazelles that they studied. The grazing conditions in Serengeti differ from those in southern Kenya in that grass swards tend to be more spatially homogeneous, irrespective of height.…”