Characterisation by fractal analysis of foraging paths of ewes grazing heterogeneous swards

On grasslands, herbivores make a trade-off between the quality and the quantity of their intake. They improve their searching efficiency by modulating their foraging velocity and/or their path sinuosity through the perception of their feeding environment. The aim of this study was to test the pertinence of fractal analyses of foraging paths in order to improve the understanding of spatial utilisation at grazing. By the analysis of grazing paths of herbivores grazing a continuously distributed and spatially limited resource, we aimed to identify the spatial scales at which the animals perceive heterogeneity of the sward, and to characterise how sheep modulate their foraging paths through resource abundance and heterogeneity. Two groups of five dry ewes, grazing two plots of contrasting areas from April to the end of September 2000, were studied. On the smaller plot (1500 m2), the application of a high stocking rate (HSR) produced a resource of good quality but low quantity, and on the larger plot (3000 m2) a low stocking rate (LSR) created a sward of low quality but good quantity. We show that on initially homogeneous swards, sheep exhibited correlated random walks, reducing movement quantities to achieve their intake. After a few weeks of grazing, vegetation structure become more complex and sheep modulated their foraging paths through resource abundance and/or sward structure. We identified a breakpoint at 5 m for which fractal dimension is always low. At lower scales (below 5 m), the modulation of sinuosity was not linked to sward abundance and structure, and sheep adapted grazing behaviour at bite and feeding station scales. At higher scales (above 5 m), modulation of grazing activity was achieved through path sinuosity in relation to perception of the environment. Grazing paths were tortuous on tall swards in summer, and straighter on heterogeneous, well-structured swards showing visual cues in autumn. Fractal dimension of grazing paths proved to be a synthetic measurement, which allowed identification of a hierarchical threshold in the spatial adaptation of foraging behaviour in domestic herbivores at grazing.