We investigated the effects of overgrazing on the foraging behavior of livestock on a Mongolian steppe, by quantifying some behaviors of cattle, sheep and goats foraging in a lightly and a heavily grazed area in summer and winter. All animal species showed higher walking velocity and tended to show higher step/bite ratio when they foraged in the heavily grazed area than in the lightly grazed area. The effect of overgrazing on the step/bite ratio was greater in sheep and goats than in cattle, and the effect on the walking velocity was stronger in winter than in summer. The results indicate that heavy grazing altered the foraging behavior of animals imposing higher foraging costs, which varied among the animal species and between the seasons. The declined quantity of preferred species in the heavily grazed area required high searching efforts and decreased the bite rate, for selecting acceptable plants and better feeding sites. The stronger effects on sheep and goats than on cattle and in winter than in summer may reflect different foraging behavior potentials among the animals and different food availabilities between the seasons. We suggest that the ongoing increase in the number of goats in Mongolia is problematic not only from the viewpoint of grassland deterioration but also from feeding cost of whole livestock animals.
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