The cattle egret is a bird that lives around herds of cattle. The only available explanation of the fact that the cattle egret follows cattle herds is that the egrets consume the insects stirred up from the grasses as the cattle herds graze. Which one of the following, if true, would most seriously undermine the claim that the explanation given above is the only available one? ['Cattle egrets are not generally known to live outside the range of large, slow-moving animals.', 'The presence of cattle dissuades many would- be predators of the cattle egret.', 'Forests are generally inhospitable to cattle egrets because of a lack of insects of the kind egrets can consume.', 'Cattle egrets are known to follow other slow- moving animals, such as rhinoceroses and buffalo.']