Most animals have little connection with animals of a different kind, unless they hunt them for food. Sometimes, however, two kinds of animals come together in a partnership which does good to both of them. You may have noticed some birds sitting on the backs of sheep. This is not because they want a ride, but because they find easy food in parasites on sheep. The sheep allow the birds to do so because they remove the cause of discomfort. So although they can manage without each other, they do better together.
大多数动物和其他种类的动物很少有关联，除非是猎食关系。然而，有时两种动物会因为对彼此有好处而结成伙伴关系。你可能会注意到有些鸟会停留在羊背上，这不是因为他们省力，而是因为在羊身上很容易找到寄生虫作为食物。羊允许鸟这样做是因为鸟会清除寄生虫，会让它们感觉舒服。因此，尽管它们彼此不依赖，但在一起可以更好。
Sometimes an animal has a plant partner. The relationship develops until the two partners cannot manage without each other. This is so in the corals of the sea. In their skins they have tiny plants which act as "dustman", taking some of the waste products from the coral and giving in return oxygen which the animal needs to breathe. If the plants are killed, or are even prevented from light so that they cannot live normally, the corals will die.
有时动物会有植物伙伴，这种关系一直发展到两个伙伴没有彼此就无法管理为止。大海中的珊瑚就是这样，在它们的皮肤里有微小的植物充当“清洁工”，从珊瑚中提取一些垃圾，并提供动物需要呼吸的氧气作为回报。如果植物死亡，或者被遮光，不能正常生存，那珊瑚也会死亡。
