Some animal partnerships are formed to satisfy craving foe sometimes sweet to eat. The best known of these is probably the relationship between ants and aphids. Aphids are tiny insects that excrete a sweet, sticky substance called honeydew. You can sometimes see dried honeydew on plant leaves, glistering in the sun. Aphids suck the juices from the plants that they live on, so gardeners considered them to be pests. But ants love aphids- because ants love the taste of honeydew.

Several kinds of ants have developed ways of keeping herds of aphids, much as people keep herds of cows. They collect aphids and bring them to a plant near their nest, so that honeydew will also be handy. Whenever an ant wants honeydew, it “milks” an aphid by tapping or stroking it. This prompts the aphid to release the sweet substance. In exchange, the ants protect the aphids.