Langur Black-faced Monkey curing cow's ear (India)
The cow is letting the monkey clean bugs from its ear. The monkey eats the tasty bugs it finds in the cow's ear. They both get something good out of it. This is called "mutualism" (wikipedia article about mutualism).
Shortly after taking this photo, i witnessed a funny scene: another cow came close to another of those monkeys, apparently because she too wanted her hear scratched and cleaned. But that other monkey was not interested, and he gave small punches to the cow's head to tell her to go away. But the cow just did not bunged, she kept getting her head close to the monkey. The monkey was clearly very upset that the cow was so insistent!