At the Institute for Marine Mammal Studies in Mississippi, Kelly the dolphin has quite a reputation.
All the institute’s dolphins are trained to hold onto any litter that falls into their pools until they see a trainer, when they can trade the litter for fish.
Kelly took this task one step further. When people drop paper into the water she hides it under a rock at the bottom of the pool.
The next time a trainer passes, she tears off a piece of paper to give to the trainer.
After a fish-reward, she goes back down, tears off another piece of paper, gets another fish, and so on.
This behaviour is particularly interesting because it suggests that Kelly has a sense of the future and delays gratification.
She has, in effect, trained the humans.