Previously I mentioned the ants and the process of sharing food known as trophallaxis, a kind of regurgitation or vomiting happening per request, to "altruistically" share the food between a hungry and a satiated ant.

The vampire bats do that as well. When a bat gets some blood and has had his full of it, upon returning home he can "altruistically" vomit a bit and share it with his bat friends.

Now one may think that such a system would tend to profit a selfish cheater kind of bat, since he could always try to get a share of the common blood from his friends when he's hungry, and refuse to give away any of his own victuals should he be the happy camper returning home with the full stomach. Better to share, expecting the same favour later on, or better to refuse, profiting from the present lucky instant?

A prisoner's dilemma kind of game, in which bats have a complex decision to make.

Luckily the nature provided them with complex brains to help in their deciding business. The fact is, vampire bats have unusual brain size and the biggest neocortex, "the thinking part" of the brain, from among all the other bat species. This is to enable them to memorise better which bat colleague wanted to share in the past and which was selfish... and so the solitary selfish cheaters, that do not return a favor, are learned about and avoided next time.

It's thanks to their bigger brain that such a sharing system can work in reality.

After Matt Ridley's "On the origin of virtue".