A vampire bat that fails to feed can ask a roost-mate to regurgitate blood for it

A vampire bat starves if it goes two nights without a blood meal, but hunting fails often enough that many bats would die without help. Biologist Gerald Wilkinson found that well-fed bats regurgitate blood to share with hungry roost-mates — not just relatives, but unrelated bats that have shared with them before. Bats that never reciprocate get cut off, making this one of the clearest documented cases of reciprocal altruism outside humans.

— Gerald S. Wilkinson, Reciprocal Food Sharing in the Vampire Bat — Nature, 1984