Vampire bats use the buddy system

I guess we’ve known for a while that vampire bats are (reciprocally) altruistic. They’ll share excess blood with the understanding that others will share with them in their own time of need:

Moreover, we found that blood sharing was not random, even among bats who had a high degree of prior association. Instead it appeared that the unrelated bats developed a buddy system, so that two individuals regurgitate almost exclusively to each other–a strong indication that their roles reverse on a regular basis.

PDF of the article is here.