I vant to share my blood

It is easy to assume that sucking another creature's life's blood is the simplest way to obtain necessary nutrients. Problem is, most creatures don't like their blood being sucked, but moreover, as anyone doing mass spectrometry on serum will tell you, blood is around 95% water. The remaining 5% contains proteins and amino acids, electrolytes, and virtually no fat. The New York Times has this interesting piece on bloodsuckers. Because blood is really a poor diet and mostly water, bats quickly expel it through urination. If they do not feed within 60 hours, they will die. Since they pretty much feed at night, they must feed at least every other night. But what if they don't?

Vampire bats exhibit reciprocal altruism. The successful hunters will share some of their blood with the unsuccessful ones. There seems to be the recognition (at least behaviorally) that bats in need today may be the successful hunters tomorrow, when the successful ones today might become the needy.

For more info, check
http://www.bio.davidson.edu/people/vecase/Behavior/Spring2002/perry/altruism.html and http://courses.washington.edu/ccab/Wilkinson%20on%20vampire%20bats%20(small)%20-%20Sci%20Amer%201990.pdf