Turns out that bats practice reciprocal altruism - basically the exchange of acts which benefit both giver and receiver over time - by sharing meals. Let's just say this involves the words 'blood' and 'regurgitate' and then leave it at that.
It occurred to me that perhaps Halloween is another example of reciprocal altruism. Consider the scenario: a young person in disguise comes to your door with a proposition - trick or treat. By this act and your subsequent response, you both potentially benefit. You're forewarned about the consequences for failing to cooperate, thereby increasing the chances that you will make the prudent choice and protect your private property, and the little manipulator will almost certainly get candy.
Notice the similarity between vampire bats and trick-or-treaters? Well, yes, I suppose they are both small creatures that come out after dark to suck the [blood, chocolate] from the helpless. But what I was really getting at is that food plays a central role in their acts. For bats, it is the means to an end: sharing their meals with members who failed to sufficiently feed themselves increases the survival of the whole colony by reducing the number of deaths. For Halloweeners, food itself is the goal: the collection of candies motivates these masked extortionists who use an undefined but understood threat of mischief to enforce sharing by those holding the desired objects.
An offering of food, whether for survival (bats) or other reasons like blackmail (trick-or-treaters), is always welcome because of its high intrinsic value as a basic necessity of life. So it's an ideal exchange tool, particularly when the giver and receiver are not well-acquainted. Case in point: every Christmas, my husband receives gifts ranging from tins of popcorn to fruit baskets from companies who want to maintain or establish new business with him. They know nothing personal about him but are rightly confident that a gift of food will not be rejected and that its acceptance paves the way to a stronger (business) relationship. Among our own circles of family and friends, in which relations are cemented by more intimate ties, edible gifts can hold extra value as expressions of love and affection.
I don't know if vampire bats are aware of success in their strategy but I know my aims in the Halloween ritual of reciprocity have been achieved if the neighbors' kids are on their way to tooth decay, my trees are not decorated with Charmin and, if I planned well, there are 4 bags of mini KitKat and Twix bars left. Reciprocal altruism: a win-win situation for all.