About 7% of 340 adult vampire bats and 33% of 258 younger bats (<2 years of age) failed to feed on a given night. But when they do feed, wild vampire bats fill up like water balloons.
Jon Flanders took this picture of a male vampire bat. He had originally caught this bat around the time of emergence and then caught it again about 3-4 hours later. As you can see, it had just fed and it’s so full it looks pregnant. He described the recapture this way:
Urinating the whole time I was getting it out of the net (I could see where its urine was on the floor underneath it as well). When we weighed it, it was still 6g heavier than when we first caught it.
This is a great illustration of how foraging in vampire bats is a ‘boom-or-bust’ phenomenon, where a successful bat gets either a large meal or none at all.
If only they could store it for later. But alas– to make matters worse, vampire bats don’t put on fat like other mammals.
So the best way to save food for later might be in the form of social capital that leads to future sharing.
Before describing the food-sharing systems of the Ache people of Paraguay and the !Kung San of the Kalahari Dessert, Steven Pinker lucidly explains the basic logic of reciprocal food sharing in How the Mind Works:
…species are driven to share when the variance of success in gathering food is high. Say in some weeks I am lucky and have more food than I can eat, but in other weeks I am unlucky and in danger of starving. How can I store extra food in the fat weeks and draw on it in the lean weeks? … I can store it in the bodies and minds of other people, in the form of a memory of my generosity they feel obliged to repay when fortunes reverse. When the prospects are risky, it pays to pool the risk.
Compared with females, adult male vampire bats do not participate much in reciprocal food sharing. So one would predict that they starve to death more often, and that males may even have slightly different physiological strategies for dealing with fasting. But I’m not sure if there’s any evidence to support this conjecture.