In my opinion, reciprocity in vampire bats has not been demonstrated unambiguously. This is because all the evidence to date has really been correlational.
My singular obsession recently had been trying to setup and perform the “perfect” reciprocity test in vampire bats. Vampire bats are a good candidate for this kind of experiment because they will voluntarily help unrelated bats using a completely natural behavior (regurgitated food sharing) under controlled conditions in the lab. The real question is, can I create and control a situation of non-reciprocation (freeloading) to test a bat’s response?
Contingent rewarding and withholding of aid is the gold standard for demonstrating reciprocity, even though when people have done that exact test, critics still won’t believe it, unless you use different terminology. Anyways…
One way of doing this with vampire bats is to create a situation where two bats (bat A and B) with a history of much food sharing are no longer able to help each other (because they are always fasted at the same time), whereas another bat (bat C) with a history of less food sharing is always able to feed bat A. In other words, I can make a preferred partner less able to reciprocate and a less preferred partner more able to reciprocate. Then, will the bat switch its preference?
Some people think this design has a flaw because the bats might have an “excuse principle”– they might not “punish” bats that don’t reciprocate because they cannot (ie because they don’t have access to food). A recent study demonstrated this in mobbing flycatchers. However, any excuse principle should simply add a delay (patience) to reducing cooperative investments. If the are really reciprocators, then eventually, the bats have to care about the lack of return on their cooperative investments. From an ultimate evolutionary standpoint, a bad cooperative investment is a bad cooperative investment regardless of the reason why. So a bat that cannot return favors for any reason should be avoided after too many defections. Otherwise, there is no long term mutual benefit. If the bats are acting as altruists however (as the result of kin selection), no cooperative return is ever necessary. That’s why it is true altruism.
So can I control who feeds who? And will a bat switch its preference? Not easily it seems. The social relationships of these bats seems well entrenched by a past history of interaction. This video below (playing at 4 times actual speed) shows two unfed (hungry) females with a previous history of food sharing. Their relatedness estimate is 0.125 (like first cousins). The two bats (A and B) groom each other and beg each other but neither can share food with the other.
The video shows only a 2 min sample (the first min and ten minutes later) from a 1 hour trial. This is the third time that these bats have been hungry and unable to share food with each other. But everytime together, they groom each other a lot and seem willing to share food if only they could.
Now here, in the video below, is one of the same females (bat A = without a wing band) placed with a new and different partner (bat C) who has a full belly of blood and who has a higher relatedness estimate of 0.2 with this bat (close to being half-siblings). But, these two females are not very close and there has only been one past case of possible food sharing from C to A.
This is the third time that bat C has had the opportunity to feed bat A, but she never does. Why? Bat C is probably more related to bat A than bat B, and they have all been living in the same cage. But some of the bats are closer with each other than others, and these social relationships do not flip over so easily.
In the wild, female vampire bats have been recorded still living together after more than a decade. And when I’ve put previously unfamiliar bats together for 2 months, they begged but did not share food. So these relationships may take awhile to form.
Like humans and other primates, social relationships in vampire bats seem to develop slowly and are relatively stable to perturbations. For instance, many primates form long-term relationships which involve reconciliation after conflicts. Ravens also do this. Reciprocity is more difficult to demonstrate if it is embedded in a long-term social relationship, because every cooperative or defective act is buffered by a long history of past interactions.
Another interpretation is that social bonds are typically linked to relatedness in the wild and the bats don’t actual perform reciprocity or enforce cooperation. They simply help bats that are familiar and hence probably related. These are the two hypotheses I’m trying to distinguish between.