Reciprocating rats! Rats more willing to help those that help them.

Humans are familiar with the concept of returning a favor if someone has helped them, this is known as reciprocity and is a favorite explanation for how co-operative behaviors evolved. In nature this behavior is difficult to study, this could be due to it being rarer then thought in nature or because it is a behavior that is difficult to measure in the field.
A study by Dolivo and Taborksy looking at Norwegian lab rats (Rattus norvegicus) found that the rats give as good as they get. Following the principle that this behavior is hard to study in the wild and therefore using a controlled laboratory environment instead; they trained the rats to pull a stick that issued food items to a rat in a adjourning cage which could see the the rat dispensing the treat through the cage. They then expanded on this by placing a rat in between two cages containing rats on either side. The cage on the left had a stick that upon being pulled dispensed a carrot treat to the central rat whilst, the cage on the right had a stick that dispensed a banana treat to the central rat. The central rats generally favored the banana treat therefore, this would suggest that the central rat would favor the right side rat that dispensed the banana treat and would be more willing to help banana providing rats than carrot providing ones; this turned out to be true with rats that provided better help (banana) receiving greater rewards as a result, indicating reciprocal behavior.

 
These results therefore clearly show that rats have the ability to recall not only which rat helped them but the quality of help received as well, adjusting their behavior accordingly as a response. Adding this onto previously studies undertaken by Taborksy's team that rats are more likely to help a partner that has aided them before compared to one that had not; it provides a interesting insight into how a species manages the challenges of co-operation.
However it is not just rats that have displayed the ability to reciprocate, a study by Melis et al (2006) demonstrated how chimps theorized which partners to choose, based on previous experiences, when embarking on tasks. It has also been seen with fish that use cleaner fish, these client fish as they are often referred as, will often favor cleaner fish that behave in a more co-operative manner. Lastly even pied flycatchers have been observed favoring other flycatchers that actively aid them in mobbing owls in their territories.

All these examples clearly show that reciprocation is used within the natural world as, it provides a benefit to both animals involved and that those who help more will receive more in return; this behavior possibly aids in forming stronger bonds amongst social groups allowing those who aid the group to be favored over those that do not.
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