"Reciprocal Altruism" is a term from evolutionary biology and it describes a form of social interaction that is present in pretty much every social species of animal on Earth, including insects, mammals and humans.
If you give something without expecting an immediate compensation, then the other individual will be more likely to help you out when you need something. It's the basis of both friendship and love. It's the whole 'giving and taking' part of a relationship.
The simple principle that is the evolutionary basis of this behaviour is that of helping each other/sharing. It sounds paradoxical, but working together is the egoistic thing to do:
Lions and wolfs, fierce predators, but also very social animals which operate in a hierarchy that is quite similar to the way in which we instinctively construct our societies. They work together and share the kill and everyone benefits individually. It's egoism from an individuals point of view but an effective strategy for a group. It's like making money with your job (egoism) while helping to maintain the economy of your country (which is social, because everyone benefits). A lone wolf has a slim chance of catching a large animal, but a pack of wolfs that shares it's kill will make it through the winter.
Cheers, Tom.
Zappy5971
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Awww. I quite agree and a dog will protect his pack (animal or human) because it is his instinct to do so. I love that picture, those pups are gorgeous.