OldWoodSpecter said:

How do you know that without society there would be no loyality? Jelousy is an emotion that apperas even at early age

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My point was that being faithfull (in monogamy and polygamy) in not always unnatural behaviour in nature

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Veritas said:

A genetically-linked trait is selected because those members of the species who possess said trait are more likely to survive and successfully raise their offspring to sexual maturity.

Of course the swans do not "choose" to do what is good for their genetics! However, if non-pair-bonded swans hatch cygnets who are more vulnerable to predators, their genetic traits are less likely to be passed on to future generations.

Perhaps they are "jealous," but nothing I have read about swan behavior conveys this image to me. They pair-bond by choice, because they are descended from swans who pair-bonded. If jealousy were the motivating factor, why would widowed swans grieve for years after the death of their life-mate? Why would they build a nest season after season, yet reject all potential suitors, as if waiting for the return of their mate? Because their grandparents did.

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OldWoodSpecter said:

so then that's not, really a choice now is it? They do it because it is in their genes, and how do these genes manipulate them to do that? By making the feel the urge to do it (like any other behaviour)
This urge also causes the anti-urge when it comes to deviation from that bhaviour. In other words, the urge to do it would make them object if the other partner decides not to do it. That's jealousy

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