What is cooperation?

Cooperation must somehow be something to increase individual fitness. Either because:

Forced cooperation (necessary): One simply cannot do it alone ("When you can't do it alone you must do it together"), like for instance sexual mating, where cooperation is a necessary condition.
Non-forced cooperation (voluntary): One can do it alone but not as economically efficient as doing it together, like division of labor.
There is probably a continuum between achieving something alone versus achieving something together. For instance, killing a mammoth has a very low probability of succeeding alone as a human being. Making children is impossible alone. However, a pin can be made alone, but it is far less efficient than a cooperatively made pin, as in Adam Smith's pin factory example.