Final answer:
Mutualists achieve greater fitness through acts of altruism that increase their inclusive fitness by aiding relatives, or through reciprocal altruism, expecting a return of favors, ultimately benefiting both parties involved.
Step-by-step explanation:
Mutualists can still have greater fitness despite giving things away because they will engage in behaviors that, while seemingly altruistic, increase their inclusive fitness by aiding relatives who share their genes, or through reciprocal relationships that benefit them in the long term.
In the case of mutualists like social insects, a sterile worker increases its inclusive fitness by helping the queen reproduce, as her offspring will carry the worker's genes. This is a natural outcome of kin selection. Moreover, seemingly altruistic acts among unrelated individuals can enhance greater fitness through behaviors such as reciprocal altruism, where each party benefits by taking turns in the role of giver and receiver.
For example, in monkey societies, individuals groom each other not out of pure selflessness, but with the expectation that the favor will be returned, thus raising the overall fitness of both parties involved and fostering cooperative relationships within the group.