Final answer:
Altruistic behavior being the random effect of living in a complex social species does NOT explain why it exists in primate societies, as altruistic behaviors are typically non-random and evolutionarily beneficial via kin selection or reciprocal altruism.
Step-by-step explanation:
The question at hand asks which of the following is NOT a reason for the existence of altruistic behavior in primate societies. The answer is:
altruistic behavior is the random effect of living in a complex social species.
This doesn't explain why altruistic behavior exists because altruistic behaviors are not generally random but often have evolutionary benefits through mechanisms such as kin selection and reciprocal altruism. Altruistic behavior is often directed toward related individuals, which supports the inclusive fitness of the altruistic individual by helping to pass on shared genes. This behavior is an example of kin selection, where individuals behave altruistically towards their kin to improve their own genetic success.
In some cases, even unrelated individuals may behave altruistically towards each other for mutual benefit, known as reciprocal altruism. This occurs when there is an expectation of the favor being returned in the future, hence making it a strategy for long-term fitness gain. This reciprocity requires a level of social structure where individuals can repeatedly encounter each other and where there are systems in place to punish cheaters, thus maintaining the mutual benefits of these altruistic acts.