How can monkeys be big if they have to be? Is diet the only influence?
You inherit the potential to grow a certain body stature from your parents. You can achieve that potential only if you have good nutritional health when you are growing/maturing. When I talked about male baboons having larger stature than females, this is a trait that is inherited. That is, it is an example of an ADAPTATION… a physical trait, in this case, that had a selective advantage for individuals, allowing them to achieve greater reproductive success and pass that on to their offspring. What is different in this case is that male and female stature can DIFFER in some species if the selection pressures on males and females differ… some alternative explanations for sexual dimorphism are described in Lewin Chapter 13.
