What reproductive pressure explains the very large testes size in chimpanzees relative to gorillas?

Answer

Intense sperm competition resulting from females mating with multiple males in short periods

Chimpanzees live in social groups where female promiscuity leads to high levels of sperm competition, favoring the evolution of larger testes capable of producing a greater volume of sperm.

What reproductive pressure explains the very large testes size in chimpanzees relative to gorillas?
reproductionanimalanatomyPrimategorilla