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Testes size indicates that we are between the chimpanzee and gorilla - not closer to the gorilla. In fact, in terms of testes size to body size ratio we are closer to the chimp; gorillas have almost no testicles. I agree with you that sperm competition was reduced in our lineage - however, understanding the degree to which it was reduced is difficult. To me, our testicle size indicates that we evolved in a system where a minority of men had the ability to practice polygyny, with other males defaulting to monogamy (with infidelity) and/or serial monogamy.