Monogamy related to testicular size

I was watching a youtube video recently about relationships, and the speaker made a comment about monogamy as it relates to testicular size. In the family great apes, the larger the testes the more promiscuous.

So, to my disbelief this morning I found myself Googling for pictures of orangutan balls.  Apparently, orangutans are not highly sexually competitive. They tend to live fairly solitary lives, and the females do not mate with many males.  As it turns out, their testes are small with respect to body weight.

maleorang

On the opposite end of the spectrum, are chimps. They are a promiscuous species. The females may mate with many males.  Male chimpanzees have large testes relative to body weight.

chimpballs

Humans, as it turns out are in-between chimps and orangutans. Our testes are larger as a percent of body weight than orangutans, but smaller than those of chimps.

I don’t think I need to include a picture of some guy’s balls.

This is interesting compared with another post I made a while back about how promiscuity is related to sexual dimorphism.

In all three cases (humans, chimps, orangs) there exist sexually dimorphic traits.  Males are larger in all species. Like human males, male orangs grow beards.  They also have those funny face pads.  I don’t believe any of these three species mates for life.  This sexual differentiation may be a function of the social organization on a spectrum from solitary to social.