Promiscuity Linked to Small Brains

In species where females are sexually promiscuous there is an evolutionary advantage for the males to have large testes capable of generating large amounts of sperm. If a male is uncertain about the constancy or duration of a female's affections, there will be strong motivation for him to ensure against some subsequent competitor sneaking in at the wire and impregnating the female, by choosing 'quantity over quality'.

Nothing portrays this clearer than the following diagram of the relative sizes of body, penis and testicle size (also showing relative body, breast, and ovary size for females)



Chimpanzees are promiscuous, and their testicles are many times larger than those of gorillas, in which a single dominant male has exclusive access to a “harem” of females. Because the male gorilla does not worry about (day to day) competition, he need not invest excessive energy creating sperm through growing large testicles. Humans testicles are intermediate in size (on average of course).

A study of testicle size in bats produced similar results. In the study, a team looked at testicle and brain size in 334 different species of bat. They determined that the size of testicles increased markedly in species with promiscuous females, and that the males' brains were correspondingly smaller.

All else being equal, an animal has to chose between making sperm and making brain cells - there is no free lunch. For those species that choose the former in order to protect their genetic interests in the face of loose females, the males brain size 'shrinkages' accordingly.

The reader can draw their own conclusions as to the relevance of these studies to identity system trust models.