The following story reminds me of this old joke:

I went to the County Fair. They had one of those “Believe it or not?” shows.

They had a man born with a penis and a brain.

Here’s the story:

Syracuse University biology professor Scott Pitnick knows a lot
about the birds and the bees (or in this case, bats). He teaches
courses in evolution and population biology, and researches topics such
as sexual selection, sexual conflict and speciation. His latest
research sheds light on a little-explored area of evolutionary biology:
the contribution of sexual selection to brain evolution. “The brain can
be considered the most important ‘sex organ,’” says Pitnick.
“Nevertheless, the relationship between breeding system and relative
brain size has received little investigation.”

A recent study by Pitnick and colleagues Kate Jones of the
Institute of Zoology at the Zoological Society of London and Jerry
Wilkinson of the Department of Biology at the University of Maryland
was funded by the National Science Foundation and used comparative
analysis to show that bat species that roost in larger social groups
and those with promiscuous females have relatively smaller brains than
species with females that are faithful to their mates. Male infidelity,
by contrast, had no evolutionary impact on relative brain size.

According to Pitnick, a likely explanation for this relationship
relates to the energetic demands of producing and maintaining both
brain and sperm cells; males cannot afford a lot of both. Under this
explanation, males with relatively large testes and small brains leave
more offspring than larger-brained, less fertile, competitors.

“When females mate with more than one male, sperm compete to
fertilize the female’s eggs. Such ‘sperm competition’ is rife in many
bat species, perhaps due in part to the unusual ability (among mammals
at least) of sperm to survive inside the female’s reproductive tract
for a very long time,” says Pitnick. “The
male who ejaculates the
greatest number of sperm may win at this game, and hence many bats have
evolved outrageously big testes—as much as 8.5% of their body mass.
Because they live on an energetic knife-edge, bats may not be able to
evolutionarily afford both big testes and big brains. We’re excited
about these results, as they may stimulate more research into the
correlated evolution of brains, behavior and the extravagant and costly
ornaments and armaments favored by sexual selection.”

The full results of Pitnick’s study have recently been published in Proceedings B (London) a biology journal of the Royal Society, the United Kingdom’s national academy of science.

You know what this means, don’t you?  If someone says you have big balls they are also saying you are pea brain and your mother was promiscuous.