Brains or brawn, which is more important? Can you have both? Well not if you’re a male bat. A recent study has shown a negative correlation between brain size and testicle size in bats; so for bats, intelligence comes at a steep price.
One theory is that the development of a large brain or large testicles is ‘expensive’ so bats can’t afford both.
Scott Pitnick, of the Syracuse University in New York conducted the research and says "The male who ejaculates the greatest number of sperm may win at this game, and hence many bats have evolved outrageously big testes. Because they live on an energetic knife-edge, bats may not be able to evolutionarily afford both big testes and big brains."
Females bats of many species mate with more than just one male and can store their sperm, leading to fierce competition. Pitnick believes that there is an important link between fertility and testicle size. After analysying 334 species of bats, Pitnick’s team found that in species with promiscuous females, males had evolved larger testicles and had relatively small brains. In species where the females were faithful to their mates, the correlation was reversed. Interestingly, male fidelity appeared to have no link to testicle or brain size.
In some bat species, the males' testicles can be as much as 8.5% percent of its body mass. In contrast, primates' testicles (including humans) is less than 1% of body mass.