Many types of organisms employ physiological trade offs during their life cycle. Jacob Dunn, an evolutionary biologist at the University of Cambridge in the United Kingdom, has found evidence of an interesting set of traits among howler monkeys. The research finds that monkeys with louder and deeper voices had small testicles and vice versa. Evidently, it is too energetically costly to have both. Please see the full story for some clever evolutionary insight into these loud primates.
http://www.sciencemag.org/news/2015/10/big-testicles-mean-softer-voice-least-howler-monkeys
