Howler monkeys are about the size of a small dog, weighing around seven kilos, yet they are among the loudest terrestrial animals on the planet, and can roar at a similar acoustic frequency to tigers.

Evolution has given these otherwise lethargic creatures a complex and powerful vocal system. For males, a critical function of the roar is for mating: to attract females and scare off rival males.

But not all male howler monkeys have been equally endowed. The bigger a male howler’s vocal organ, and the deeper and more imposing roar they possess, the smaller their testes and the less sperm they can produce.

Dr Jacob Dunn from Cambridge's Division of Biological Anthropology describes this evolutionary 'trade-off' and how it relates to Darwin's work on sexual selection.

Find out more here: http://www.cam.ac.uk/research/news/calls-vs-balls-monkeys-with-more-impressive-roars-produce-less-sperm