It's something most women intuitively knew: the louder the man, the smaller the genitals. Now science has more or less ticked off on the assumption - in monkeys, at least.

According to a study published by the Current Biology journal, male howler monkeys with a louder and deeper roar are more likely to have tinier testicles.

You know what they say about deep voices: a black howler monkey. Credit:Mike Lane / iStock

The researchers, from Cambridge, Utah and Vienna universities, said it marked the first evidence in any species of an "evolutionary trade-off" between the vocal tract and the testes.

"This means that different species of howler monkeys either invest in one of these traits or the other, but not both," said Jacob Dunn, a biological anthropologist at the University of Cambridge.