She and her team found that howler monkeys face an anatomical trade-off: a species can either have a really big hyoid bone, or it can be very well endowed. But not both. That finding is in the journal Current Biology. Those species with bigger hyoids live in single-male groups, so it makes sense to invest in what the researchers call "precopulatory traits," that is, their courtship strategy of attracting the female's attention. But in a species where groups include multiple males, the best way to ensure passing on more genes is to make more sperm—by building bigger testes. The researchers say the finding is the first evidence for an evolutionary tradeoff between mating calls and sperm production. And they warn that while human women do tend to prefer males with deeper voices, we men advertise ourselves in many other ways as well. So don't go clicking that pop-up ad that promises to enlarge your hyoid.