Males with fewer sperm shout louder

It's sometimes claimed that men buy fast cars and large camera lenses, amongst other things, to make up for deficiencies in other areas. But this behaviour may not be unique in the animal kingdom: now scientists at Cambridge University have shown that howler monkeys, which are natives of South and Central America, do something similar with the sounds they produce. Connie Orbach went to meet researcher Jacob Dunn to hear how...

Jacob - What we found was that there was a really close relationship between the size of this hyoid bone which a sort of big cup-shaped bone in howler monkeys which they use for resonating sound around, and making a really low frequency call. That closely matched the number of males that they had in each of the species. And so, those species that just had one male usually in a neat group tend to have a very big vocal organ whereas those species that have multiple males tend to have a very small vocal organ. What we also found is that there's a really close relationship between the size of this bone in the throat and the size of their testes. So, species that had a very big vocal organ had very small testes and species that had a very small vocal organ had very big testes.

Connie - So, let me get this right. Loud road (recording), smaller testicles, but big harem. Small roar (recording), bigger testicles. So, it seems the howler monkey just can't have both. But what's it got to do with how many other males there are?

Jacob - We know species that tend to live in groups with just one male. They try and defend groups of females very aggressively and in different species that have more males in the group, there's no longer the need for this male-male shouting competition. But because females will often copulate with several males within the group, so the sort of pressure shifts towards them having to compete for each other after they've copulated with the females. And so, they evolve bigger testicles so that they can produce more or faster sperm.

Connie - Okay, so that makes sense. Now, this hyoid bone must be quite something to have such a big effect. What does it even look like?

Jacob - In humans, it is a very simple little bone. It fits on top of the larynx just under the chin in the throat it's a little horseshoe-shaped bone. But howler monkeys have evolved this different kind of shape to their bone. They've turned it into a sort of bulbous expanded cup-shaped thing which they actually have some soft tissue as well that they blow into, it's a sort of air sac. But it varies hugely from species to species. So, the males have much bigger hyoids than females. But even within the males across the species, there's 10-fold variation from the smaller species to the biggest.

Connie - Is this a model you've got here or is it a real skull?

Jacob - No, this is a cast.

Connie - And this is the hyoid bone.

Jacob - Yup.

Connie - It's a really, really weird-looking bone. It's half a balloon-shaped almost and it seems to be sat kind of right underneath the jaw there. So, why can't they have both? Why do they have to have one or the other?

Jacob - We don't know the answer to that fully yet, but there are two different ways or at least two different ways that this might work and it could be that both of them are working at the same time. One of them is that there is sort of limited energy to be able to invest in different things. The expression of these kinds of traits are costly. There might just not be energy available to do both of those things during development. That will be one mechanism. The other system would be that once you're sort of down, one evolutionary trajectory, there's kind of no point in investing in both. So, as you become a species that has a big hyoid bone which means that you produce a really low frequency call, you become so dominating that your social system becomes that of just one male in the group.

Connie - There's those classic arguments about big cars or flashing their cash is maybe compensating for something else, but do we actually see this in people ever, a trade-off?

Jacob - It has been only once that is as far as I know where they've looked at this kind of trade-off. They found something really interesting actually. They found that the voices of men when played back to women that were judged to be more attractive also had low quality sperm. Which suggests that there might also be this sort of similar kind of trade-off, but it certainly suggests that something might be going on and I wouldn't be surprised if there really is.

(Sounds copyright The Macaulay Library at the Cornell Lab of Ornithology)