Nav: Home

Calls vs. balls: Monkeys with more impressive roars produce less sperm

October 22, 2015

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. New research on howler species has revealed an evolutionary "trade-off" between investments in the size of the male hyoid - the bulbous, hollow throat bone that allows the howlers' guttural roar to resonate - and in the size of reproductive organs, namely the testes.

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.

Researchers found that the trade-off corresponds to the mating systems of different howler species. Males with large hyoids and deeper roars but more diminutive testes live in small social groups with often only one male dominating a number of females - a "harem" social model.

Males with bigger testes and smaller hyoids live in large groups with up to five or six males, and females mate with all males in the group. These males don't have exclusive access to females, and the battle for reproduction is geared more towards "sperm competition": quantity and quality of sperm.

The findings, published today in the journal Current Biology, are a further example of sexual selection say the researchers - a theory first proposed by Charles Darwin in 1871 - and in particular the evolutionary trade-off between "pre- and post-copulatory reproductive strategies": traits that help males compete for access to mates versus those that help males compete to fertilise eggs.

"In evolutionary terms, all males strive to have as many offspring as they can, but when it comes to reproduction you can't have everything," said Dr Jacob Dunn, from the University of Cambridge's Division of Biological Anthropology, who led the new study.

"There is evidence in other animals that when males invest in large bodies, bright colours, or weaponry such as horns or long canines, they are unable to also invest in reproductive traits. However, this is the first evidence in any species for a trade-off between vocal investment and sperm production," he said.

In biology, trade-offs are said to exist when one trait cannot increase without a decrease in another. However, Dunn says that it's not yet clear exactly how the evolutionary trade-off in male howler monkeys works.

"It may be that investment in developing a large vocal organ and roaring is so costly that there is simply not enough energy left to invest in testes. Alternatively, using a large vocal organ for roaring may be so effective at deterring rival males that there is no need to invest in large testes," he said.

Along with collecting data on the average testes size across howler species, the researchers also used 3-D laser scans to analyse the size of over 250 hyoids - finding a ten-fold variation from the smallest to the largest howler throat bone. The team also conducted in-depth acoustic analyses of a number of howler roars.

"The vocal folds of a howler monkey are three times longer than a human's, yet they are ten times smaller than us, with a hyoid bone uniquely adapted to resonate sound and exaggerate their size," said Dunn.

"The results of our acoustic analyses show that howler monkeys produce roars at a similar frequency as tigers, which is far lower than we would have predicted from their body size, yet exactly what would be predicted from measuring their giant vocal folds."

This vocal system means that howlers give the acoustic impression of animals with much larger bodies, and can indeed roar louder and deeper than creatures ten times their size. The unnerving sound of a howler chorus ringing out across forests of Central and South America has long fascinated humans - from ancient Mayans to modern primatologists - and can carry as far as five kilometres through dense rainforest.

Charles Darwin was fascinated by the "wonderfully powerful" vocal organs of the howler monkey, despite describing their chorus as a "dreadful concert" in The Descent of Man.

The new Cambridge research continues to show just how accurate Darwin was when he wrote in On the Origin of Species: "The whole organism is so tied together that when slight variations in one part occur, and are accumulated through natural selection, other parts become modified."
-end-


University of Cambridge

Related Sperm Articles:

Paleontology -- The oldest known sperm cells
An international team of paleontologists has discovered giant sperm cells in a 100-million year-old female ostracod preserved in a sample of amber.
Research captures how human sperm swim in 3D
Using state-of-the-art 3D microscopy and mathematics, Dr Hermes Gadêlha from the University of Bristol, Dr Gabriel Corkidi and Dr Alberto Darszon from the Universidad Nacional Autonoma de Mexico, have reconstructed the movement of the sperm tail in 3D with high-precision.
Overweight and obesity are associated with a low sperm quality
Researchers from the Rovira i Virgili University in collaboration with researchers from the University of Utah have carried out a systematic review and meta-analysis evaluating the association between adiposity (normal weight, overweight, obesity, and low weight) and the sperm quality.
Novel switch protein that 'turns on' sperm for fertilization
Researchers from Osaka University and Baylor College of Medicine discovered a signaling cascade in which the testicular protein NELL2 travels through the lumen to induce differentiation of the epididymis, secretion of the protease OVCH2, and subsequent sperm maturation.
Diet has rapid effects on sperm quality
Sperm are influenced by diet, and the effects arise rapidly.
Sperm may offer the uterus a 'secret handshake'
Why does it take 200 million sperm to fertilize a single egg?
Long duration of sperm freezing makes no difference to live birth rates in large sperm bank study
Despite a time limit imposed in many countries on the freeze-storage of sperm, a new study from China has found that the long-term cryopreservation of semen in a sperm bank does not affect future clinical outcomes.
An important function of non-nucleated sperm
Some animals form characteristic infertile spermatozoa called parasperm, which differ in size and shape compared to fertile sperm produced by single males.
DNA of sperm taken from testicles of infertile men 'as good as sperm from fertile men'
Scientists have found that sperm DNA from the testicles of many infertile men is as good as that of ejaculated sperm of fertile men.
Let the sperm races begin
For best chances of in vitro fertilization success, the most motile sperm are chosen from semen.
More Sperm News and Sperm Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Listen Again: The Power Of Spaces
How do spaces shape the human experience? In what ways do our rooms, homes, and buildings give us meaning and purpose? This hour, TED speakers explore the power of the spaces we make and inhabit. Guests include architect Michael Murphy, musician David Byrne, artist Es Devlin, and architect Siamak Hariri.
Now Playing: Science for the People

#576 Science Communication in Creative Places
When you think of science communication, you might think of TED talks or museum talks or video talks, or... people giving lectures. It's a lot of people talking. But there's more to sci comm than that. This week host Bethany Brookshire talks to three people who have looked at science communication in places you might not expect it. We'll speak with Mauna Dasari, a graduate student at Notre Dame, about making mammals into a March Madness match. We'll talk with Sarah Garner, director of the Pathologists Assistant Program at Tulane University School of Medicine, who takes pathology instruction out of...
Now Playing: Radiolab

What If?
There's plenty of speculation about what Donald Trump might do in the wake of the election. Would he dispute the results if he loses? Would he simply refuse to leave office, or even try to use the military to maintain control? Last summer, Rosa Brooks got together a team of experts and political operatives from both sides of the aisle to ask a slightly different question. Rather than arguing about whether he'd do those things, they dug into what exactly would happen if he did. Part war game part choose your own adventure, Rosa's Transition Integrity Project doesn't give us any predictions, and it isn't a referendum on Trump. Instead, it's a deeply illuminating stress test on our laws, our institutions, and on the commitment to democracy written into the constitution. This episode was reported by Bethel Habte, with help from Tracie Hunte, and produced by Bethel Habte. Jeremy Bloom provided original music. Support Radiolab by becoming a member today at Radiolab.org/donate.     You can read The Transition Integrity Project's report here.