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Opinion: ̽��ֱ��biggest sperm come in the smallest packages – and other odd facts about male sex cells

Anonymous — Thu, 19 Nov 2015 14:32:59 +0000

Most people probably think of sperm as the microscopic tadpole-like things wriggling around in human semen. But there is an astonishing amount of diversity in the size, shape and number of sperm produced by male animals. In fact, despite performing the very same function in all animal species (fertilising eggs), sperm are the most diverse cells found among animals.

This diversity is a product of evolution. Every animal’s sperm has evolved to meet the needs of the individual animal that produces it. For example, new research published in Proceedings of the Royal Society B shows that the number and size of sperm produced by a mammal depends on the size of the female reproductive tract. Studying these kinds of adaptations helps us to better understand the incredible diversity we see in sperm across animal species.

Tiny animals can have massive sperm

Sperm length varies by several orders of magnitude across species, from the tiny sperm of the porcupine (0.0003 cm) to the gigantic sperm of the fruit fly (6 cm), which is more than 20 times the length of the fly. ̽��ֱ��fruit fly’s sperm looks like a wound-up ball of string that unravels once inside the female’s even longer reproductive tract.

̽��ֱ��number of sperm produced by different animals also varies enormously. Humans produce approximately 100 million sperm per ejaculate, while rams can produce 100 billion. Groups of sperm can even work together. Sperm in some species are known to team up and form a “train” that swims faster than individual sperm.

Promiscuous females mean more sperm

Much of the variation we see in the size, shape and number of sperm produced by different species is thought to be the product of competition for fertilisation among the sperm of different males. This is a type of sexual selection, only relatively recently described, known as “sperm competition”. In species whose females mate most promiscuously, there is strong pressure on males to invest more heavily in sperm, to ensure that one of their own little soldiers is the one that wins the battle for fertilisation.

This has led to an extraordinary array of different warfare tactics. These species, in general, produce more sperm, as more soldiers on the ground gives you a numerical advantage. It may also be advantageous to produce bigger sperm, which are faster and able to outcompete the sperm of other males in the race to the eggs.

Bigger females mean smaller sperm

Sperm can also vary depending on the size and shape of the female body. To accommodate this, sperm have to be able to swim far and fast enough, to successfully reach the eggs. In general, bigger sperm swim faster, so males should produce numerous, large sperm. But males only have finite resources to allocate to sperm production and may face trade-offs.

This means that evolutionary pressure to increase sperm size will inevitably lead to a reduction in number, and vice versa. As mentioned, recent research found that among mammals, males of smaller bodied species tend to invest in fewer, larger sperm, while males of larger species tend to invest in more, smaller sperm. This is because the females of larger species have bigger reproductive tracts and so more (but smaller) sperm can spread across the greater space and have more chance of encountering an egg.

Manlier males have lower quality sperm

As sperm can be costly for the body to produce and resources are limited, males can also face trade-offs between producing sperm and other characteristics useful for reproduction. For example, species in which males invest more in big bodies and horns, or deep voices have been shown to produce less sperm.

In humans, men with more attractive voices have been shown to have worse quality sperm. It seems that males are faced with a trade-off between investing in traits that are useful for competing with rivals, or those that increase the chance of fertilising an egg. They can’t have everything.

Males can control sperm quantities

Amazingly, males seem to be able to control the amount of sperm they produce. There is evidence that males vary the amount of sperm in ejaculates, depending on the quality of the female, or the risk of sperm competition. In humans, men looking at explicit images of two males and one female (“sperm competition images”) have been shown to produce more mobile sperm than those looking at explicit images of three females.

Female animals who mate with more than one partner are also thought to have some control over the sperm that fertilises their eggs. So-called, “cryptic female choice” occurs when females use physical or chemical mechanisms to control each male’s chances of fertilisation. This is well described in a number of animal species, providing a mechanism by which females can bias the outcome of reproduction. For example, in some species, females will mate with several males and then selectively fertilise eggs with only the largest sperm or sperm from males with more compatible immune system genes.

Jacob C Dunn, Lecturer in Human Biology, ̽��ֱ�� of Cambridge

This article was originally published on ̽��ֱ��Conversation. Read the original article.

̽��ֱ��opinions expressed in this article are those of the individual author(s) and do not represent the views of the ̽��ֱ�� of Cambridge.

Jacob Dunn (Division of Biological Anthropology) discusses why sperm are the most diverse cells found among animals.

Joyce Harper, UCL, Wellcome Images

Sperm cells

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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

fpjl2 — Thu, 22 Oct 2015 16:02:52 +0000

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.

̽��ֱ��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.

̽��ֱ��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 ̽��ֱ�� 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. ̽��ֱ��team also conducted in-depth acoustic analyses of a number of howler roars.

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

“ ̽��ֱ��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. ̽��ֱ��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 ̽��ֱ��Descent of Man.

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

Evolutionary ‘trade-off’ between size of throat and testes discovered in howler monkeys furthers Darwin’s theory of sexual selection and corresponds to mating systems: males with larger throats but smaller testes often have exclusive access to females, while those with larger testes share mates.

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

Jacob Dunn

Calls vs. balls: An evolutionary trade-off

Mariana Raño

A black howler monkey chorus

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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