How do sperms compete for an egg?

09 October 2018

Competing sperm

Competing sperm

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Question

What tricks do sperms have to reach the egg first? Is it like gamete wacky races?

Answer

It was over to evolutionary biologist Jacob Dunn to answer this question from listener James...

Jacob - That’s a brilliant question. I don’t know how long I’ve got but I could talk about sperm for a long time.

Right. Sperm don’t have sleeves to start with, but they do have all sorts of amazing tricks - if they did have sleeves and they would be up them. So to start with, in lots of species there are different kinds of sperm tricks evolved, but particularly in those species where multiple males breed with the same female because then the kind of level of evolution shifts. The level of the selection shifts to the sperm and then it’s all about who gets there first. In those species we tend to see that the males have much bigger testes so they can produce more sperm so it’s more likely just by sheer chance that the sperm from a given male beats the sperm from another male, but they may also evolve faster sperm. They also evolve to have special enzymes, chemicals that kill off the sperm of other males, so if there’s the sperm of one than one male in the reproductive tracts of the female, the chemicals kill off the other sperm.

And lots of other amazing thing but, for example, there are special enzymes that form what’s called a copulatory plug. So a male copulates with a female and then basically this plug forms sealing off the female reproductive tract so that no other male can reproduce with her afterwards. Even the shape of the penis has thought to have evolved for this kind of chemical warfare. The human penis, for example, the gland on the end is thought that it’s perhaps evolved so that it can pull out sperm from other males from inside the female.

But that’s not to say that females are kind of just waiting for this fantastic sperm to arrive. By no means is that the case, so the egg is very active in this process and females are very active in this process as well. Females have evolved brilliant strategies in some species to be able to eject sperm or store sperm and, essentially, choose sperm from the right male to fertilise the eggs, and other kinds of behaviour. So selectively for example to abort a foetus if it’s not from the right male and various other tricks.

There are textbooks about this kind of sperm competition and what’s called cryptic female choice. It’s a fascinating area of evolutionary biology.

Chris - Thank you very much Jacob. It certainly is fascinating. I learned recently that bees actually can store sperm for years and years and years. They mate it once in their life and they use this stored sperm for years to produce all of their offspring. It’s fascinating thing to learn about.

Jacob - Absolutely! Lots of insects store sperm for a very long time and from lots of different males sometimes too.

Chris - They can take their pick.

Jacob - Yeah, exactly.

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