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Speed counts with attractive females

Science | Evolution

Jonathan Wood | 16 Jul 09

Two male red jungle fowl

Male red jungle fowl can adjust the quality of sperm they produce, depending on how attractive the female fowl is.

The story, reported last week by Discovery Channel online, goes on to explain that males of many promiscuous species in the animal kingdom – including humans – can mate with many females, but they adjust the quality of their sperm to improve chances of fertilization when the female is more attractive.

The Oxford University research published in Proceedings of the Royal Society B involved red jungle fowl, the ancestor of all modern chicken breeds. The team showed that males transfer seminal fluid to matings with attractive females that boosts sperm swimming velocity.

The results provide crucial evidence of what causes variation in sperm quality, which has important implications for fertility and the evolution of sexual strategies.

I caught up with Dr Charlie Cornwallis of the Edward Grey Institute in the Department of Zoology to learn more.

OxSciBlog: Why might cockerels adjust the quality of their sperm when mating with different females?

Charlie Cornwallis: Females vary in their attractiveness with some hens having large fleshy combs on their heads whereas other females are barely ornamented at all. The size of a female’s comb relates to size of the eggs she lays. So by inseminating higher quality sperm into these females males have a greater chance of fertilising eggs that result in superior offspring.

OSB: How do they do it?

CC: They do this by allocating different amounts of seminal fluid to attractive and unattractive females. Sperm cells are the machines that swim, but to do so they need fuel and seminal fluid provides a crucial energy source. By allocating more seminal fluid to ejaculates males are able to increase the performance of their sperm.

OSB: How were you able to find this out?

CC: Since female attractiveness can easily be determined by measuring the size of the fleshy comb on top of their heads, we fitted attractive and unattractive females with a small harness that enabled us to collect sperm. After males had copulated with females we analysed ejaculates using computer aided sperm analysis software attached to a microscope that was originally designed for IVF programs. This software measures the speed at which sperm swim and this relates to how likely they are to fertilise eggs.

OSB: What are the implications of your findings?

CC: It has previously been shown in a number of species, including humans, that males can adjust the quality of sperm they ejaculate, but how they do this has remained a mystery. Our results show in chickens that cockerels do this by allocating more seminal fluid. This takes us one step further to understanding the factors that determine fertilisation success. We now have to assess what it is in the seminal fluid that makes sperm tick and if this is the case for other species.

OSB: What do we know about humans? Do men use similar strategies to chickens and is it even possible to do experiments to find out?

CC: The ability of males to adjust the quality of sperm they allocate to ejaculates has been shown to be extremely widespread. In fact it has been demonstrated in insects, fish, birds and mammals, including humans. However, it is not possible to tell whether males across all these species adjust their sperm quality using seminal fluid without conducting experiments specifically tailored to each species. I know that some colleagues are carrying out some experiments on humans at the moment so hopefully they will be able to answer part of this question soon.

A male red jungle fowl. Credit: Charlie Cornwallis. 

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