Sloppy seconds and reproductive strategy
Males of promiscuous species that wait until they are last in line to mate may gain a sexual advantage by "parasitizing" the sperm of other males, according to new research.
In the majority of insects, and many other species, research has shown that males that are the second or last sexual partner of a female, are likely to father a higher proportion of her offspring.
Species where a "second mate advantage" has been proved, include seed beetles, yellow dung flies, drosophila, parasitic wasps and butterflies – as well as primates and ground squirrels in mammals.
This advantage can be partially explained by forms of "sperm competition", whereby second mates produce larger volumes of sperm, or where the sperm of second mates incapacitates the sperm of earlier lovers inside the female.
Many animals also have cunning methods of removing their predecessors' sperm – some dragonflies, for example, have barbs on their penises that dig out the sperm of rivals.
Hostile place
Now, evolutionary biologists David Hosken and David Hodgson at the University of Exeter, UK, have discovered a further explanation. The pair scoured the literature for evidence that the ejaculate of first mates may unintentionally streamline the pathway to fertilisation of sperm of second mates.
The female reproductive tract is a hostile place for sperm in most species, says Hosken. It has an acidic pH and a sperm-unfriendly complement of immune cells.
Testament to this is the fact that many species produce such prodigious quantities of sperm cells, and that very few of these make it to the fertilisation site. Mammals, for example, produce hundreds of millions of sperm in each ejaculate, but less than 0.001% make it to the female's egg.
Environmentally friendly
Seminal fluid has a range of properties that help to mitigate the inhospitable nature of the vagina. The researchers argue that the seminal fluid expended by first mates, helps to improve the environment, but many sperm are lost in the process.
The sperm-friendly environment that results, however, means that second mates need expend less energy-draining seminal fluid and produce more sperm. This helps them pip earlier rivals to the post, or rather, the egg.
It's a kind of parasitism of males on males,says Hosken. Several insect studies show that increasing the time between first and second mating decreases the advantage the second male gains, helping to back up the idea.
Now Hosken intends to experimentally test the idea in crickets, which attach sperm in packets to the outside of females. Attachment times can therefore be manipulated, he says.
Journal Reference: Journal of Theoretical Biology (DOI 10.1016/j.jtbi.2006.06.024)
Males of promiscuous species that wait until they are last in line to mate may gain a sexual advantage by "parasitizing" the sperm of other males, according to new research.
In the majority of insects, and many other species, research has shown that males that are the second or last sexual partner of a female, are likely to father a higher proportion of her offspring.
Species where a "second mate advantage" has been proved, include seed beetles, yellow dung flies, drosophila, parasitic wasps and butterflies – as well as primates and ground squirrels in mammals.
This advantage can be partially explained by forms of "sperm competition", whereby second mates produce larger volumes of sperm, or where the sperm of second mates incapacitates the sperm of earlier lovers inside the female.
Many animals also have cunning methods of removing their predecessors' sperm – some dragonflies, for example, have barbs on their penises that dig out the sperm of rivals.
Hostile place
Now, evolutionary biologists David Hosken and David Hodgson at the University of Exeter, UK, have discovered a further explanation. The pair scoured the literature for evidence that the ejaculate of first mates may unintentionally streamline the pathway to fertilisation of sperm of second mates.
The female reproductive tract is a hostile place for sperm in most species, says Hosken. It has an acidic pH and a sperm-unfriendly complement of immune cells.
Testament to this is the fact that many species produce such prodigious quantities of sperm cells, and that very few of these make it to the fertilisation site. Mammals, for example, produce hundreds of millions of sperm in each ejaculate, but less than 0.001% make it to the female's egg.
Environmentally friendly
Seminal fluid has a range of properties that help to mitigate the inhospitable nature of the vagina. The researchers argue that the seminal fluid expended by first mates, helps to improve the environment, but many sperm are lost in the process.
The sperm-friendly environment that results, however, means that second mates need expend less energy-draining seminal fluid and produce more sperm. This helps them pip earlier rivals to the post, or rather, the egg.
It's a kind of parasitism of males on males,says Hosken. Several insect studies show that increasing the time between first and second mating decreases the advantage the second male gains, helping to back up the idea.
Now Hosken intends to experimentally test the idea in crickets, which attach sperm in packets to the outside of females. Attachment times can therefore be manipulated, he says.
Journal Reference: Journal of Theoretical Biology (DOI 10.1016/j.jtbi.2006.06.024)
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