GIRL power has reached a new level, in the animal kingdom at least. When it comes to demanding quality sperm, female birds can force males to compete not just against other males, but against themselves.
It is now widely acknowledged that females of certain species expel inferior sperm in favour of seed from a more promising mate. But researchers have discovered that female kittiwakes in monogamous relationships eject earlier deposits of semen from their partner in the hope he will produce fresher, newer stuff.
Richard Wagner at the Konrad Lorenz Institute in Vienna and his colleagues observed 634 copulations between 188 pairs of black-legged kittiwakes, Rissa tridactyla. Then they recorded whether a female ejected her mate鈥檚 semen shortly after.
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They found that post-copulatory sperm ejection 鈥� in which the semen is violently expelled 鈥� declined steeply about 16 days before the first egg of a clutch was laid. This suggests that females were unloading sperm from early trysts and hanging on to more recent specimens.
That fits with the 鈥測oung sperm hypothesis鈥�, which suggests that discarding stale sperm will help a female bird raise more chicks. Female kittiwakes who ejected sperm less frequently laid more eggs that failed to hatch, and the chicks that did hatch tended to be more unhealthy (Biology Letters, DOI: 10.1098/rsbl.2003.0142).