THE SEX ISSUE
monopolise females for an additional half hour or so , to maximise his chances of his sperm being used to fertilise the female ’ s ova , while keeping other males at bay .
The final ( for now ) male adaptation is their sperm cells . Early on in the study of sperm competition , it was assumed that sperm were undifferentiated ; what counted was numbers . In fact , we now know that both quantity and quality count . Sperm vary both within and between males , both in design and performance . As you might expect from their highly promiscuous mating arrangements , male Dunnocks produce huge numbers of uniformly sleek , Porsche-like sperm . The Bullfinch , on the other hand , produces a limited , motley collection of Trabant-like sperm that are highly variable and not especially well made , although amongst them are sufficient good ones to ensure fertilisation . If the chances of having to compete with another male are low , why bother investing in quality control ? The male Bullfinch ’ s strategy is simply to ensure that it makes enough decent sperm to ensure that his partner ’ s ova are fertilised . For the Dunnock , every sperm counts and quality control is essential .
One of our most extraordinary findings in the avian sperm department relates to another species that is fairly monogamous , the Zebra Finch Taeniopygia castanotis . Like the Bullfinch , the Zebra Finch produces rather variable sperm . Some male Zebra Finches produce long sperm with a long midpiece ( essentially , the sperm ’ s energy supply ),
0 The Eurasian Bullfinch Pyrrhula pyrrhula appears to be strictly monogamous and has the smallest testes , relative to body size , at just 0.29 % of male body mass . Photo Francis Franklin
7 The Red-billed Buffaloweaver Bubalornis niger breeds in a polygynous harem-like arrangement , with two males sharing a group of up to twelve females . Photo Greg Tee
WHEN IT COMES TO SPERM WE NOW KNOW THAT BOTH QUANTITY AND QUALITY COUNT : LONG SPERM SWIM FASTER
others have a long sperm with a short midpiece , and yet others produce short sperm with a short midpiece . These different designs are genetically determined ; they dictate the speed at which the sperm swim , and hence their competitive ability when pitted against the sperm of another male . Overall , long sperm swim faster , and are most likely to fertilise a female ’ s ova .
So far , my examples have focussed predominantly on males . What about females ? For a long time it was assumed that females were merely passive receptacles and conduits for male gametes ; sexual selection operated predominantly on males . Certainly , there was nothing as obvious as the variation in relative testes size to offer an anatomical clue to female promiscuity . It had however , suggested that promiscuous females might be able to decide which of several males ’ sperm they would use to fertilise their ova . Where might we find such ” cryptic female choice ” going on ? The most obvious opportunity lay in those circumstances in which females do not appear to have any pre-copulatory choice of inseminating partner . The most obvious situation in which cryptic female choice (” cryptic ” because the choice , would occur out of sight within the female ’ s oviduct ) might be found was in Huxley ’ s ducks , where males , armed with a phallus , effectively rape females .
I decided to explore this and , with a post-doctoral researcher , Patricia Brennan , examined the reproductive system of female ducks . To our surprise , they were different from the large
18 BIRDLIFE • JUNE 2017