KEY POINTS:
Normal sexual stereotypes seem to be turned on their head in the case of the African topi, an antelope where the male of the species likes to say "no".
Observers have reported extraordinary behaviour during the courtship rituals of the topi of the Masai Mara game reserve in Kenya - where some male antelopes have literally had to fight off over-amorous females eager to be inseminated. Far from being standoffish, female topi exhibit an unabashed sexual predatoriness.
The topi has a mating system based on an open arena of land where the males gather to defend little territories called "leks", where each individual advertises his sexual availability.
Females show a distinct preference to mate with the males with the most centrally positioned lek, and competing males fight over these territories the most. So far, so stereotypical.
But sometimes being a male with a central lek can be exhausting. So exhausting that they often find themselves having to turn away familiar females in favour of unfamiliar individuals visiting for the first time.
Jakob Bro-Jorgensen of the Zoological Society of London, who led the study published in Current Biology, said it was extraordinary to see males rejecting female advances so vehemently. It was as if male topi were in the unusual position of having to ration their valuable sperm.
To explain the latest skirmish in the battle of the sexes takes some doing. It has long been established in biology that there is a fundamental difference in the strategies of the sexes. Males are supposed to distribute their sperm far and wide, whereas females are noted for being more picky over whom they choose to fertilise their precious eggs.
"When biologists talk about the 'battle of the sexes', they often tacitly assume that the battle is between persistent males who always want to mate, and females who don't. However, in topi, we've found a reversal of these sex roles," said Dr Bro-Jorgensen.
When analysing sex strategies in the animal kingdom, zoologists like to invoke the notion of economic investment. Males invest little in each sperm cell - which is why sperm are small and exceptionally numerous. A male does not have to rear all the young he sires, so it pays to distribute his investment far and wide.
However, the female starts out with a more substantial investment. Each of her eggs is a relatively valuable commodity that needs to be carefully managed. It would pay her, for instance, to invest even more in terms of time and effort to ensure that her fertilised egg has a good chance of reaching adulthood.
This explains why females of so many different species stick around to rear their young - and why they have to be choosy over which male to mate with.
The lek system of mating is not unique to topi. Leks are common in birds - they are a useful way of letting females play the field and choose the best male, who is usually the one with the most centrally located lek.
The males of some lek-mating birds, such as the capercaillie grouse, have also been observed to reject females after a bout of overindulgence with a line of females.
"They get shagged out. But the females just go away and come back the next day," said Professor Tim Birkhead of Sheffield University.
Why doesn't the topi female do the same? The answer seems to be because she is only in oestrus for a day or so and cannot afford to risk being barren for the entire breeding season, according to Dr Bro-Jorgensen.
"The females have just a single day to ensure that they become pregnant, and preferably with a hotshot male, so they must focus all of their energies into ensuring that males mate with them in that time.
"The males, however, must focus on maximising the potential of their sperm to ensure they impregnate as many females as possible."
Topi and the capercaillie exemplify a system of mating called polygyny, where a male mates with more than one female.
A lek system of mating is just one expression of polygyny and is a supreme example of female choice - they are free to mate with any male, but are naturally attracted to those that other females find attractive.
The harem system is another form of polygyny. Here, males dominate their females and guard them against other males - examples range from sealions to gorillas.
Monogamy is more common, but even here it is not always what it appears to be. Since DNA fingerprinting was invented 20 years ago, biologists have discovered that supposedly monogamous species engage in sneaky "extramarital" sex.
Studies of offspring have shown that socially monogamous birds ranging from blue tits to albatrosses are not sexually monogamous.
Which only goes to show that everything is not what it often seems when it comes to the sexual games animals play.
As Dr Bro-Jorgensen said: "We should not regard coyness as the only natural female sex role - just as we should not expect that it is always the natural male sex role to mindlessly accept any mating partner. Nature favours a broader range of sex roles."
- INDEPENDENT
