Zoologists studying the mating rituals of the African Topi species of antelope, in the Masai Mara game reserve of Kenya, have reported some extraordinary behaviour.
It has long been established in biology that there are fundamental differences in the strategies of the two sexes. Males are supposed to distribute their sperm far and wide, to all comers, whereas females are noted for being choosier about whom they decide to let fertilise their valuable eggs.
Yet, the Topi have reversed this sexual stereotype, and it seems, according to Jakob Bro-Jorgensen (Zoological Society of London) that males literally have to fight off the females, who vehemently wanted to mate with them.
Topi antelopes have breeding grounds in which the males defend small territories called “Leks” (a word meaning “to play” in Swedish,) where each male advertises his sexual availability. Female Topis express preference for mating with males with the most centrally-positioned lek, and males compete to occupy these central territories. This, so far, is stereotypical of animals that breed in a lek.
However, sometimes a male with a centrally-positioned lek can find this to be an exhausting experience, and then often find that they have to turn away familiar females in favour of unfamiliar females, who are visiting for the first time.
Dr Bro-Jorgensen said:
I was interested to see that in cases where the male antelope was free to choose between females, he deliberately went for the most novel mate, rather than the most high ranking. However, some pushy females were so aggressive in their pursuit of the male that he actually had physically to attack them to rebuff their advances
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 antelopes, where females are known to prefer to mate with males in the centre of mating arenas, we’ve found a reversal of these stereotypic sex roles
When analysing sex strategies in the animal kingdom zoologists like to utilize the idea of economic investment, i.e. males invest relatively little in each sperm cell, so provided a male does not have to rear all the young he sires (in reproductive terms) it’s best for him to distribute his investment far and wide, in the hope that some of it will pay off.
Females, however, begin with a more substantial investment. Each of her eggs is a relatively precious commodity that needs to be carefully managed. It would pay her, for instance, to invest much more (in terms of time and effort) to ensure that her fertilised egg has a good chance of reaching adulthood. This explain why females of so many different species stick around to rear their young, and why they are choosy about which male they decide to mate with.
The lek system of mating is not unique to Topi antelopes. Leks are especially common in birds, where they are a useful way of letting females “play the field” and choose the best male, usually the one with the most centrally-located lek.
Similarly to the Topi, the males of some lek-mating birds, such as the Capercaillie Grouse (Tetrao urogallus) have also been observed to reject sex after excessive amounts with the queue of females. But the females just go away and come back the next day.
So why doesn’t the Topi female just 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 quality 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. It was not uncommon to see males collapsing with exhaustion as the demands of the females got too much for them
The Topi and the Capercaillie demonstrate 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 an extreme example of female choice. Females are free to mate with any male, but are psychologically drawn to those that other females are interested in.
A harem system is another form of polygyny. Here, males dominate the females and guard them against the advances of other males, examples being Sea Lions or Gorillas. Some polygynous species are also territorial, where defending a resource-rich plot of land means that females will mate, so that they can be allowed to stay in that male’s territory.
The other form of polygamy is polyandry; here a female has more than one male as a mate, although this is rare. One of the best examples is the Dunnock (Prunella modularis), here females can have two “husbands” at the same time to help rear their young. And, this system has been shown to favour females because they can rear more young with two mates. The males however are probably less happy because they have to compete for access to the female. Indeed, it has been shown that a male Dunnock that is not allowed frequent-enough access to his shared “wife” will not feed the resulting offspring.
Even monogamy is not always what it seems to be. Since the invention of DNA fingerprinting, biologists have discovered that most supposedly monogamous species engage in what has come to be known as extra-pair copulations. DNA studies of offspring from species of socially monogamous birds have shown that the species are not sexually monogamous, as once thought. Both males and females will go for extra-pair sex. Amazingly, only a very few species are truly monogamous.
So, everything is not what it often seems, especially when it comes to the sexual games that animals play.
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.
Said Dr Bro-Jorgensen.
Posted by Jonathan in Sociobiology