Emancipation on the Prairie
The female pronghorn go on a complicated and exhausting search for a partner every year. For this to be worthwhile, something has to jump out. But what?
Women are emancipated. Women are picky. After all, they're looking for the man of their dreams, and he's not waiting on every street corner. This seems to be a problem that not only we humans, but also many animals can sing a song about, for example the female pronghorn (Antilocapra americana).
Pronghorns are the only extant species of an otherwise extinct order within the even-toed ungulates. But there is something else that makes them unique among even-toed ungulates: In a certain way, the women wear the trousers because they decide who is allowed to be the lord of the election. Just before she gets in the mood to mate, she begins sprinting across the vast expanses of the North American prairies in search of him. She spares no effort: She visits many gentlemen, who often maintain their harems far from each other, and sometimes tests a candidate several times. All of this happens in just two weeks-fortunately, these ruminants are America's fastest land mammals.
How does this testing work? John Byers of the University of Idaho and others noted some time ago that the pronghorn females first look at whether the master can keep a band of wayward women in line - how he convinces them not to return to him to run away. This happens whenever he has trouble stopping an opponent from encroaching on his territory.
But the females often leave the males for reasons that are still unknown to the researchers. The size of the existing harem is important in any case, whether a female also decides for this male - apart from a few very clever ones. They think he's safe, but then try to escape to a rival at the last minute, provoking a fight between the two. Only those who win can end up with these women. In any case, in the end almost all females opt for a small select group and most of the other males get nothing.
But what do these gentlemen offer that the others don't have? Do they offer protection? No, because the emancipated females take care of themselves. Are the gentlemen particularly handsome? At least according to human judgement, that may not be the case either, because pronghorns actually all look the same to our eyes. So are the females looking for a sperm donor who will give them perfectly he althy offspring? A possibility that no one had previously investigated in animals in the wild.
Byers and his colleague Lisette Waits therefore lay in wait for four years in the National Bison Range in Montana and monitored the mating behavior of the pronghorn. To distinguish between the animals living there, they marked each individual by the ear and also sequenced ten places in the genome that are characteristically composed for each individual. Then they caught as many newborns as possible annually and repeated these procedures. Now they were able to compare their genome with that of around fifty potential fathers and thus once again confirm at a genetic level what their observations in the field suggested: In fact, only 14 of the male pronghorn had fathered almost sixty percent of the offspring. And these hatchlings did survive more often than those of the less favored males.
What quality made this offspring particularly fit for life? Byers and Waits suspected: rapid growth. Because the faster the animals are big and strong enough to sprint away from their biggest enemies, the coyotes, the more likely they are to survive. The researchers were able to confirm this with comparison photos of the hind feet of the young animals, which they took immediately after birth and then again at a later point in time. From this they calculated how quickly the hind foot had grown. This happened much faster in the offspring of the attractive males.
But the researchers experienced a surprise in one thing: Offspring of premium males were cared for and nursed less by their mothers than offspring of second-rate partners. At second glance, it's not really that surprising: the females, who were only able to get a less good sperm donor, are probably trying to make their genetically disadvantaged little ones more suitable for the fight for survival in this way.
The pronghorn females have actually developed a good method to recognize their dream man - the one who gives them the he althiest offspring. And if the strategy doesn't work out, they stick to it like many people do: always make the best of the situation.