“She’s saying hold it, I’m not going to mate with any old bloke, you’ve got to show me you’re worth it,” says Dr Sue Healy, a behavioural scientist from the University of St Andrews.
It’s a question biologists have pondered for years. How do females choose which male to mate with?
“The male might try and display that he’s really healthy, which could mean that he’s not going to give her anything horrible like a disease, or it could mean that any babies she’d have were going to be healthy too,” says Healy.
The choice might be easy when it’s between an unhealthy male and a healthy one. But what about when the choice is between two good mates? Can females really tell the difference between better and best?
All male animals have tricks to attract mates, from huge antlers to elaborate mating calls. “Look at peacocks, they have a train that’s completely ludicrous … it’s brightly coloured, it’s got eye spots on it,” she says. “Scientists for hundreds of years wondered what on earth is going on here and the suggestion was that it has something to do with females going round and looking at males.”
Biologists believe elaborate traits like the peacock’s tail reveal whether or not that animal has good genes. These genes will be passed onto any children and will help determine whether they are healthy and successful at having offspring of their own. Biologists also thought that to attract a female, size mattered and that these traits (whatever form they took ) needed to be big.
But back to our original question: can females tell the difference between two potentially good mates?
It seems not. Scientists have found that females might not always find it obvious which male has the biggest or best trait.
The experiment, led by Dr Karin Akre from the University of Texas, studied female Túngara frogs. These are small, mottled brown and yellow animals found in humid areas from Mexico to Venezuela. At night the male frogs try to attract females with a mating call consisting of a “whine” followed by a “chuck” sound. The males attempt to add one more chuck to their song than their neighbours and it was thought that the call with the most ”chucks” would win a mate.
The researchers wanted to see whether the females would notice any difference between two long mating calls.
Akre’s team put the females in a box with two hidden loudspeakers on opposite sides. They played two different long calls and waited to see which direction the frogs would head towards. Their results, published in the journal Science, showed that the females weren’t always able to choose the call with the most chucks.
It seems that when it comes to differences of only one chuck the females don’t notice, putting a limit on the length of the male frogs’ calls. “There’s a trade-off because as they are adding more and more [chucks] it’s harder and harder for the females to distinguish between the different males … so there’s no benefit because as they increase the chuck number even more the relative increased attractiveness gets lower,” said Akre.
This happens because of Weber’s Law, which states that it’s easy to notice a small difference to something small, for example one person added to a group of three people. But if you have a group of 20 people, and one more person arrives, you probably wouldn’t realise. You would need a much bigger addition of people to notice.
So the female Túngaras would need a much bigger difference than one chuck between two long calls to notice the difference.
Akre also found that frog-eating bats have the same problem. She repeated the experiment with fringe-lipped bats, one of the Túngara frog’s main predators. The bats are more attracted to long calls, so the longer the frog’s call the more likely it is to get eaten by a hungry bat.
To their surprise, the researchers found that the bats didn’t always choose the call with the most chucks. This means that the frogs could increase their call length gradually without lowering their chances of survival.
But the frogs’ calls don’t go on all night so something else must be stopping them. The fact that females don’t notice or don’t care when a male frog adds an extra chuck means that there isn’t much incentive to do it, keeping the calls relatively short.
The question of whether the female can’t hear the extra chucks or if she’s just ignoring them is difficult to answer. Dr Akre thinks that it’s likely the females simply don’t notice the difference.
Dr Healy suggests a different explanation. “The female has the problem that when she turns up in the same area as all these males who are making a lot of noise she might also become dinner, so she doesn’t want to hang around,” she tells me.
“So potentially what’s going on is that she could tell, she really could if she had all the time in the world … she just hasn’t got the time to spend listening to the difference.”