Do you think you're sexy?
15 July 2011
Most of us know if we pass muster when we leave the house, but how do animals know how attractive they are? Nick Royle and Tom Pike turned to zebra finches for an answer.
What do we mean when we talk about attractiveness in animals? An 'attractive' individual is one that can arouse interest in a potential mate, so in general the most attractive individuals are the most successful at reproducing. There has been lots of research in behavioural and evolutionary ecology, looking at variation in attractiveness and reproductive success, but very little is known about how animals actually work out how attractive they are to others.
Why is this important? It's interesting from a biological point of view, because individuals can use this information to modify the 'decisions' they make during reproduction. For example, in species where males invest their time and energy in parental care, knowing their own attractiveness may affect how much effort males put into finding a new mate relative to the effort they put into raising their existing offspring: more attractive males are expected to put more effort into mating than parental care. But do males know how attractive they are and, if so, how do they find out?
For us humans these questions are also intrinsically fascinating. Whole industries are founded on making ourselves more attractive - cosmetics, perfume, fashion - and it's the main focus of countless magazines and newspapers. Attractiveness is a constant human preoccupation, and for good reason. Studies on humans have shown that couples that have similar levels of self-perceived and partner-perceived attractiveness have more successful marriages, for example. So there are potential benefits to having an accurate perception of your own attractiveness compared to others.
Nick Royle in with the finches.
Our brains are sufficiently sophisticated for us to look in a mirror and compare our own image with that of other humans in our mind's eye. But how do we know how other people would rate our attractiveness against others'? One way of finding out is through social feedback. If others are keen to spend time with you this can provide a 'behavioural mirror' that reflects how attractive you are.
There is evidence that some other animals adjust their courtship behaviour in response to such social feedback, but we don't know whether animals can use feedback to gain information about their own attractiveness.
To find out, we did an experiment on zebra finches in captivity to test whether social feedback from females can provide accurate information to males about their attractiveness. Female zebra finches have been shown to prefer males with brighter red beaks, and this general preference for red extends to the addition of coloured leg rings - red leg rings make males more attractive to females. In contrast, green makes males deeply unattractive. We took advantage of this quirk of female zebra finch perception, to manipulate the attractiveness of male finches independent of their actual 'quality'.
Am I hot or not?
We gathered a group of females that showed clear, unambiguous preferences for males wearing red leg rings. We then showed male birds to them under four sets of experimental conditions. First, we gave the males a red leg ring, and showed them to females on the other side of a transparent partition. This allowed the birds to see each other, so the male could see the female's response to him - her social feedback. Then we changed the partition, this time putting mirrored film on it so the male could see the female but she couldn't see him; in this set-up the males didn't receive any social feedback at all. Then we changed the male's red leg ring for a green one, and ran the experiment again with both kinds of partition.
Attractiveness in zebra finches is mainly a social construct.
We measured the females' interest in each male by how long they spent on the perch closest to the partition, as this has previously been shown to be a good indicator of mating preference in zebra finches.
By doing this we could make sure the actual quality of the male stayed constant while we varied the factors we were interested in: social feedback and attractiveness.
When the males and females could see each other, they gave each other more attention when males were wearing their attractive red leg ring - the female gave positive feedback and the male responded by courting her more. When the males wore green leg rings they got less positive feedback and spent less time courting. But when the females didn't respond to the males (because they couldn't see them) the amount of time the males spend courting wasn't related to which ring he was wearing - in other words his behaviour wasn't related to how attractive he thought he was.
This shows that males behave differently according to how attractive females think they are: females provide a behavioural 'mirror' that reflects male attractiveness. Males effectively find out about their own attractiveness from the females' behaviour, and this is independent of their quality as a potential mate. So it looks like attractiveness in zebra finches is mainly a social construct.
It's a neat result, but we are still not sure whether males are simply learning what responses are effective in courting particular females, or whether they somehow remember the information for use in future social interactions. Other research on social feedback among male zebra finches suggests that repeated behavioural interactions can lead to long-term changes in social status. So it seems likely that repeated behavioural interactions with females could lead to long-term changes in males' perceptions of their own attractiveness too. If so, this opens the door for future studies looking at whether males can use what they learn about their attractiveness to 'decide' how much effort to put into mating and parental care to maximise their success. For now, though, the answer to the question of how male zebra finches know how attractive they are can be broadly summarised as 'females tell them so'.
Dr Nick Royle works at the Centre for Ecology and Conservation, University of Exeter. Dr Tom Pike is a NERC research fellow at the University of Lincoln. Email: N.J.Royle@exeter.ac.uk
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