Do you think you’re sexy? Most of us check the mirror at least once before we step out of the house, and we know instantly if we pass muster or if we’re having a bad hair day. But how do animals know how attractive they are? Nick Royle and Tom Pike decided to find out. 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. 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 Attractiveness in zebra finches is mainly a social construct. 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’. AmIhotornot? 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. We measured the females’ interest in 06 PLANET EARTH Summer 2011 Nick Royle