The article I discuss below came out last year, but it really is a rather cool experiment, so I felt I had to share it.
Bowerbirds, perhaps best known from Sir David Attenborough’s documentaries, are unique in their elaborate constructions of bowers. My description of them won’t do them justice, so I’ll have to hand it over to the master here:
There are twenty species of bowerbird, all building different types of bower. In species such as the Great bowerbird (Ptilinorhynchus nuchalis), males decorate the bowers to attract females. As these males offer no parental care at all to their offspring, literally all they give to the female is sperm. So, if you are a female trying to decide which male’s sperm to use, how good a bower-builder could be a deal-breaker.
The female inspects the local males and their bowers, viewing them through a long avenue (similar to the Western bowerbird in the video above). Having the female placed in this particular position when checking out the male creates a unique opportunity: the male can alter what the female sees and how she sees him (whether he knows he is doing this is a whole other question, and one I’ll come back to later).
So, how might a male take advantage of this opportunity? Well, males decorate their bowers with grey and white objects. These objects have been called gesso (as the artists among you will know, this is a word for describing the under layer or primer used before adding paint to a portrait, and is appropriate here as the bowerbird males add coloured objects on top of these grey and white ones). Endler and colleagues1 -observed that Great bowerbird males placed their gesso objects in order of increasing size with distance from the avenue, which would create the illusion to a visiting female that when standing at the end of the avenue he is bigger than in reality.
In order to see how deliberate this placing of the gesso by the male really was, the researchers then changed the gesso around, placing the small objects further back and the larger objects further forward. This would make the male look smaller than he actually was when the female looked down the avenue at him (and who wants to look like a pocket-sized bowerbird?). Of fifteen bowers that they did this to, most of the males switched back their gesso after a few days, and all had done so after two weeks.
Photo of the original bower, and the bower after the researchers had altered it. Photo taken from Endler et al. 2010 1
However, we know that some bowerbird males will correct the changes that humans make to their bowers, so that if we move an object the male will tend to put it back in its original location. So how do we know that the male is actually correcting this ‘illusion’ that he has created, and not just wanting everything in its right and proper place? Well, the scientists then went on to look at whether moved objects were being placed back in their original location or not. They found that males moved the objects towards the other end of their display to which they had been moved to, thus correcting their illusion, but not necessarily into the exact same original location.
So why should females choose males who build better bowers? Well, a good bower might mean that a male is good at defending his bower from other males and keeping it in good shape, showing he is probably pretty strong and healthy. It might also show that he himself is good at stealing bits and pieces from other males’ bowers to decorate his own. The ability to have a nicely built and decorated bower might not only show good physical condition, but also superior cognitive skills: you may have to be fairly quick-witted to multitask building an elaborate bower with cleverly placed gesso, whilst also defending it from other males, stealing bits of their bowers to make them look less worthy, not to mention also trying to attract a few females to come and have a look. All these traits are ones females would like to have in their sons, and so by mating with males with better bowers, they ensure that their sons will hopefully be good bower-builders, increasing their chances of carrying on the family line.
Finally, the researchers made what I think is a really interesting observation. When males are re-arranging objects around their bowers they frequently go around to the other end of the bower avenue, and look down it, as a female would if she were there. This indicates that he is perhaps rearranging objects so that they look ‘right’ from the perspective the female would have. Of course this is only an observation, but it is observations like this that lead to exciting new experiments to determine whether our hunches are correct or not.
1 Endler, J.A., Endler, L.C. & Doerr, N.R. (2010) Great bowerbirds create theaters with forced perspective when seen by their audience. Current Biology.