Returning to the Fisher/Lande runaway theory, what is needed now is evidence from real animals. How should we go about looking for such evidence? What methods might be used? A promising approach was made by Malte Andersson, from Sweden. As it happens, he worked on the very bird that I am using here to discuss the theoretical ideas,the long-tailed widow bird, and he studied it in its natural surroundings in Kenya. Andersson's experiments were made possible by the invention of a recent advance in technology: superglue. He reasoned as follows. If it is true that the actual tail length of males is a compromise between a utilitarian optimum on the one hand, and what females really want on the other, it should be possible to make a male super-attractive by giving him an extra long tail. This is where the superglue came in. I'll describe Andersson's experiment briefly, as it is a neat example of experimental design.
Andersson caught 36 male widow birds, and divided them into nine groups of four. Each group of four was treated alike. One member of each group of four (scrupulously chosen at random to avoid any unconscious bias) had his tail feathers trimmed to 14 centimetres (about 5 inches). The portion removed was stuck, with quick-setting superglue, to the end of the tail of the second member of the group of four. So, the first one had an artificially shortened tail, the second one an artificially lengthened tail. The third bird was left with his tail untouched, for comparison. The fourth bird was also left with his tail the same length, but it wasn't untouched. Instead, the ends of the feathers were cut off and then glued back on again. This might seem a pointless exercise, but it is a good example of how careful you have to be in designing experiments. It could have been that the fact of having his tail feathers manipulated, or the fact of being caught and handled by a human, affected a bird, rather than the actual change in length itself. Group 4 was a 'control' for such effects.
The idea was to compare the mating success of each bird with its differently treated colleagues in its own group of four. After being treated in one of the four ways, every male was allowed to take up its former residence on its own territory. Here it resumed its normal business of trying to attract females into its territory, there to mate, build a nest and lay eggs. The question was, which member of each group of four would have the most success in pulling in females? Andersson measured this, not by literally watching females, but by waiting and then counting the number of nests containing eggs in each male's territory. What he found was that males with artificially elongated tails attracted nearly four times as many females as males with artificially shortened tails. Those with tails of normal, natural length had intermediate success.
The results were analysed statistically, in case they had resulted from chance alone. The conclusion was that if attracting females were the only criterion, males would be better off with longer tails than they actually have. In other words, sexual selection is constantly pulling tails (in the evolutionary sense) in the direction of getting longer. The fact that real tails are shorter than females would prefer suggests that there must be some other selection pressure keeping them shorter. This is 'utilitarian' selection. Presumably males with especially long tails are more likely to die than males with average tails. Unfortunately, Andersson did not have time to follow the subsequent fates of his doctored males. If he had, the prediction would have been that the males with extra tail-feathers glued on should, on average, have died younger than normal males, probably because of greater vulnerability to predators. Males with artificially shortened tails, on the other hand, should probably be expected to live longer than normal males. This is because the normal length is supposed to be a compromise between the sexual selection optimum and the utilitarian optimum. Presumably the birds with artificially shortened tails are closer to the utilitarian optimum, and therefore should live longer. There's a lot of supposition in all this, however. If the main utilitarian disadvantage of a long tail turned out to be the economic cost of growing it in the first place, rather than increased dangers of dying after it has grown, males who are handed an extra-long tail on a plate, as a free gift from Andersson, would not be expected to die particularly young as a result.
