Peacocks are spectacular for any zoo-goer to behold. But when Marion Petrie and her colleagues in England began intently watching a large group of peafowl in 1987, they had a very specific goal in mind. The birds' large and elaborate tails, called trains, developed through natural selection. Do the females influence the evolution of the train, wondered Petrie, by choosing and mating with males that have the most ornate trains?
It was the beginning of a classic exercise in the scientific method of gaining new knowledge about the world. The first phase, observation, had started as far back as Charles Darwin, who had pondered the peacock's train question but hadn't specifically studied it. More recently, scientists had suggested that in other bird species, females seemed to select males with certain body features.
This led Petrie and her group to pose their hypothesis, a tentative explanation for the question under study. A hypothesis should be testable: It can be proven wrong, but it can't be proven right with absolute certainty. Petrie's hypothesis was that peahens preferred males with more elaborate trains, probably because they had "better" genes to pass on to offspring. Prediction is the next phase. Based on the hypothesis, the scientists predicted that some males would be more successful at mating than others, and those males would be the ones whose tails contained the most "eyespots," or ornamental designs.
The experiment is a test of one or more predictions, and is designed to prevent chance, self-deception, or differing conditions from biasing the results. The English researchers weighed and measured 111 peacocks and recorded the number of eyespots on the trains of all the males. They then hid and watched as males and females gathered to "lek," or court in groups.
Over a breeding season, the scientists observed that a small minority of the males were chosen more often by the females for copulation. Then, the critical observation: Were the more-successful males the ones with the most eyespots in their trains? The answer: In most cases, yes. The prediction proved true. (The process was not as simple as it sounds. The scientists performed statistical tests to make sure the findings could not have occurred by chance.)
For findings like these to be accepted by other scientists, they need to be replicated in independent experiments. And Petrie pointed out that her results didn't reveal why the females chose males with bigger trains -- that was the subject of another investigation.