Symmetry and ornamentation in a peacock's train

Issue 4, December 1995

In 1991 two researchers, Manning and Hartley*, published an article describing work they had carried out on peacocks (Pavo cristatus). Mature male peacocks have a long train of tail feathers and when the male opens the tail he reveals a highly colourful pattern. One of the most obvious features is that some feathers end in an apparent eye or ocellus (plural ocelli). The ocelli are spread over the whole train but 4-7 are usually found directly above the bird's head. Manning and Hartley used this to divide the tail into two and counted the number of ocelli on both sides of this mid-line. They then defined train symmetry based on this: train symmetry was "the number of ocelli on the side with the greatest number minus the number of ocelli on the side with the least". If a bird had a train symmetry score of 0 then it would have an equal number of ocelli on either side of the mid-line and be symmetrical with respect to the number of eyes on its tail. It has been suggested that female animals tend to prefer mates who have symmetrical bodily characteristics. For example, it has been found that female barn swallows (Hirundo rustica) preferentially select males with symmetrical tail feathers, i.e. symmetrical in terms of the length of the two tail streamers.

To carry out their study the researchers visited zoos, parks and gardens in North West England and photographed each male when it directly faced a camera with its tail fully open. It was then possible to see the n-@d-line and count the ocelli. The data relating to seventeen birds are illustrated in Figure 1. Manning and Hartley found, by statistical analysis, that train symmetry was very strongly correlated with the number of ocelli per train (r =0. 84, N= 1 7).

Figure 1. The relationship between train symmetry and the number of ocelli per train.

1. Why did the researchers wait unto the bird faced the camera before taking the photograph? (1)
2. The researchers probably used a camera with a telephoto lens to take the photographs. Explain why it would have been advantageous to do so. (1)
3. Identify one limitation of this study.
4. The researchers counted the number of ocelli in the train of each bird. What scale, or level, of measurement is this and state one characteristic of this scale of measurement. (2)
5. The researchers were interested to see if a significant correlation existed between the two variables. Suggest a suitable experimental (alternative) hypothesis that could be tested to determine if the correlation is significant and state whether your hypothesis is one-tailed or two-tailed. (3)
6. The Spearman rank correlation coefficient was used in this study to analyse these data. What other correlation coefficient can be used to determine the the strength of the correlation between two variables and when should this one be used rather than the Spearman coefficient? (2)
7. Figure 1 and the correlation coefficient that they calculated both indicate that a positive correlation was found between the variables. Explain what interpretation you would put on this finding, in terms of the two variables under investigation. (1)

* Manning, J.T. and Hartley, M.A. 1991. Symmetry and ornamentation are correlated in the peacock's train. Animal Behaviour, 42, 1020-102 1.

ACKNOWLEDGEMENTS We would like to acknowledge the help given by Dr Kathy Krebs (Headington School, Oxford) and Sheila Bennett (Hills Road Sixth Form College, Cambridge) and their students in devising the exercises in this issue of FEEDBACK. We also want to thank the Royal Society for the Protection of Birds for allowing us to use the photograph of the cuckoo and the illustration of the blue tit, Linda Gray (Homerton College) for the illustration of the peacock, Stephen Tomkins (Homerton College) for the cartoon and Academic Press for permission to use material in Animal Behaviour for the A Level Psychology exercise.