Comparing cross-sectional with longitudinal analyses

In a recent paper Gil et al. (2001) investigated agerelated changes in song characteristics of male willow warblers, Phylloscopus trochilus. The authors particularly focus on the implications of using longitudinal (following the same individual through its lifetime) versus crosssectional (comparing individuals of different age) data. When comparing the results obtained by these two methods, Gil et al. make some far-reaching conclusions about mortality in relation to singing performance and state that their findings are consistent with handicap models of sexual selection theory. Their data are definitely very interesting and they unequivocally show that song characteristics are age dependent. However, with regard to their interpretations, I would like to contribute a few considerations. First, it should be realized that the discrepancies between the two approaches may simply result from sampling errors, a point that I think is disregarded in many studies. Second, some of their results seem to be not in line with, but directly opposite to, what is normally expected from the theory of condition-dependent signalling. Gil et al. studied age-related changes in song characteristics because this may shed some light on the role of song as a potential indicator of male quality or condition. It is assumed that song is a sexually selected male ornament that is costly to produce and that is therefore expressed in a condition-dependent manner. It is further argued that old and experienced birds may typically be in better body condition than young birds, for instance, because they may be more efficient in foraging or more successful in competition for food. Thus, if singing performance is condition dependent one might expect it to improve with age. Although it might be argued that an age-related increase in performance may occur for reasons other than sexual selection, this is not the point I want to raise here. A main focus of their paper is on a methodological issue, namely on how one should interpret results stemming from longitudinal versus cross-sectional analyses. While longitudinal studies yield reliable estimates of age-related changes, cross-sectional data may be confounded, if mortality is not independent of the trait under consideration. For instance, if individuals with larger song repertoires were longer lived, this would create an apparent increase in repertoire size with age, even if there was no intraindividual change. Although this issue is not a new one in principle, it may be worthwhile discussing the implications that follow from this methodological problem. In their longitudinal analysis Gil et al. found that two measures of singing performance significantly increased with age (repertoire size and element rate), while three parameters did so only nonsignificantly (song length, song versatility and repertoire fixation; see Table 1). Some of these results were also found in the cross-sectional analysis: there was a significant age effect on repertoire size, and again there were no significant changes of song length and repertoire fixation with age. However, two parameters deviated from what was found in the longitudinal analysis. First, there was a significant age effect on song versatility, which may not be too surprising, as there was already such a trend in the longitudinal data (P=0.096). Second, element rate showed no significant age-related increase in the cross-sectional analysis (P>0.2), while the longitudinal analysis had revealed such an increase. Gil et al. interpret this latter difference between the two approaches as follows: ‘This pattern implies that there is a negative correlation between element rate and survival in first-year males, suggesting that high element rate has a survival cost. the higher mortality of first-year males with high element rates suggested by our data would be evidence for a handicap associated with this trait (Zahavi 1975; Iwasa et al. 1991)’. I have two reservations about this. (1) The conclusion that there must be a negative correlation between Correspondence and present address: W. Forstmeier, Department of Animal and Plant Sciences, University of Sheffield, Alfred Denny Building, Western Bank, Sheffield S10 2TN, U.K. (email: w.forstmeier@sheffield.ac.uk).