S22.Summary: Commodity selection and coloniality: Exaggerated traits are produced by processes of choice

1Université Pierre et Marie Curie, Laboratoire d'Ecologie, CNRS-UMR 7625, 7 quai Saint-Bernard, Case 237, 75252 Paris cedex 05, France, edanchin@snv.jussieu.fr; ²Department of Biology, York University, North York, Ontario, M3J 1P3, Canada, e-mail rwagner@yorku.ca

Colonial species show an obviously skewed distribution of nests in space, with nests being clumped in some areas while equally favourable adjacent areas often remain unsettled. It is therefore convenient to define colonies as skews in breeding habitat selection, a definition that is implicitly used by field ornithologists. Although the standard approach to studying coloniality is to search for the costs and benefits of high density breeding, we might ask alternatively, what produces skews in breeding habitat selection?

Danchin and Wagner (1997; see also discussion between Tella, J.L., Hiraldo, F. & Donazar, J.A. 1998. Trends in Ecology and Evolution 13: 75-76 and Danchin, E., Wagner, R.H. & Boulinier, T. 1998. Trends in Ecology and Evolution 13: 76.) proposed to unify those approaches into the general framework of ‘Commodity Selection’. Commodities are the resources that are necessary for reproduction. They include nesting habitat, shelter, food, mates and extra-pair fertilisation. The commodity selection hypothesis proposes that coloniality is not an adaptation per se but rather is the by-product of nesting habitat and sexual selection which are both based on conspecific cues. Individual and habitat spatial and temporal heterogeneity strongly selects for sophisticated mechanisms of sexual and habitat selection. In this context, relying on conspecifics is a parsimonious strategy to select reproductive commodities: it is simpler for animals to base their decisions on a few conspecific cues than to try to evaluate a complex matrix of environmental and social factors that determine habitat and mate quality. Commodity selection provides a true mechanism of nesting aggregation. Indeed, by cueing on conspecifics, animals will tend to settle close to successful and high quality conspecifics in order to benefit from the same environmental conditions and to mate with or obtain extra-pair fertilisations from them. Its aggregative effect may provide an initial seeding of nest clumping, paving the way to coloniality.

Empirical evidence for aggregation via habitat selection comes from a wide range of species while studies of other species provide evidence that mechanisms of sexual selection can produce aggregations of monogamous birds (Table 1). Theoretical treatments have also shown the general viability of a strategy in which individuals exploit conspecific RS (Boulinier and Danchin 1997; Switzer, P.V. 1997. Behavioral Ecology and Sociobiology 40: 307-312). Finally, results of a recent comparative study of coloniality in birds support the commodity selection framework of coloniality. In that study, Rolland et al. (Rolland, C., Danchin, E. & de Fraipont, M. 1998. American Naturalist 151: 514-529) showed that in marine birds, coloniality chronologically preceded the passage to marine life. Thus, contrary to the standard interpretation of the association between coloniality and marine life, it seems that it is the acquisition of coloniality that allowed species to exploit the marine environment and not the opposite. This leads us to seriously question some classic assumptions of the economic approach.

A provocative thought about coloniality is to consider the highly skewed spatial distribution of nests as an exaggerated trait. It is interesting to note that exaggerated traits seem to be always produced by processes of choice. For instance, exaggerated secondary sexual characters are commonly considered as produced by mate choice. [e.g. Fisher's (1930) runaway process (Fisher, R.A. 1930. The genetical theory of natural selection. Oxford; Clarendon Press); Handicap principle; (Zahavi, A. 1975. Journal of Theoretical Biology 53: 205-214; Zahavi, A. & Zahavi, A. 1997. The handicap principle. Oxford; Oxford University Press) Parasite hypothesis (Hamilton, W.D.& Zuk, M. 1982. Science 218: 384-387.)]. Exaggerated ornaments of chicks of the American Coot Fulica americana have been shown to be produced by their parents' choice of feeding the more brightly coloured of their offspring (Lyon, B.E., Eadie, J.M. & Hamilton, L.D. 1994. Nature 371: 240-243). Predator choice is also responsible for the bright colour of distasteful aposematic species. Similarly, in marine invertebrates, skews in adult distribution are produced by habitat choices by larvae just before they settle and begin metamorphosis (Meadows, P.S. & Campbell, J.I. 1972. In: Russell, F.S. & Yonge, M. (eds). Advances in Marine Biology. Vol 10; London; New-York; Academic Press: 271-382; Pawlik, J.R. 1994. Nature 370: 511-512; Chase, M.E. & Bailey, R.C. 1996. Malacologia 38: 19-31). Thus, processes of choice are likely to naturally generate exaggerated traits under a wide variety of circumstances.

We believe that commodity selection offers a promising framework for the study of coloniality. We hope that it will help resolve this long-standing evolutionary enigma. The purpose of this symposium is to explore the potentialities of this approach. We asked participants to provide some evidence in favour of a role of commodity selection in coloniality.

A key aspect of Charles R. Brown's and Mary Bomberger Brown's approach is their assumption that the commodities necessary to breed differ among phenotypes. They illustrate this point with the role of colony size as a potential cue for breeding habitat selection. The two following papers concern the importance of fitness components as cues in habitat selection. Thierry Boulinier, Etienne Danchin and Stéphane Durand present experimental evidence of conspecific attraction based on the presence of chicks in the nests, and resulting activities, in breeding habitat selection in the Kittiwake. Thomas J. Valone and Craig W. Benkman then review the evidence that animals use fitness components of their conspecifics in foraging and breeding habitat selection. The last two papers deal with the sexual selection aspects. Richard H. Wagner describes the hidden lek hypothesis and offers some new developments. Herbert Hoi finally presents a tentative synthesis of the variable mating systems observed in several monogamous semicolonial species (Herbert chose not to provide a written version of his oral communication).