VARIATION IN MATING SYSTEM AMONG BIRDS - ECOLOGICAL BASIS REVEALED BYHIERARCHICAL COMPARATIVE-ANALYSIS OF MATE DESERTION

Since most bird species are socially monogamous, variation among speci es in social mating systems is determined largely by variation in the frequency of mate desertion. Mate desertion is expected to occur when the benefits, in terms of additional reproductive opportunities, outwe igh the costs, in terms of reduced reproductive success from the prese nt brood. However, despite much research, the relative importance of c osts and benefits in explaining mating system variation is not well un derstood. Here, we investigate this problem using a comparative method . We analyse changes in the frequency of mate desertion at different p hylogenetic levels. Differences between orders and families in the fre quency of desertion are negatively associated with changes in the pote ntial costs of desertion, but are not associated with changes in the p otential benefits of desertion. Conversely, differences among genera a nd species in the frequency of desertion are positively associated wit h increases in the potential benefits of desertion, but not with chang es in the potential costs of desertion. Hence, we suggest that mate de sertion in birds originates through a combination of evolutionary pred isposition and ecological facilitation. In particular, ancient changes in life-history strategy determine the costs of desertion and predisp ose certain lineages to polygamy, while contemporary changes in the di stribution of resources determine the benefits of desertion and thereb y the likelihood that polygamy will be viable within these lineages. T hus, monogamy can arise via two very different evolutionary pathways. Groups such as albatrosses (Procellariidae) are constrained to social monogamy by the high cost to desertion, irrespective of the potential benefits. However, in groups such as the accentors (Prunellidae), whic h are predisposed to desertion, monogamy occurs only when the benefits of desertion are very limited. These conclusions emphasise the additi onal power which a hierarchical approach contributes to the modern com parative method.