The evolution of egg size in socially polyandrous shorebirds

In classical and multi-clutch polyandry, females lay multiple clutches duri ng a breeding season for more than one mate. The production of multiple clu tches may be energetically demanding. We used comparative analyses to inves tigate three possible ways of reducing such egg-laying costs in polyandrous shorebirds: (1) reduction in egg size, (2) reduction in clutch size, and ( 3) evolutionary increase in female size. Paired comparisons of polyandrous and non-polyandrous taxa showed that females of polyandrous shorebirds lay smaller eggs than females of closely related monogamous and polygynous spec ies. Directional analyses corroborated this result by indicating a signific ant decrease in egg size after phylogenetically independent origins of poly andry. The comparative analyses uniformly rejected the two alternatives, i. e. neither clutch size nor female size is related to social mating pattern. We also tested and rejected three alternative explanations for reduced egg size in polyandrous taxa, First, we found no evidence that polyandrous fem ales have evolved smaller egg sizes in response to selection to match small er size of males, which provide the parental care in these species. Second, reduction in egg size was not related to longer breeding seasons (and henc e more opportunity for re-nesting). Third, reduced egg sizes were also not related to rates of clutch predation (another potential correlate of multip le clutch production). Our results are thus consistent with the hypothesis that selection for reducing laying costs explains small egg size in sociall y polyandrous shorebirds.