LOVE, O.P.*; WILLIAMS, T.D.; Simon Fraser University, British Columbia, Canada; Simon Fraser University, British Columbia, Canada: Sex-specific trade-off between immune function and growth in a sexually-size dimorphic bird

In birds, a trade-off between growth and immune function in offspring results from a conflict for the allocation of parental resources during development. This may be especially true for offspring of sexually-size dimorphic species, where nestlings of the larger sex must choose to invest in growth (where adult size may be important for mate competition) versus immune function (which can significantly affect survival). Based on this, one would expect that if immune function is competing with growth, inter-sexual difference in immunocompetance should be higher in species in which male and female offspring can exhibit differential growth patterns and mortality. To test this, we examined cell-mediated immunity over two breeding seasons in nestling European starlings (Sturnus vulgaris) from a wild colony where males have higher growth rates and fledge at a larger mass and structural size than their female siblings. In accordance with sex-specific allocation theory, we found that while male offspring exhibited faster growth rates and larger structural size at fledging, they also exhibited lower cell-mediated immunity compared with female siblings. As the breeding season progressed and food availability/quality decreased, immune function in female offspring decreased, eventually matching that of male offspring; immune function in male offspring remained consistent throughout the season. These novel results reveal inter-sexual differences in physiological strategies early in life and suggest that the larger-sex-offspring of sexually-size dimorphic species trade-off structural size with immune function. This may have important sex-related consequences such as susceptibility to disease and therefore ultimately survival.