Lower adaptive immunity in invasive Egyptian geese compared to sympatric native waterfowls

Successful invasive species increase their spreading success by trading-off nutritional and metabolic resources allocated to reproduction and range expansion with other costly body functions. One proposed mechanism for the reallocation of resources is a trade-off with the immune function and the regulation of oxidative status. Relying on a panel of blood-based markers of immune function and oxidative status quantified in an invasive species (Egyptian goose) and two native competing species (mallard and mute swan) in Germany, we tested the hypothesis that the invasive species would have (i) lower investment in immune function, (ii) lower levels of oxidative damage, and (iii) no higher antioxidant defences compared to the native species. We found lower levels of adaptive immune markers (lymphocytes and immunoglobulin Y), in the invasive species compared to the two native species. Innate immune profile was generally similar between Egyptian geese and mallards. By contrast, mute swans showed higher levels of heterophils and lysozymes, and lower levels of bacteria killing ability compared to both Egyptian geese and mallards. Mute swans also showed higher levels of haemolysis and haemagglutination, but lower levels of monocytes and haematocrit compared to Egyptian geese. Reactive oxygen metabolites, a marker of oxidative damage, were higher in mallards and lower in Egyptian geese compared to the other waterfowl species, while levels of antioxidants were generally similar among the three species. Our results point to a reduced investment in adaptive immune function in the invasive species as a possible resources-saving immunological strategy due to the loss of co-evolved parasites in the new colonised habitats, as observed in a previous study. A lower investment in immune function may benefit other energy-demanding activities, such as reproduction, dispersal, and territoriality, while maintaining relatively higher innate immunity is beneficial since invasive species mainly encounter novel pathogens. Results pointed out also other important species-specific differences in baseline immune status, supporting previous findings on the relationship between species' body mass and immune profile.