Lower baseline immunity in invasive Egyptian goose compared to sympatric native waterfowls

Successful invasive species out compete native species for vital resources. To increase their spreading success, invasive species need to trade-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. According to the evolution of increased competitive ability hypothesis, a reduced investment in immunity would be favoured by parasite loss in the colonised habitats. It is also suggested that invaders would reallocate resources among different immune effectors depending on the new pathogens they encounter in the new habitat. Reallocation of resources may also involve the regulation of oxidative status given its fundamental link with the immune system. 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 a lower investment in immune function compared to the native species. We predicted lower levels of baseline immune markers associated with systemic inflammatory response, and higher levels of certain low-cost immunological effectors (e.g. humoral effectors) in the invasive species compared to the two native species. If Egyptian geese reduced their investment in immune function, we would also expect that geese generated less oxidative damage and had a lower expression of antioxidant defences than native species. We found lower levels of several immune markers associated either with inflammatory response or humoral innate and adaptive immunity in the invasive species compared to the two native species. Bacteria killing ability of plasma was the only immune marker that was upregulated in Egyptian geese. Markers of oxidative status were higher in mallards compared to the other species. The results of our study point to an overall reduced investment in immune function in the invasive species as a possible energy-saving immunological strategy due to the loss of parasites in the new colonised habitats. Thus, a lower investment in immune function may benefit other energy-demanding activities, such as reproduction, dispersal, and territoriality. ### Competing Interest Statement The authors have declared no competing interest.