Effects of haemosporidian co-infection and parasitemia on reproductive strategies in a polymorphic species

Host-parasite interactions play a significant role in evolution. Parasite infections impose fitness costs that can trigger trade-off responses in host reproductive strategies. Individuals can invest limited resources in parasite defense, such as stimulating the immune system, or perform terminal reproductive investment. Here, we investigated how haemosporidian co-infection and parasitemia influence different reproductive strategies in a polymorphic bird species (the white-throated sparrow, Zonotrichia albicollis). We provided an account of the impacts of co-infection and parasitemia on host reproductive success and reproductive strategies in response to parasite infection. We tested the effect of co-infection and parasitemia on reproductive success (fledgling number, extra-pair paternity, ratio social/genetic offspring) and the effect of parental infection on nestling quality (mean nestling growth rate and body condition). We found that co-infection increases reproductive extra-pair paternity and nestling mean body condition. Parental high parasitemia positively impacts the ratio of genetic offspring belonging to the social father and has mixed results on nestling quality. We observed that co-infection in females and parasitemia in males might trigger a switch in reproductive strategy towards a terminal investment for co-infected individuals. In contrast, high parasitemia in females negatively impacted their offspring fitness, possibly due to the reallocation of resources for fighting the infection. ### Competing Interest Statement The authors have declared no competing interest.