Sarah A. Budischak, Dawn O’Neal, Anna E. Jolles and Vanessa O. Ezenwa
Parasites, by definition, steal resources from their hosts, but how infection relates to host fitness is not well understood, particularly in wild populations. We combine experimental deworming and a 4-year study in African buffalo to better quantify fitness costs of infection and examine their underlying mechanisms. For one parasitic worm, Haemonchus, we found significant negative associations with host body condition, which is the primary predictor of buffalo reproduction and survival. Hosts elevated immune responses to resist infection by this blood-sucking worm, known to have serious effects on livestock. Conversely, the other common parasitic worm, Cooperia, is less harmful in livestock, and surprisingly, was positively associated with buffalo body condition, reproduction and survival. Additionally, elevating immune defences had no effect on Cooperia abundance. Thus, tolerating Cooperia infection (i.e. maintaining health despite high parasite loads), seems to be a more successful strategy to minimize the fitness costs of this only mildly-harmful parasite. More broadly, our study highlights that how hosts respond to infection plays a key role in the fitness costs of infection. Our data show that fitness is highest when hosts pick their battles based on how harmful a particular parasite is and how well they can defend against it; mismatches lead to lower condition, reproduction, and survival.
Image caption: Buffalo with words.