Can hosts tolerate avian brood parasites? An appraisal of mechanisms

Theoretical work has long stressed the need of studying in concert defenses based on resistance (i.e., mechanisms minimizing the frequency of effective parasite attacks) and tolerance (i.e., mechanisms minimizing the impact of parasites after a successful attack) to achieve a full understanding of host-parasite evolutionary dynamics. The study of tolerance can be particularly illuminating in the comprehension of avian brood parasite-host interactions because if hosts can tolerate parasitism this may resolve the long-lasting paradox of why some hosts do not reject parasite eggs despite costly parasitism. Surprisingly, although the study of host defenses against brood parasites is a hot spot for research in behavioral ecology, empirical studies of tolerance are very rare. Here, I first identify the main reasons explaining reluctance to incorporate tolerance in the study of avian brood parasite-host interactions. Tolerance defenses have been neglected because: 1) behavioral ecologists have primarily targeted on antagonistic coevolution which is most likely selected by resistance; 2) because tolerance (contrary to resistance) cannot be easily measured on host individuals; and 3) because there is a limited knowledge about the mechanisms of tolerance. In a second step, I review current evidence about tolerance in hosts and propose yet unexplored mechanisms to be studied based on parental investment theory. Finally, I propose an experimental framework based on well-established knowledge about phenotypic plasticity that can help detecting the effects of tolerance in future studies. I urge behavioral ecologists to embark on suggested mechanistic approaches to study tolerance defenses to achieve a better comprehension of avian brood parasite-host coevolution.