A paradoxical effect of brood-parasitism intensity on host defence.

GLOAG R; FIORINI VANINA D.; REBOREDA JUAN C; KACELNIK A

Lund

Congreso; 14th International Behavioral Ecology Congress; 2012

International Society for Behavioral Ecology

The extent to which host broods are multiply parasitized can have strong influences on the evolution of adaptive strategies of both parasites and hosts. We investigate with field experiments and theoretical models two of these influences, focusing on the interactions between the South American shiny cowbird (Molothrus bonariensis) and its hosts. First, we show that multiple parasitism can reverse the sign of selection for of egg rejection by the host, because, since female parasites destroy pre-existing eggs prior to laying, the eggs of hosts that reject parasite eggs are at greater risk of being destroyed in subsequent parasite attacks than the eggs of those that accept them. Paradoxically, higher intensity of parasitism promotes host tolerance. Second, we show that multiple parasitism can also reverse selection pressure for parasite virulent behaviors (e.g. chicks killing nestmates or adult parasites destroying pre-existent eggs). This arises because the presence of nestmates has two opposing functional consequences for parasite nestlings: it increases host provisioning rate but reduces the fraction of provisions consumed. The competence of parasite nestlings to elicit food provisioning and to compete for these provisions determine when and to what extent parasites benefit by nest sharing. These examples demonstrate that parasitism intensity is a fundamental factor for host-parasite co-evolution, and that its influences are not easy to intuit: what is adaptive when parasitism intensity is low may be maladaptive when parasitism intensity is high.