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The cost of begging: Does it exist? And if so, is it necessary to guarantee honest parent-offspring communication? Several theoretical models on the evolution of begging predicts that begging must be costly to be an honest, and evolutionarily stable, signal of need. However, the empirical search for begging cost has been unsuccessful, with proposed costs of begging, such as energy, attraction of predators, or reduced growth, not being universal or sufficient to explain the evolution of begging. Nonetheless, recent research points to a cost of begging in the form of reduced immunocompetence. Laboratory experiments with an array of bird species repetitively support the contention of a negative correlation between begging effort and immunocompetence. Moreover, my co-workers and I have shown that the negative impact of begging on immune system is not compensated by the increased ingest of food in fiercely begging nestlings. Furthermore, in a long-term field experiment, we have modified begging behaviour of pied flycatcher (Ficedula hypoleuca) nestlings, finding that begging level is under natural selection, and that its trade-off with immune system is key for understanding the impact of begging on fitness. In overall, although the subjacent cause of the relationship between begging and immune system remains unclear, recent findings suggest that offspring begging exaggeratedly may incur in an immunological cost, which has shaped the evolution of begging. However, these empirical results, demonstrating that begging is associated with a “cost”, do not demonstrate that such a cost is necessary for begging to be honest and evolutionarily stable. Begging, as other behaviours, might be inevitably associated to physiological processes, conducing to unavoidable trade-offs. In fact, several mathematical models predict that cheap begging may be honest and evolutionarily stable. The model in which I am working, as other models, predicts that begging does not require be costly to be honest, but the cost increases information contained in the signal.

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