Immunocompetence handicap hypothesis in tree frog: trade-off between sexual signals and immunity?

The ICHH assumes that only males of high genetic quality can afford the immunosuppression caused by testosterone that in turn allows them to express costly sexual signals. Using a multimodal signal approach, we demonstrate that both acoustic and visual signals are testosterone dependent in a tree frog. Contrary to expectation, a challenge to the male immune system showed that testosterone was not the direct mediator of the trade-off between sexual signal production and immunocompetence.The immunocompetence handicap hypothesis (ICHH) assumes that testosterone plays a dual role in mediating both sexual signal expression and immunosuppression. Although there is good evidence of the ICHH in birds, it was primarily tested using only one sexual signal approach. By focusing on the European tree frog Hyla arborea, a species for which mate choice is based on both acoustic and visual signals, we test for the first time the ICHH in a multimodal communication system. The ICHH may be viewed as a universal way to explain signal honesty no matter the sensory modality used by animals. After studying the effects of testosterone on the acoustic and visual sexual signals through a daily testosterone supplementation, the effects of androgen on the immune system response were measured through a phytohaemagglutinin injection. In H. arborea, we demonstrate that both the acoustic and visual sexual signals were testosterone dependent. However, the phytohaemagglutinin injection did not provoke immunosuppression for testosterone-supplemented males. Surprisingly, the testosterone-supplemented males with a high body mass presented an immunoenhancement. Therefore, it seems that ICHH is not an integrating mechanism that is able to explain signal honesty in the multimodal signaling system used by H. arborea in mate choice. On the contrary, our results support the idea that multiple mechanisms (e.g., cost-added and index signals) operating simultaneously may explain signal honesty of both acoustic and visual signals.