Low costs reinforce the mutualism between bats and pitcher plants

Whether mutualisms persist or vanish during the course of evolution depends on the cost-benefit ratio for the species involved. In the interaction between the pitcher plant Nepenthes hemsleyana and the bat Kerivoula hardwickii both partners mutually benefit: N. hemsleyana offers high quality roosts in exchange for nutrients from the bats’ faeces. Here, we tested the hypothesis that the partners should also incur costs from their interaction. In the field, we examined potential costs that are likely to occur in our system and that are already known to be present in other bat-plant interactions. Regarding the plants, the bats could injure the tissue of the fragile pitchers and thus affect the pitchers’ longevity. Regarding the bats, the ephemeral nature of N. hemsleyana pitchers and incoming rainwater could force them to switch these roosts more frequently than if using an alternative roost type. Our results suggest that neither the pitcher plants nor the bats accrue substantial costs from their interaction. We conclude that the ratio of high benefits to low costs strengthens this mutualism and promotes reciprocal adaptations and specialisations.