For most pitcher plant species, ants constitute the majority of the prey. Ants are attracted by copious nectar secreted at the inner edge of the pitcher rim, and under the roof-like pitcher lid. The trap is safe to visit during dry times, but its rim turns into a deadly slide when it is wetted. Ant colonies have been shown to be capable of evaluating benefits and risks associated with a food source – so unless the colony gains a net benefit, the ants should avoid visiting pitcher plants. We have shown in the field that the ON/OFF bimodality of the trap ensures the survival of ‘scout’ ants, thereby boosting recruitment and eventually prey capture. During dry times – up to 8 hours per day – ants can safely collect nectar and potentially derive substantial food intake from the plant. What looks like a predator-prey relationship might in fact be a ‘fair trade’ of nitrogen-rich ant workers for energy-dense sugary nectar, to the mutual benefit of plant and ants.
Polyrhachis pruinosa ant harvesting sweet nectar from the pitcher rim of Nepenthes rafflesiana in Brunei Darussalam, Northern Borneo.
We aim to quantify this nutrient exchange for the first time by continuously monitoring the traffic between an ant colony and a pitcher plant in our lab. Ants are not only continuously filmed and automatically counted, but they also have to cross a miniature weighing bridge on their way to and from the plant. An automated computer algorithm notes the direction of travel as well as the mass of the ant worker. From the cumulative mass difference of outgoing and incoming ants, we can estimate the rate of nectar intake for the colony. By counting the number of ants going out and coming back, we know how many ants fell prey to the plant. Follow-up experiments will measure growth rates of ant colonies with and without access to pitcher plants, and look at natural ant-plant interactions in the field in Borneo.
In a fully climate-controlled growth room at the University of Bristol, we can continuously monitor and weigh ants as they interact with a pitcher plant and harvest the nectar from its traps. By integrating the data feed from the highly sensitive weigh bridge with the footage of a video camera, we can determine exactly how much nectar the ants bring back to the colony, and how many workers fall prey to the plant.