Wireless sensor networks (WSNs) have been widely studied in the context of many-to-one communication, in which multiple data sources send messages to a dedicated sink. However, there has been little research in the area of many-to-many communication. Many-to-many communication in WSNs is a growing application area, with examples including fire detection in both natural and urban areas, and the monitoring of heating and air conditioning within buildings. In this paper, we propose a scalable many-to-many routing protocol that makes use of Ant Colony Optimisation (ACO) that is applicable for an arbitrary number of sources and sinks. The protocol aggregates data sent from multiple sources into a single, shared backbone of nodes to reduce the total number of packets sent and so increase network lifetime. Results from simulations using the Cooja Network simulator show that the protocol is able to achieve packet delivery ratios above 95%, with the algorithm becoming more efficient with larger networks, sending fewer packets relative to the size of the networks, as well as involving fewer nodes in routing.