In wireless sensor networks nodes forward the data they acquire from the environment towards the base station by relaying through multiple intermediate sensor nodes used as relays. If multiple sensor networks are deployed in close proximity then they can help each others' data forwarding so that all parties involved in such cooperation benefit from this collaborative effort (i.e., energy consumption for packet relaying reduces and the lifetime of each network is prolonged). There are many parameters that affect the impact of cooperation on sensor network lifetime such as number of domains, node density, network area, propagation environment, network topology, and base station deployment. In this paper we investigate cooperation strategies for prolonging sensor network lifetime in multi-domain wireless sensor networks through a linear programming framework. While our model is detailed enough to capture the essence of the multi-domain cooperation we intentionally avoid implementation specific details. Hence, we use our framework to determine almost achievable performance benchmarks in idealized yet practical settings. Our results show that under certain conditions (sparse network deployment and harsh propagation environment) multi-domain cooperation can extend the wireless network lifetime more than an order of magnitude when compared to non-cooperating domains of wireless sensor networks. (C) 2009 Elsevier B.V. All rights reserved.