Public key cryptography can introduce significant processing delays in vehicular communication platforms. This can lead to serious performance issues, especially in the case of multi-hop Inter-Vehicle Communication. In this paper we propose Adaptive Message Authentication (AMA), a lightweight filtering scheme that reduces the number of cryptographic operations performed by the nodes. Based only on local observations and with no additional communication channel, our scheme achieves global improvement of network performance. We show through simulation that the scheme successfully adapts the number of cryptographic operations to the locally observed number of adversaries.