Mobile ad hoc networks are self organizing, infrastructure-less, low power networks, design to deploy bandwidth-shared radio channel communication and to work under vulnerable environment. Security is primary concern in MANETs and in order to achieve high security (confidentiality, integrity, authentication, availability and non repudiation), several techniques have been explored in which biometrics with cryptography or intrusion detection has gained a momentum in recent years. This paper critically reviews and investigates the present biometric based security models works for MANETs, and along with security challenges and direction of further research are proposed.