Biological snakes can perfectly move on almost all natural environments. Snake robots should be adaptive changing environment in order to have these motion abilities of their biological counterparts. This feature is important for potential applications of snake robots. This work investigates optimal gait parameters for energy efficient locomotion of a wheel-less snake robot under environments with different ground friction conditions. Adaptive gait parameters are obtained using symbiotic organism search (SOS) algorithm by considering both minimizing the average power consumption and maximizing the forward velocity of the robot. To show the validity of proposed method in finding optimal gait parameters for adaptive locomotion of the snake robot, a fairly wide range of surfaces from glass to wood is used. The obtained results indicate that the snake robot can maintain optimality of locomotion in different environmental condition. This paper contributes to improving the environmental adaptability of the snake robot.