Numerous MAC protocols have been proposed for stationary wireless sensor networks (WSNs). However, there have been very few approaches proposed to make the MAC layer in WSNs suitable for mobile scenarios. We propose a new MAC layer protocol called MOBMAC to support mobility in WSNs. MOBMAC uses an adaptive frame size approach to overcome the effect of frame losses caused by the Doppler shifts in mobile scenarios. An extended Kalman filter (EKF) is used to predict the frame size for each transmission, which enhances the energy efficiency of the system and minimizes latencies. Our study shows that under mobile scenarios, the MOBMAC reduces energy consumption by 60% and shows decrease of 25% in latency in comparison with the well known base protocol $SMAC. Our study also includes the contribution of a more realistic physical layer implementation model in ns-2, which processes the received frame based not only on the fading characteristics of the signal but also on the SNR and relative velocity between the communicating sensor nodes.