Using low duty-cycle is the most common technique to extend the system lifetime in WSNs. However, it also implies limited throughput and long delay and the penalty is even higher under variable traffic patterns. In this paper, we present iQueue-MAC, a hybrid CSMA/TDMA MAC that adapts to variable/bursty traffic. With light load, iQueue-MAC uses a contention-based CSMA mechanism that provides low delay with scattered transmissions. When traffic increases, detected by a forming backlog in the sender, iQueue-MAC changes to a contention-free TDMA mechanism allocating transmission slots. Thus, iQueue-MAC mitigates packet buffering and reduces packet delay, combining the best of TDMA and CSMA. In this paper we also show how iQueue-MAC can operate in both single and multi channel modes. We implemented it on SIM32W108 chips together with other reference WSN protocols for comparison. iQueue-MAC exhibits similar figures during light traffic. However, with bursty traffic its throughput can be five times that of CoSenS and Ri-MAC-MC and its delay 20 times lower. Finally, iQueue-MAC is able to effectively use multiple channels, duplicating its throughput when compared to single channel operation.