Wireless sensor networks may support heterogeneous applications ranging from classical environmental monitoring, to more demanding integrated surveillance systems based on multimedia traffic. To this extent, we argue that the specific MAC layer solutions must be flexible enough to provide differentiated services based on the context of the specific WSN. In this work, we leverage the SPARE MAC protocol for WSN based on a receiver-oriented TDMA protocol, and we propose adaptive techniques to dynamically adjust the resource (slots) assignment which are able to track the changes in the network status (traffic increase/decrease, contention increase/decrease). We comment on the trade-off between performance gain and increased signalling complexity, by resorting to numerical results derived through simulation.