Several Medium Access Control (MAC) protocols have been proposed for wireless sensor networks with the objective of minimizing energy consumption. For example, Sensor-MAC (S-MAC) was proposed to reduce energy consumption by introducing a duty cycle. Under variable traffic load, the fixed duty cycle of S-MAC results in energy loss due to idle listening along with higher latency and lower throughput compared to an adaptive duty cycle. Timeout-MAC (T-MAC) introduced such an adaptive duty cycle to handle variable traffic loads. However, nodes that do not take part in data exchange waste energy because of continuous renewal of their timeout values. In this paper we propose Advertisement MAC (ADV-MAC), a MAC protocol for wireless sensor networks that eliminates this energy waste by introducing the concept of advertising for data contention. ADV-MAC minimizes the energy lost in idle listening while maintaining an adaptive duty cycle to handle variable loads. Additionally, ADV-MAC enables energy efficient MAC-level multicasting. We provide detailed comparisons of the ADV-MAC protocol with S-MAC and T-MAC through extensive simulations. The simulation results show that ADV-MAC efficiently handles variable load situations and provides substantial gains over S-MAC and T-MAC in terms of energy (reduction of up to 45%) while faring as well as T-MAC in terms of throughput and latency.