Advances in miniaturization of sensor nodes enable a wide range of novel application scenarios. At the same time, however, this miniaturization drastically reduces the energy available for communication. We focus on wildlife monitoring applications for bats, which set a weight limit of 2g for the sensor node including the battery. Here, the protocol design is complicated by the need to recharge a capacitor before each communication attempt. For communications with ground stations, wake-up receivers are used that inherently help mitigate synchronization demands and to provide a superframe structure. We study the not obvious choice of transmission slots within these synchronized superframes. Our findings clearly indicate that slotted access outperforms simple random channel access. Well-planned TDMA schedules only bring little gain compared to random slot selection.