In intelligent sensing systems with wireless sensor nodes, energy efficiency is one of the most important research issues. In this paper, we focus on energy efficiency for monitoring a large-scale object such as gas and chemical material diffusion and spread of radioactive contamination and wild fire. For monitoring of a large-scale object, a great number of sensor nodes might be participated in object detection and tracking. Thus, general functions of such huge quantities of sensor nodes like sensing and message exchanging could be sources of energy exhaustion and shorten network lifetime. Therefore, we firstly adopt the sleep/wakeup state switching to restrict active sensor nodes for object tracking. That is, since an object dynamically alter its own shape by wind or geographical condition, we support that only the sensor nodes around the current boundary of the changeable object actively function while the others are on the sleep mode. In addition, we also propose that active nodes are steadily held as a small set of sensor nodes collaborated for detecting and tracking of the current boundary. A variety of computational simulations proves that our proposal is able to provide high energy efficiency as well as to trace accurate boundary shapes