Really small tracker using wifi only?
I don't know if it fits your data throughput needs, but you might want to look at lower power protocols such as Bluetooth Low energy (BLE), Zigbee, or Xbee. They tend to be lower power consumption, and lower throughput than wifi, but also various range options (some longer than typical wifi). In general the lower the data throughput, and lower the range, the lower the power requirements. If you know within 1-100 meters of where your bat is going to end up in general, I'm imagining a known cave, you might be able to lower the range and power requirement quite a bit and download data from where they rest.

If you drop the RF requirement altogether, and just data collect in very small amounts, something like an MSP430 with FRAM could take very little power.