Please note the text and sub-pages below is for reference only as we officially ceased operations now.

GPS/GSM trackers are very common indeed. 

Nowadays, manufacturers such as SiRF, uBlox, Mediatek, and Skytraq offer standalone receivers which exhibit low power and high sensitivity. So tags can become really small and still pack quite a bit of functionality and have long battery life!
A quick survey shows that there are several Companies using GPS to track wildlife:

We only do exotic stuff, so here is a rather uncommon GNSS tag with GSM. The tag idea and sponsorship came from Stichting Otterstation Nederland (that's why it's called SvoTracker), check the article in attach!

This time there is NO on board GPS receiver as such, just a front-end and a microcontroller to store the data in the uSD card, or send them to the modem. In practice the GNSS receiver bit is done with a snapshot, post-processing software. The advantage is a tag which sleeps most of the time and consumes very little power!