In this paper, we describe a novel animal-tracking-system, solely powered by thermal energy harvesting. The tracker achieves an outstanding 100 $\mathsf{\mu }\mathrm{W}$ of electrical power harvested over an area of only 2 times $20.5$$\mathrm{c}$$\mathrm{m}$${}^2$, using the temperature difference between the animal’s fur and the environment, with a total weight of 286 $\mathrm{g}$. The steps to enhance the power income are presented and validated in a field-test, using a system that fulfills common tracking-tasks, including GPS with a fix every 1.1 $\mathrm{h}$ to 1.5 $\mathrm{h}$, activity and temperature measurements, all data wirelessly transmitted via LoRaWAN at a period of 14 $\min$. Furthermore, we describe our ultra low power design that achieves an overall sleep power consumption of only 8$\mathsf{\mu }\mathrm{W}$ and is able to work down to temperature differences of $0.9$$\mathrm{K}$ applied to the TEGs.