@dd33 Dustin, just more clarification on your question as you are dealing with Galapagos:
The left upper leg has indeed been standardized as the PIT tag/microchipping site for Galapagos tortoises in the captivity and the wild. There are now several thousand tortoises microchipped in the wild plus those raised at the breeding centers in the Galapagos.
Some microchips do migrate in a few tortoises and if high in the rear leg and the chip migrates, it will usually migrate down the leg where still accessible to read rather than into the abdominal cavity.
One of our fellow directors of the Galapagos Tortoise Alliance while in the Galapagos Islands, was teaching some of the park rangers to microchip in the rear leg.
OF interest:
On the January 2021 expedition to work with C. vandenburghi on Alcedo Volcano, the crews marked 4723 tortoises! 1745 females, 1794 males, and 1184 juveniles! The estimated population of this species is now 12,000-15,000 tortoises! That is considerably more (2x - 3x) than any previous estimates and probably the largest species population in the Galapagos.
