Oh those Giga's...

Three times my tribe and I have tried to tranq a giga. Three times we have failed.

The first time, we were completely inadequately equipped - so gave up. The second time, it was going perfect. Four of us shooting darts at it, then suddenly it jumps up and bites my Quetzal on the ass - and promptly melts under the map... The third time, I gathered my equipment and tribe-mates... Only to watch it walked into the mountain and disappear...

This has been over the space of a few weeks - purely because they never seem to spawn!

Is there a special inclusion/exclusion to when/how a Giga spawns? Or is it purely a case of munching some steak-holders and freeing the spawns up..?