Did you check your gpu memory usage? (so not the core usage that you mentioned?). A friend of mine also has a 1080 and in a small zoo ALL the 8gb of his memory is in use.So here is my system:
3700X - Usually in the 4125 to 4300 MHZ range
EVGA 1080Ti FTW3 - Fans at 90%, power at 75 to 100% depending on zoo
Reason I say depending on zoo is the following:
4500 person, 116 animal zoo - 50% CPU and 40 to 50% GPU utilization, 35ish FPS, GPU @ 1974 MHZ, 55C
850 person, 31 animal zoo - 55 to 60% CPU, GPU is used 90 to 100%! 100 FPS, GPU @ 1860 MHZ, 60C+
Case fans are at 100%, 70F ambient
Why am I getting a lot more GPU usage with a much smaller zoo than I am with a much larger zoo? The GPU is most certainly waiting on things in large zoos and the usage along with temps shows. This is not imagined, this is me watching over the past few minutes and using MSI Afterburner for an OSD in game.