As far as these horses are concerned, humans don't exist.
Now, they live in partnership with a different animal - Seals!
Every winter half a million grey seals crowd onto the sands to give birth.
And this huge gathering of sea mammals keeps the horses alive.
'So we have seals coming onto the island for weeks where they're pupping, they're mating, and while they're here they're fertilizing the grass.
The influx of nutrients is quite impressive and the horses are responding to this.'
It's a remarkable connection between two mammals who couldn't be less alike.
The seals bring nutrients to these barren dunes and the horses feast on the grass.