Sharp as a fox

In the UK series 'Bill Brand', 'there they were, sharp as foxes' was used to mean 'vividly there'. This ties in with 'sharp as a fox' meaning very clever. My query is where the phrase comes from and why a fox should be 'sharper' than, say, a swordfish. Or,is it a corruption of 'cunning as a fox' and 'sharp as a tack'? Any ideas?