Sharp as a fox

Posted by Parthian on June 01, 2007

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?