Yes it has been 'posed', the rat is in the foreground nearer the camera to make it look bigger. The photographer has used a wide angle lens (short focal length) close up to make the effect even more pronounced. Standard photographic technique, wide angle lens close up with a small f stop for greater depth of field.
Rats really are a lot bigger than most people realise ( ask ABs own 'Ratter' for confimation)
Sherlock Holmes once mentioned The Giant rat of Sumatra. In the same way Cane Toads were brought to Australia could someone, a rat fancier, have brought giant rats to NI and they've escaped?
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