It depends if they have always been like that or if they have lost weight recently .I always weigh my chins on a monthly basis so I can see any fluctuations in their weight .

To be honest if they have lost weight recently and their ribs are prominent I would suggest you have them X Rayed as there is no other reliable way to ensure that they do not have any teeth/ root problems .

To build underweight chins up I give a teaspoon of oats, wheatgerm & wheatbran mix a couple of times a week and a healthy diet of pellets and lots of good hay and readigrass a couple of times a week . You can also get the 'naturals' range in pet shops these days for chins which are healthy treats .

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Chinchilla rescue and rehoming http://www.chinsrus.co.uk