Is the question the "How to prevent additional cracks?", or is the question "How to prevent existing cracks from additional cracking?" If the latter, the best product that I've used, and I've been involved in cabinetry and restoration work for 18 years, is polyurethane glue. Inject it into the crack a bit shy of the surface as it expands a little, and it wont crack there again.
If the former, as mtry and strat have written, it is definitely a moisture balance issue. If you can seal the carving with tung oil without damaging the carving's value, I would do so immediately. Untreated wood that is not kept in a consistent environment will absolutely shrink and swell.