A scanning project at Manchester Museum and the University of Manchester has revealed that about a third of the bundles of cloth are empty inside.
Researchers believe there was a huge appetite for these religious offerings, and demand for the mummies may have outstripped supply.
The research team has been conducting the largest scanning project of its kind.
More than 800 mummies, ranging from cats and birds to crocodiles, have so far been analysed using X-rays and CT scans.
About a third of those scanned contain complete animals, which have been remarkably well preserved.
Another third contain partial remains - but the rest have been empty.
Quote:Unlike human mummies, which were created to preserve the body for the afterlife, animal mummies were a religious offering.
Quote:Scientists estimate that up 70 million animals may have been mummified by the Egyptians.
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