Usually it's gravid females, but sometimes other animals will do this too. I've seen snakes going into shed lay inverted as well.
--Donna Fernstrom 16.29 BPs in collection, 16.11 BP hatchlings Eclipse Exotics http://www.eclipseexotics.com/ Author Website http://donnafernstrom.com Follow my Twitters: WingedWolfPsion, EclipseMeta, and EclipseExotics
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