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Learn more about Outcome-based care, our holistic approach to wildlife care.

Oak trees are iconic. They're also threatened with extinction. San Diego Zoo Wildlife Alliance is committed to protecting these keystone species.

Cells vary in shape and function, and while some are enormous (the ostrich egg, the largest single cell in the animal kingdom, averages six inches long and weighs about three pounds), most cells can only be seen with the help of a microscope. The smallest object the human naked eye can see is approximately one-tenth of a millimeter (0.10 mm) in size—roughly the thickness of a piece of paper (0.15 mm), and the approximate diameter diameter of a Southern white rhino egg (0.14 mm). Let's zoom in and see how other cells measure up.