Every animal born small, then grow larger and larger.
The same is true for parts of the animal bodies.
Is there any cases, when body or part of the body diminishes in size and mass, while staying healthy?
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$\begingroup$ Do any example of structure existing in insect's larvaes but not in adults would answer your question? What about the thin skin that human baby have between their fingers and get lost later? $\endgroup$ – Remi.b May 6 '14 at 20:09
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In humans at least (I'm not sure about other mammals), the thymus grown until puberty and then shrinks gradually, a process called involution. I'm not sure if this meets your criteria. The thymus remains "healthy," but it's function greatly diminishes over time.
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Not a mammal, but the paradoxical frog (Pseudis paradoxa) starts life as a small tadpole that grows into a large tadpole and ends up as a much smaller frog. There are bound to be other examples of this phenomenon, but this is the one that immediately sprang to mind.
