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It seems that at any given moment in a creatures life, it either grows bigger or has stopped growing.

The only exceptions I know of are extreme events like insects undergoing metamorphosis or evolutionarily irrelevant effects like humans shrinking a bit at old ages due to their spine getting compressed (I consider it irrelevant because for most of human history, people old enough for this effect to be significant enough were either dead or long done with reproduction).

Are there any animals that are known to "grow smaller" over some period of their lives as a purposeful part of their biological functions?

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  • $\begingroup$ Are you asking about decreases in length, volume, weight, or all of the above? $\endgroup$ – fileunderwater Jan 3 '17 at 23:45
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In holometabolous insects it is quite common for the final larval stage to be heavier than the adult stage. For instance, Goliathus beetles, which are among the largest insects on earth, the adult stage often weighs about half of the final larval stage (which weigh 80-100 grams).

The same situation can be seen in the Hercules beetles, which this picture of both larvae and adult of Dynastes tityus clearly shows:

enter image description here.
(picture from http://harrellhouse.com/in-the-museum-eastern-hercules-beetles/)

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    $\begingroup$ Sorry, I just saw that you mentioned metamorphosis in your question, which I managed to overlook. I'm leaving the answer either way, since some might find it useful. $\endgroup$ – fileunderwater Jan 3 '17 at 23:04

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