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Size growth after birth seems to be the norm in the animal kingdom (as well as among plants). This makes sense. It's evidently physically challenging for an animal to produce an offspring of its equal size, both if born through gestation or eggs.

But, is this always the case? Are there exceptions, i.e. animals that are born at their "normal", adult size? I don't seem to find any (nor that I am a biologist or expert on the topic).

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    $\begingroup$ You may perhaps want to more specifically ask whether, for instance, there are any animals (e.g. placental mammals) that do not grow in size after birth. There are terms for this, such as altricial babies (babies that are less developed in some ways, or require additional care) or precocial babies (those that develop less post-birth). You could ask for extreme examples of precociality! It may also help to read up on neoteny which is related but not directly linked; you may get some good ideas there. $\endgroup$ – S Pr Jan 23 at 13:24
  • $\begingroup$ Also, birth is something that we can interpret fairly easily for many vertebrates. However, what counts as birth for a moth or butterfly? The embryonic egg is certainly very small. The larvae grow. Then they pupate, and undergo eclosion (finishing metamorphosis) and come out of their pupa usually completely adult sized. It is not unusual for flies to eclose as very large imagos (adults) and then slightly shrink during the rest of their lifetime. $\endgroup$ – S Pr Jan 23 at 13:31

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