The actual size of most mammals – including humans – is mostly set from birth. Our growth rate is influenced by the environment, but our skeleton stops growing once we reach adulthood. But it’s true about kangaroos — they indeed never stop growing. The skeletons of kangaroos and the larger wallabies continue to grow – slowly – throughout their lives.
There are many other kinds of animals with the capacity for unlimited growth. For example, invertebrates, such as corals, never stop growing. This pattern is called “indeterminate” growth – adult size depends largely on environmental conditions.
Most fish, amphibians, lizards, and snakes are indeterminate growers. In theory, they can get as big as their environment and diet allow. So why don’t we see huge creatures in nature? It’s mainly because the longer an animal lives, the more likely it is to come in contact with predators, diseases, and natural disasters that end its life before it gets very big. And, for many species, there may be structural constraints – where a single set of organs can only support a body of finite size.
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