Which is true when it comes to animal growth?
A. Reach a particular size and then stop growing
B. Keep growing indefinitely
C. All of the above
If you guessed C, congratulations! The skeletons of most mammals reach a certain size and then stop growing. However, many animals, including some mammals, keep growing throughout their lives. Kangaroos, for example, just keep growing and growing until they die. Most fish, amphibians, lizards, and snakes are also indeterminate growers. Until something, disease, a predator, or old age-takes them down, these animals know no bounds when it comes to size.
Why don't you regularly see dinosaur-sized lizards, fish and snakes? Part of it has to do with the law of the jungle, so to speak. Simply put, most animals are eaten or killed before they get really big. Also, the longer an animal lives the greater the chance it might catch some deadly disease. In short, most animals die or are killed before they reach truly gargantuan proportions.
What about that goldfish in your home aquarium? With plenty of food, no predators, and a disease free environment, what's to stop it from growing right out of the tank? Only its innards. The fish's skeleton may keep growing, but its organs can only keep things going until the body gets too big for its own good. The size of the body is constrained by the ability of the organs to support it.