What stops a creature from growing?

[RobertBurden]

When a creature of any sort mammal or insect grows to adult, what stops the body from growing any larger? Could it ever be possible to grow an insect say to a foot long? Or a butterfly to a bigger proportion?
Any ideas welcome...

[jedimistress]

From what I remember at school, an insect's exoskeleton would become too heavy for the animal to function. This is a factor which restricts an insects growth.

[Siamsal]

Doesn't something like human growth hormone have something to do with it? If it goes wrong, dwarves or giants are the result (are we allowed to say dwarves, now-a-days). I'd guess, after a certain time, this whatever it is stops, and the body stops getting any bigger and just goes into maintenance mode.

As jm says, there are physical limits as well. Things with exo-skellingtons can't get bigger than a certain size, things without lungs ditto, (big) whales couldn't live out of the sea or they just splodge etc

[Clanad]

In animals, at least, the reason for maturation has to do almost equally with genetic limitations and environment. It's been observed that animals of the same species and even the same generation with some of the animals experiencing severe climatic conditions with greatly reduced food availability can be noticably smaller than those with normal, unrestricted food. If the conditions continue for extended periods of time, genetic limitations can develop that affect the entire species as to size.
This interesting question also has something to do with the surface area to volume ratio of the body. Each species has an average body size that works well in its environment, and the body's organs, such as the lungs and heart, are designed to work best at that size and weight. Growing larger could lead to a loss of efficiency. This is especially important for warm-blooded animals, whose food needs are high. Also, when the animals become mature, the extra energy that was used in growing can be redirected toward reproduction and the care of their young. Size appears to be inherited: micro-evolution would select animals that grew to an optimally efficient size and then stopped the growing process... In my opinion....

[doglady]

In captivity, lions and tigers sometimes cross-breed. The growth inhibitor hormone is in the female tiger, and the male lion (I think - could be the other way around. Anyway!). When a male tiger and a female lion are bred, the tigon doesn't have a growth inhibitor, so they get humungous. I saw a TV program once where this guy was leading one along on a chain, and it was the size of a pony. Massive!!

[wildwood]

Yes, mostly what clanad said. lol

There are some animals that never stop growing though. Some of the better known ones include the Shark, crocodile, many species of snakes etc. but as they get older the increase increments per year do get smaller.
I don't know of any mammals that continue to grow after adulthood.

Doglady you probably saw Hercules from the Institute of Greatly Endangered and Rare Species in Florida. And yes, the parents were the other way around - lion boy and tiger girl had a date and the result was a Liger. You may be slighly off-centre with the growth hormones though. The growth hormones are definitely effected with this hybrid but there is a limit in all mammals afaik. Anyone also a big cat enthusiast, google for 'Institute of Greatly Endangered and Rare Species' to be amazed.

Back to your question which was mostly covered by Clanad, if a mouse were to grow to 12" it would not be able to walk because its legs would be far to weak to carry the increased bulk of the body. Instead of the legs being 4 times thicker, they should be 12 times thicker.
If you can imagine a shadow (all black) picture of an elephant and scale it down to the size of a mouse, you'll see that the legs are still a lot thicker than a mouse's. This also prevents the elephant from growing any bigger because their legs would be so thick, they couldn't move them.

I know the dinosaurs spring to mind. Forget what you see in the movies. The ones that were larger than 10 tonnes spend most of the time in the water to lighten the pressure on their bulk.

[RobertBurden]

Cheers Everyone food for thought!!
Robson