MadSci Network: Engineering

Re: Why does an exoskeleton have a mech. adv. over an endoskeleton?

Date: Wed Apr 3 15:17:46 2002
Posted By: Aurelio Ramos, Grad student, Computer Engineering, Not a member of any institution
Area of science: Engineering
ID: 1010754142.Eg

This is a difficult question, partly because it is very open, secondly 
because it is a little off from my field of expertise.

There is no absolute advantage to either kind of skeleton. As you have 
found, the lobster can at least crush a human finger joint, possibly even 
cut off a small human finger, but at the same time, a human can rip a 
lobster's claw right out. So the advantage of the situation seems to 
depend more on who grabbed first than on the specific type of skeleton.

Now, all other things being equal, If a lobster were to attack a 
vertebrate its own size, I'd put my money on the lobster. The mechanical 
advantage in this scenario stems from the fact that the lobster simply 
has lots more bone. to protect it (in proportion to its own size), and 
the bone in the claws is shaped in a curved way that enhances its 
strength. A small vertebrate would likely have very little bone, 
surrounded by much softer tissue. So, in short, the advantage is all 
because of the large amount of bone, and the fact that it is on the 

Exoskeletons are generally better for smaller animals because it provides 
the best protection. On the other hand, an exoskeleton requires molting 
for growth (the old skeleton is shed away periodically. Until the new 
crust hardens, the crustacean is very vulnerable to attack. 

For larger animals, an exoskeleton would have to be excessively heavy to 
be of an advantage. The molting cycles would have to speed up as the 
animal grows to maintain a particular rate of growth, but at the same 
time, the amount of material required to create the new skeleton 
increases, which seems to require slower molting cycles. So, there has to 
be a maximum size at which growth can no longer be sustained effectively. 
(Comment by reviewer:
while weight certainly is an important factor, it does not limit the 
seizes to what we see today. A much more limiting factor are the problens in 
getting oxygen and nutrients dristributed throughout the body, as well as getting 
rid of breakdown products and carbon dioxide. All animals with exoskeleton do 
have a comparatively simple heart, open blood circulation and a system of trachea 
for gas transport. I once learned that these are the limiting factors for the 
growth of animals with exoskeletons.)

If we observe in nature, all those facts seem to be reflected in the 
evidence: animals with exoskeletons tend to be smaller than animals with 
endoskeletons. At the same time, there are many more animals with 
exoskeletons on the planet than animals with endoskeletons. So there is 
clearly an advantage, as long as they are small animals.

An endoskeleton is not as protective, but it allows larger species to 
grow, and thus be able to escape total domination by spiders crabs and 

I have gathered some good resources you may find useful, about lobsters 
and their claws.

Your mad scientist,

-Aurelio R. Ramos

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