Why Do Land Animals Have Denser Bones Than Sea Animals?

[Hereformore]

Given the greater pressure underwater, i.e. at the bottom of the ocean, to survive there youd think that those animals would have stronger/denser bones, but we find that land animals have denser bones to withstand the higher net weight (gravity).

But that seems counter intuitive because underwater the pressure is much higher at lower depths (like in the movies when submarines start to creak and get crushed under the pressure).

So what's the deal? I know its a pressure vs force distinction but I am having trouble seeing it.

 

[russ_watters]

Submarines are filled with air at low pressure. Bones and fish bodies aren't. So there is nothing for the pressure to push against to crush. If there were, human divers wouldn't be able to dive more than a few feet!

So the fluids are continuous and the pressures are equal, so nothing to push against.

[edit] Actually, even worse, when lying in bed it would feel like you'd have 10,000 lb of weights stacked on top of you due to the air pressure!

 

[Drakkith]

Bones are used to support an animal against uneven forces (among other things), like having to stand up against gravity. Water pressure is evenly distributed so bones don't have to do any work against it.

 

[A.T.]

Given the greater pressure underwater, i.e. at the bottom of the ocean, to survive there youd think that those animals would have stronger/denser bones

You do realize that there are aquatic animals, which don't have any skeletons? In fact, as life developed in water none of it had skeletons. The musculo-skelettal system developed mainly to enable certain types of movement.

 

[PJC01]

I can explain the reason for this apparent contradiction. While it is true that the pressure increases with depth underwater, it is important to understand that pressure is a force applied over an area. In the case of animals living in the ocean, the pressure is distributed evenly across their entire body, including their bones. This means that the bones are not subjected to any significant net force that could cause them to compress or break.

On the other hand, on land, the force of gravity is constantly acting on animals, causing them to experience a net downward force. This force is distributed unevenly across the body, with the bones bearing the majority of the weight. In order to withstand this force and prevent the bones from breaking, they need to be denser and stronger.

In addition, land animals also need to support their own body weight while moving, jumping, and running, which requires even stronger and denser bones. This is especially important for larger animals, as they have a greater body weight to support.

Furthermore, the density and strength of bones are also influenced by the environment in which an animal lives. Land animals have evolved to have denser bones in order to survive on land, while sea animals have evolved to have lighter bones to aid in buoyancy and movement in the water.

In conclusion, while the pressure may be greater underwater, the force of gravity and the need to support and move on land are the main factors that contribute to the denser bones of land animals compared to sea animals.