Re: In animals, why does brain size increase if learning ability does not.

what a great question...

one thing to remember about brains, they do more than just 'remember' 
things.  a brain is really a collection of different bits of neuronal 
tissue that works together to do a number of things.  One thing that 
it must do is to collect sensory information from the environment and about 
the internal condition of the animal.  For example, a mouse's brain must be 
able to receive information that a predator is coming.  It does that 
through vision, hearing, smell, etc.  All of that information is coded in 
the brain.  It also needs to know where all of the various parts of its 
body are so that it can properly place a foot for the next cycle, or place 
a piece of food in its mouth and know that its foot is nearing where its 
mouth is.  Thus, the brain has a 'sensory' role.

The brain also has a 'motor' role.  It must be able to send signals down to 
its feet and legs that says 'run, predator is coming', or a signal down to 
its heart that says 'beat faster, predator is coming'.  Thus, the brain has 
to have an ability to send output to all of the various tissues of the 
body.

In between those two functions is something scientists don't understand 
very well.  How sensory input comes into the brain is fairly well 
understood.  How signals are sent out to the brain is fairly well 
understood.  What is not well known is how something like visual 
information representing SNAKE is taken into the brain and RECOGNIZED as a 
potential prey item and then CONVERTED into the properly timed signals that 
get sent out to the muscles that say RUN!  Somewhere in between there is 
something that we know of as memory.  It's obviously terribly important for 
survival (whether its not getting eaten or whether its passing your next 
history exam).  However, memory is not the only thing that the brain does. 
An elephant brings into its brain a lot more information relative to that 
of a mouse (there's lots more of its body that sends information to the 
brain), and, consequently, there's more muscle that needs signals to move. 
 There is a lot more muscle mass of the various organs (such as the heart) 
that receive signals to beat slower, or breath faster, etc, so all of that 
increase in body needs nerves to control it.

I hope that helps.  Don't hesistate to ask if you want more information.  
Best wishes.