Hello, So I have heard that there are certin spieces of fish that can change their gender when the female is longer apaet of their group

Also that some animals become alot more agressive when they are the Alpha.

This may sound a little off topic to these but can an Alpha from a species grow a lot bigger in size due to being a leader of the pack due to chemicals in its brain of genes or what ever?  Thanks! :)

To answer your points:

1) Yes, some fish species do change sex. Some of these are protandrous (start out male, become female) and some are protogynous (the other way around). A short article about it is here:
http://evolutionfaq.com/articles/sex-ch … -reef-fish

2) This can be true but it depends on the social system. In systems where there is clearly a highest ranked individual (actually not that common but does occur in some high profile animals e.g., wolves, meerkats, some primates) aggression and "punishment" can be used by the dominants to maintain their position.

3)  Certainly there are physiological differences between dominants and subordinates and these can be manifest as hormonal differences and  - v likely - different neurotransmitter profiles. It is also very possible tha, for instance, hormonal changes could have consequences for body size. I do not know of any specific examples where becoming a dominant is shown to cause increased growth. Bear in mind that we do know individuals often become dominant because they are the biggest/strongest - so working out the direction of causality here is crucial.