[liberalsurfer]

If you think about what is required for a mammal to survive multiple environments it seems intuitive the more environments a mammal can survive the more genes is needed to create different phenotypes (observable characteristics of an organisms due to the interaction of its genes with the environment). Just looking at the organisms themselves a mouse is capable of surviving harsher and diverse environments than humans. Due to this resiliency the mouse must have more phenotypes and thus have more genes than humans.

This would also perhaps explain the reason as to why some plant species have far more genes than mammals as without the ability to move they need to be able to code for multiple environment changes throughout their life.