In the poem "To a Mouse," when he calls the mouse "fellow mortal," what is the speaker acknowledging?

In the Burns poem, "To A Mouse," when the speaker refers
to the mouse as a fellow mortal, he is saying that we are all mortal, we all die. Mice
and men alike live their short time on earth then die.  The speaker is acknowledging
that in this respect, mice and men are equal.  We put in our time and then we
perish.

To be mortal is to end our time with death.  This
is a fundamental connection men have with mice in the poem.  Between birth and death
mice and men alike suffer hardships (like having our homes destroyed, specifically) and
having our best laid plans go awry (in general). 

Burns
uses local language (his Scottish dialict) to present local people, common people with
common problems.  Burns brought poetry to the masses, to the commoners.  Rather than
deal with society's large issues, he deals with everyday existence.  We live our lives
and then we die, sharing our state of existence with the natural
world.