Mouse heart beating? /shaking

Question
Hi Natasha, I wrote you previously & thanked you for your "antiobiotic fish tetracycline fix for eye infection", which I've already used two different times. Recently I've noticed that whenever I look at Cheerio "at rest", his little body is moving or "throbbing", like, as in "to (watching) his little heart beat".
It's NOT shaking (like he's cold), it's just like watching his heart beat, but he seems content, & I just wondered if it might be something simple I needed to change or fix? I know although he doesn't look it (ha, ha), he is getting older--I've had him for about 13 mos., & not sure about the previous owner? He stopped climbing the walls of the cage long ago, & seems to have a harder time of reaching up for his water, but he still seems "pretty busy" otherwise. Perching up with his little face peeking out, looking & waiting for a "cheerio", he is "just the CUTEST THING"!
What do ya think?

Answer
Dear Cheerio, and his person friend,

Cheerio is getting older. Mice do start to shake some as they age. He may also soon begin to hunch his back a little or get rougher fur where he has trouble cleaning himself. He will get more quiet as well. The difficulty getting to his water bottle may be arthritis. The good part is, as mice get older they tend to get more and more loving. And he may still have quite a while to go.

Make absolutely sure he can still get his water. Maybe it needs to be lower. If you aren't sure, leave him wet vegetables such as cucumber (don't let it rot), carrot, celery, or green pepper) in the cage. Make sure he isn't having trouble cracking seeds.

When it is time for him to go, friendly mice often prefer to die in your hands. Often they will wait until you pick them up. A dying mouse can kick like it is having a fit, but that is normal.

Much love to Cheerio. Sounds like he has a good life with a very loving human companion.

squeaks,

Natasha