Which mammal has the shortest life span?

The giant sunda rat has the shortest lifespan of all mammals.

https://a-z-animals.com/blog/8-shortest-living-animals-in-the-world/

The Giant Sunda Rat is considered to be the mammal with the shortest lifespan. Their life expectation is somewhere between six and twelve months.

Found in Indonesia, these mammals begin breeding at one month of age, with young produced just three weeks later.

Natural selection amongst rodents (about half of all mammals) helps these animals avoid predation through prodigious breeding rates and short lifespans.

https://a-z-animals.com/blog/8-shortest-living-animals-in-the-world/