Here's a very straightforward study, "Evidence That Lifelong Low Dose Rates of Ionizing Radiation Increase Lifespan in Long- and Short-Lived Dogs" [1].
Two studies on beagles:
"One exposed the dogs to whole-body cobalt-60 γ-radiation."
"The other evaluated dogs whose lungs were exposed to α-particle radiation from plutonium."
For both studies, excess radiation improved their lifespans by 20-50 percent. It is a substantial beneficial effect. Above a beneficial level of radiation their lifespans shortened to the level of dogs who were not exposed at all and then to substantial reductions in lifespans.
Two studies on beagles:
"One exposed the dogs to whole-body cobalt-60 γ-radiation."
"The other evaluated dogs whose lungs were exposed to α-particle radiation from plutonium."
For both studies, excess radiation improved their lifespans by 20-50 percent. It is a substantial beneficial effect. Above a beneficial level of radiation their lifespans shortened to the level of dogs who were not exposed at all and then to substantial reductions in lifespans.
This graph illustrates it best: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5347275/figure/...
We don't know why low radiation exposure is good, but it does seem to be.
[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5347275/