He also noted the unexpected finding of longer lifespans among the exposed male rats. “This may be explained by an observed decrease in chronic kidney problems

He also noted the unexpected finding of longer lifespans among the exposed male rats. “This may be explained by an observed decrease in chronic kidney problems that are often the cause of death in older rats,” said Wyde. Looking at the source data in the report for male rate survival probability (https://ntp.niehs.nih.gov/ntp/htdocs/lt_rpts/tr595_508.pdf https://ntp.niehs.nih.gov/ntp/htdocs/lt_rpts/tr595_508.pdf, page 66), the increase in survival appears to be dose-dependent. In other words, the higher the dose of radiation, the better the survival rate after 76 weeks. This is what I'd expect to see for a real effect as opposed to an artifact. The lack of error bars makes it hard to give significance to the finding, though. The female group (second graph on page) show no clear difference. Looking at the table on the preceding page, the main effect appears to be reduction of moribund state. There might be multiple explanations here. For example, an animal that can't move can't feed itself properly and so will succumb to malnutrition sooner.