In older male mice, genes play a bigger role in how long they live — by the time they reach super old age, about half the reason one lives longer than another comes down to their DNA, not their environment.

In mice with different genes, scientists found 29 specific gene spots that affect how long they live — but these genes act differently depending on whether the mouse is young or old, and whether it's male or female. Some genes only matter at certain life stages, and some even switch from helping to hurting (or vice versa) as the mouse ages.

In male mice, being heavier as a young adult is linked to a shorter life — each extra gram might take away up to 14 days. This effect is much stronger in males than in females, suggesting their genes handle weight and aging differently.