The energy is gathered from nectar, and the butterflies that make the trip, those born in the early fall, are able to convert nectar into fat.
Those born in September know to fill up on nectar. Their abdomens get really large. Unlike the other generations, these monarchs have a little area of fatty tissue where the sugar of the nectar is converted into fat. They can live off this cushion in winter and need only water to rehydrate their bodies.
Monarchs born in September or late August live seven, eight, or sometimes nine months. Their children, grandchildren, and great-grandchildren live just a month.
By the time you get to the great-greatgrandchildren, you’re back at September.