Which folk tune does Gustav Mahler transform into a funeral march in Symphony No. 1?

In the third movement of his Symphony No. 1 in D Major, Gustav Mahler transforms the old French folk song "Frère Jacques" into a dark, minor-key funeral march.1

Mahler conceived this movement based on an art print that depicted forest animals escorting a hunter to his grave, using the irony of the familiar nursery tune to accompany the procession.1