At the Kaiser Wilhelm Institute for Medical Research, under Ludolf von Krehl, he worked on the metabolism of warm-blooded animals.
He studied the comparative morphological and genetic differences between mainland and island forms to draw conclusions about the lizard's speciation.
Because this ability requires an inner clock to measure the time of day, Kramer sought cooperation with Jürgen Aschoff.
[1] On 19 April 1959, while trying to remove young rock pigeons from their nests in the mountains of Calabria, he fell and was killed instantly.
The obituary written by Konrad Lorenz in the Journal of Ornithology stated: "his authorized worldwide fame as the initiator of experimental analytical orientation research" should not be forgotten.