From the moment he arrived at Auschwitz as a medical researcher and doctor in 1943, Dr. Josef Mengele relished the "selection" process that occurred when transports of prisoners arrived at the camp. If he was on duty, Mengele would methodically sort long lines of new arrivals to the left or right. If you were sent to the right, you would be allowed to enter the camp and survive, at least in the short-term; the left meant extermination in the gas chambers within a matter of hours.
Even when he was off-duty, Mengele was enthusiastically present on the railroad platform as the trains rolled in, there to ferret out twins and other people who appeared medically anomalous to him, like dwarves or giants. Twins were an obsession of Mengele's, and while they were not the only group of prisoners he grossly exploited, they were his most prized. The twins under Mengele had a special status and regimen. If Mengele deemed them "valuable" for his research, he allowed them to keep their clothes and hair, and if they were not being subjected to experimentation, he allowed the twins access to rudimentary education and to play soccer.
However, the twins routinely had their blood drawn, with especially small children having blood extracted painfully from their necks. Physical dimensions were diligently examined, measured, and recorded. If the poking, prodding, and measuring didn't prove fruitful for him, Mengele would decide that a particular set of twins was not tall enough or physically fit enough and send them to the gas chamber.
Mengele was also obsessed with blue eyes. Children received extremely painful eye drops or injections of various substances to artificially change their eye color that proved both ineffectual and toxic. Injections of tuberculosis, typhus, and other infectious diseases were administered to one twin and not the other, for example. Eventually, both would be euthanized with injections to the heart with phenolic acid, and their cadavers would be meticulously autopsied. Spinal taps and spinal experimentation were routinely administered without anesthesia. Other complicated surgeries involving amputation, removal of organs, and even castration were also performed without anesthesia.
One survivor described the process enacted upon his brother by Mengele:
One day, my twin brother, Tibi, was taken away for some special experiments. Dr. Mengele had always been more interested in Tibi. I am not sure why - perhaps because he was the older twin. Mengele made several operations on Tibi. One surgery on his spine left my brother paralyzed. He could not walk anymore. Then they took out his sexual organs. After the fourth operation, I did not see Tibi anymore. I cannot tell you how I felt. It is impossible to put into words how I felt. They had taken away my father, my mother, my two older brothers - and now, my twin.
Adding to the diabolical mystique surrounding Josef Mengele, despite his stature as one of the most wanted criminals of the 20th century, he evaded capture and died of natural causes in Brazil in 1979.