The year is 1849. The setting is Göttingen, Germany. A physician and anatomist named Arnold Adolph Berthold is staring at four roosters, a scalpel, and a very specific scientific hunch.
What he’s about to do — with four birds, a simple observation, and four pages of notes — will eventually become the founding act of modern endocrinology. It will explain the basis of hormones. It will, eventually, explain testosterone. It will, eventually, explain why men act the way they do.
He just doesn’t know any of that yet. He’s a guy with roosters.
What Berthold Actually Did
Berthold’s experiment was strikingly simple. He took four roosters. He castrated all of them. Then he split them into two groups:
- Two roosters he left alone — no testes, full stop.
- Two roosters received an ectopic transplant — one testis from another bird, re-implanted into a random part of their abdominal cavity, completely disconnected from its original nerves.
Then he watched. For weeks, he just… watched.
The results were hard to argue with. The two castrated roosters who got no transplant became what you’d expect: quiet, docile, and indifferent to hens. Classic hypogonadism, though that word didn’t exist yet. They lost their crow, they lost their fight, they lost their drive. Classic castration effects.
The two roosters with the transplanted testes? They crowed lustily. They picked fights. They showed, as Berthold put it in the dry language of 19th-century German science, "a normal inclination to hens."
The kicker: the transplanted testes had no nerve connection to anything. No original neural pathways. No direct physical connection to the brain or reproductive organs. Berthold reasoned that whatever was making those roosters cocky again — literally — had to be traveling through the blood. The testes were secreting something into the bloodstream. That something was acting on the whole organism.
He had, in four pages and four roosters, just described the concept of internal secretion. The concept that would become the foundation of endocrinology.
Why Nobody Noticed for 50 Years
Here’s the frustrating part: nobody cared.
Berthold’s rival at the University of Göttingen, a professor named Rudolf Wagner, tried to replicate the experiment and failed — apparently because he didn’t wait long enough to observe the effects. Rather than question his methodology, Wagner declared Berthold’s findings “rubbish.”
Wagner then became the full professor of physiology. His opinion prevailed. Berthold’s paper fell into complete obscurity for half a century.
This is science. One jealous senior faculty member can erase a generation of progress.
Berthold himself was too reserved to fight for his work. He published his four-page masterpiece in 1849 and moved on. He died in 1861, never knowing his experiment would eventually be called the founding moment of endocrinology.
The Idea That Eventually Came Back
It wasn’t until the early twentieth century that researchers started repeating Berthold’s experiment and confirming what he’d found. Moritz Nussbaum confirmed it in frogs. Eugen Steinach confirmed it in rats. A researcher named Pézard confirmed it in cocks — the actual bird kind — completing the full circle back to where Berthold started.
By then the concept was undeniable: the testes produced a chemical substance, that substance traveled through the blood, and that substance had systemic effects on the entire organism. They called the theoretical substance “Androkinin” — the hypothetical male hormone — before they knew what it actually was.
What Berthold had done was not just describe testosterone. He had described the concept of a hormone itself. The idea that glands talk to distant organs through the bloodstream — that’s endocrinology. That’s what Berthold gave the world, with four roosters and a scalpel, in 1849.
What This Means for Modern TRT
When you understand what Berthold discovered, the logic of testosterone replacement therapy becomes obvious. The testes produce a substance that travels through blood and governs physiology. When testes produce less of that substance — through age, disease, injury, or other causes — the effects are systemic. Energy, muscle, mood, libido, bone density: all affected.
Berthold’s roosters told us this before we had the vocabulary. The biology was clear. The treatment logic was clear. It just took science almost another 90 years to get the actual molecule right.
The rooster crowed. Nobody listened. Eventually, the world caught up.
What’s Next
In the next post, we’ll meet the man who took Berthold’s concept, threw scientific caution completely out the window, and injected himself with dog testicle juice at the age of 72 — then reported miraculous results to the Lancet.
Science has always been a little messy. But this was something special.
