The Rooster Experiment That Invented Endocrinology

Almost a century before anyone isolated testosterone, Berthold linked the physical and behavioural changes of castration to a substance secreted by the testes.[1] He did it without a chemistry lab, without knowing the word "hormone," and without being able to name the thing he had just proved existed.

That is what makes the experiment worth telling. It is a demonstration of something invisible, run by inference alone.

What everyone already knew, and what nobody could explain

The effects of the testes had been recognised for as long as anyone had been keeping animals or castrating people. The biological consequences were known since antiquity, long before testosterone was identified as the active agent.[2]

So the observation was ancient. The mechanism was the mystery, and there were two candidate explanations:

The nerves. The testes might act through nervous connections to the rest of the body — the standard assumption of the period.

Something carried in the blood. A substance released by the organ, travelling to distant tissue.

Nobody could tell these apart by observation, because in an intact animal both are present at once.

The design that separated them

The elegance is in the transplant. Move a testis to a completely different part of the body, where its original nerve supply cannot follow, and see whether the effects survive the move.

They did. The comb and wattles developed, the behaviour returned — in animals whose transplanted tissue had no nervous connection to where it had come from. Whatever the testes were doing, they were doing it through the bloodstream.

Berthold linked the changes of castration to a secreted substance,[1] and that inference is the founding move of endocrinology as a discipline.

He was not quite first, and the sequence matters

Hunter had performed testicular transplantation experiments in 1767, while studying tissue transplantation techniques.[1] The tissue-moving was not new.

What Berthold added, almost a century later, was the interpretation — connecting the surviving effects to an internal secretion rather than treating the graft as a curiosity of surgery. The experiment was similar; the conclusion drawn from it was not.

The 86-year gap

Here is the part that should give anyone pause about the pace of science. Berthold's inference was correct, and it sat there.

It was not until 1935 that Ernst Laqueur's group in Amsterdam isolated testosterone, and Adolf Butenandt in Gdansk and Leopold Ruzicka in Zürich synthesised it.[2] Between the proof that a substance existed and the substance in a bottle: the better part of a century — an interval traced end to end in how testosterone was found.

And the gap was not empty. It was filled with treatments built on the correct premise and the wrong pharmacology — testicular extracts and transplants used on a large scale for symptoms of hypogonadism, which turn out to have had only placebo effects.[2] Berthold was right, and being right did not stop the field from spending decades on things that did not work. That story runs through the 72-year-old who injected dog testes and the monkey-gland craze.

Why this still matters clinically

Because the thing Berthold established — that the testes act on distant tissue through a substance in the blood — is the assumption underneath every testosterone measurement taken today. When a lab reports your total testosterone, it is reporting the concentration of a circulating signal, and the entire logic of that number is the logic of the rooster experiment.

What modern practice added is the part Berthold could not do: measure it. And measurement turned out to be more complicated than a single number, because much of what circulates is bound and unavailable — the subject of free vs total testosterone.

Frequently asked questions

Did Berthold discover testosterone? No. He established that the testes secrete a substance that acts at a distance.[1] The molecule itself was isolated and synthesised in 1935.[2]

Why did it take so long to isolate it? Testosterone circulates in extremely small quantities, and the analytical chemistry needed to isolate and characterise it did not exist for most of that period. The 1935 breakthroughs came from the pharmaceutical industry and academic chemistry working on the problem directly.[2]

Were the transplant treatments that followed effective? No. Testes were used in organotherapy and transplanted on a large scale as treatment for hypogonadism symptoms, and these practices had only placebo effects.[2]

Next in this series

The idea that the testes held something powerful was correct. What people did with that idea for the next fifty years was not — starting with a distinguished physiologist injecting himself in front of an audience.

Continue with the 72-year-old who injected dog testes.

References

2 sources

1. Freeman ER, Bloom DA, McGuire EJ. A brief history of testosterone. J Urol. 2001;165(2):371–373. doi:10.1097/00005392-200102000-00004 · PMID 11176375
2. Nieschlag E, Nieschlag S. ENDOCRINE HISTORY: The history of discovery, synthesis and development of testosterone for clinical use. Eur J Endocrinol. 2019;180(6):R201–R212. doi:10.1530/EJE-19-0071 · PMID 30959485