The Origins of Endocrinology: How Roosters and Hormones Shaped Medical Science

In 1849, German physiologist Arnold Adolph Berthold made a groundbreaking discovery in the field of endocrinology. He conducted a unique experiment by castrating roosters and observing their physical and behavioral changes. While his results were revolutionary for science, the concept itself wasn’t new.

Historical records show that castration of male animals dates back to 2000 BCE, where farm animals were neutered to make them more docile for work. Even in ancient Rome, emperors like Constantine (4th century) surrounded themselves with eunuchs, as castrated men were seen as less aggressive and more trustworthy.

Berthold’s Experiment: Linking Gonads to Behavior

At the University of Göttingen in Germany, Berthold conducted controlled experiments on young male chickens:

• He removed their testicles before maturity, which resulted in stunted growth, lack of crowing, and reduced aggression—traits typical of roosters.
• When he castrated adult roosters, they stopped fighting, lost their sex drive, and no longer crowed.
• However, when Berthold reinserted the testicles into the body cavity, the roosters regained their natural behaviors.

This experiment proved that the gonads (testes) played a crucial role in the development of secondary sex characteristics—a finding that laid the foundation for the field of hormonal studies.

Charles-Édouard Brown-Séquard: The Search for the "Elixir of Youth"

Nearly 40 years later, the field of endocrinology took another step forward when Charles-Édouard Brown-Séquard, a Mauritian-born physiologist, expanded on Berthold’s findings.

Brown-Séquard was already a well-respected neurologist and physiologist, with teaching positions in London, Paris, and Harvard University (Cambridge, MA). His research on the spinal cord led him to propose that the body produces chemical messengers that travel through the bloodstream to regulate bodily functions—a concept later confirmed with the discovery of hormones.

The "Rejuvenation" Experiment: Placebo or Breakthrough?

In 1889, Brown-Séquard published a controversial paper in The Lancet, a leading medical journal. He claimed that by injecting himself with testicular extracts from dogs and guinea pigs, he experienced:

• Increased energy levels
• Enhanced cognitive function
• Physical rejuvenation, feeling decades younger despite being 72 years old

However, modern science later revealed that his perceived improvements were due to the placebo effect. Despite this, Brown-Séquard’s experiments sparked interest in hormone therapy, paving the way for future discoveries in testosterone and other vital hormones.

Legacy: The Birth of Endocrinology

Berthold’s work proved that glands regulate bodily functions, while Brown-Séquard’s bold experiments pushed scientists toward hormone therapy research.

Interestingly, Brown-Séquard was a neighbor of Robert Louis Stevenson, the famous author of Dr. Jekyll and Mr. Hyde. Many believe that Brown-Séquard’s obsession with youth and transformation inspired Stevenson’s legendary novel.

How Roosters and Hormones Shaped Modern Medicine

From rooster castration to early hormone injections, the pioneering work of Berthold and Brown-Séquard set the stage for modern endocrinology. Today, hormone therapy, testosterone treatments, and medical endocrinology continue to shape healthcare and anti-aging research worldwide.

The British artist Francis Smith (1722–1822) painted Kisler Aga, Chief of the Black Eunuchs and First Keeper of the Serraglio between 1763 and 1779. The word eunuch typically refers to a castrated man who accordingly has little testosterone. In ancient times, eunuchism was often practiced to render slaves less aggressive and more servile.