Female birds need a ‘male hormone’ for sexual development 

Why does the rooster crow while the hen remains silent? This intriguing question led scientist Arnold Adolph Berthold, in the mid-19th century, to castrate a rooster. The result was telling: the rooster’s morning crow ceased. 

While Berthold did not know the exact substance responsible, we now understand that the sex hormone testosterone plays a key role in such male-specific behaviors.

Testosterone and the androgen receptor

Although testosterone is present in females and has important functions, it is classically referred to as the “male hormone.” It significantly contributes to sexual development, appearance, and aggressive behavior in males. 

To exert its function, testosterone binds to the so-called androgen receptor. This binding activates the receptor and triggers the production of certain proteins within the cell. 

However, testosterone can also be metabolized into estrogen – the “female hormone” – which binds to a different receptor. This complexity raises the question: when we talk about testosterone and its effects, what is the role of the androgen signaling pathway?

Hormone signaling in birds

A team of scientists led by Benjamin Schusser (Technical University of Munich) and Manfred Gahr (Max Planck Institute for Biological Intelligence) delved deeper into this question using birds as a model. 

The researchers noted that, unlike mammals, avian cells possess cell autonomous sex identity, allowing somatic cells to develop sexual traits independent of hormonal influence due to the absence of sex chromosome dosage compensation. 

“This unique characteristic has significant implications, potentially leading to the manifestation of sexual traits differing from gonadal sex expectations,” wrote the study authors. 

The experts pointed out that male chickens with experimentally reversed gonads develop complete sexual ornaments typical of roosters, challenging the notion that these traits are driven by testicular hormones. 

“Our study aims to unravel the intricate interplay between hormonal and cell autonomous mechanisms in avian sexual development by investigating the role of androgen signaling through androgen receptor knockout in chickens.”

Focus of the research

Using the CRISPR-Cas method, the researchers created genetically modified chickens lacking the androgen receptor. This allowed them to study, for the first time, the effects of androgen signaling on the development, appearance, and behavior of chickens.

Chickens were particularly well-suited for this study. They are intelligent and social animals exhibiting gender-specific behaviors, such as roosters crowing in the morning. This made them ideal subjects to observe the consequences of disrupting the androgen signaling pathway.

Surprising outcomes in modified chickens

As expected, examinations of the young roosters showed they were infertile. Additionally, some typical external sexual characteristics were underdeveloped, including the comb, wattle, and earlaps. 

Interestingly, other traits remained unaffected by the genetic modification: the tail feathers and the spurs were comparable to normal roosters.

“We were surprised to find that the male traits were only partially lost. The outer appearance of roosters is therefore not solely determined by the androgen signaling pathway,” explained Mekhla Rudra, a co-lead author of the study.

Young hens without the androgen receptor showed a very similar pattern. They were also infertile, and their typical head ornaments were much smaller than normal. 

Consequently, young roosters and hens were almost indistinguishable in appearance – quite unlike chickens that carry the androgen receptor.

The role of hormones in birds

Surprisingly, although adult females continued to produce testosterone, without the androgen receptors they never laid eggs nor ovulated, showing that egg formation and laying are androgen-dependent.

The results demonstrate that testosterone plays an important role in both sexes. It is therefore too simplistic to describe it as a pure male hormone. The way the hormone works is complex and not yet fully understood.

Complex interplay in sexual development

The study provides general insights into the sexual development of birds, revealing a complex interplay of hormone-dependent and hormone-independent mechanisms. 

By understanding these processes, scientists can better comprehend how hormones influence development and behavior across different species.

“These findings highlight the significance of androgen receptor mechanisms in fertility and sex-specific traits in chickens, challenging the concept of a default sex in birds and emphasizing the dominance of androgen signaling in avian sexual development,” the authors concluded.

The research is published in the journal Nature Communications.

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