There’s a single hidden gene that explains why some female birds are absolute giants while the males stay tiny.
April 13, 2026
Original Paper
Retention of a female-specific growth hormone receptor gene correlates with reverse sexual size dimorphism in birds
bioRxiv · 10.64898/2026.04.10.717760
The Takeaway
In most species, males are larger, but some birds flip the script. Researchers found a specific growth hormone receptor tucked away on the female chromosome that acts as a genetic "smoking gun" for this rare size reversal.
From the abstract
In bird species, larger males are the most common, followed by equal-sized sexes; larger females are rarest. Although multiple forces have been proposed to select for larger female body size, the molecular mechanisms determining this trait remain uncharacterized. The avian gene for growth hormone receptor (GHR) on the Z chromosome is a known factor in sex-specific development. Male birds (ZZ) have two copies of this gene, while females (ZW) typically have one. In addition to the Z copy of GHR fo