Fascinating Research On Testosterone Variation Within Chicks Of The Same Clutch
Dr. Hubert Schwabl, a German scientist working at the Rockefeller University Field Research Center in Millbrook, N. Y., discovered that the canary hen doles out the vital hormone testosterone in carefully tailored inequitable amounts that gradually increase from the first to the last egg laid.
While the egg is developing inside the hen, she adds, in addition to the standard accretion of protein, fat and nutrients a lacing of testosterone, the male hormone. Dr. Schwabl discovered the the hen donates the testosterone to her eggs early in their development, while they are swelling up from the follicles surrounding her ovaries but before they have been inseminated by a male's sperm. During that rapid egg production, a layer of cells is laid down around the egg yolk able to produce testosterone. Later, during incubation, as the chicks bloom within the shelter of the shell, the testosterone seeps through the yolk and enters their bloodstream.
If eggs are not removed daily as laid, the asymmetry of hatching creates a huge size disadvantage for the last chick hatched. To help level the playing field, each egg in succession is given a tailored larger amount of testosterone to create an advantage over the previous chick. In fact the amount of testosterone in the fifth egg is 20 times that given to the first egg! She does this without regard to whether the recipient is a he or she chick, but rather to lend a head start on their development.
Testosterone is well recognized by body builders and football players as a source of strength, stamina, and surliness. For canaries, testosterone increases begging, body mass, growth rate, and aggression. Chicks that fail to beg will not get fed. If they don't beg their parents will ignore them and they will die.
Chicks hatching from the last egg laid, regardless of whether they are male or female are the most aggressive! Testosterone may also play a role in the maturation of the spinal cord, allowing the chicks to coordinate their movements, lift their heads and demand their dinner.
I found this research very helpful as I always wondered why some chicks were so aggressive and how the growth rates within a clutch were so uneven. I have long known that animal protein in the diet promotes aggression but even then there was variation amongst chicks of the same clutch! I also had observed that chick vents appear somewhat male-like even when barely feathered but of course they can't be all cocks!
Lesson: Not all Canary Chicks are Created Equal, it's true, but you can help equalize the playing field too by removing the eggs as laid!
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