PermeateFree said:
mollwollfumble said:
Bubblecar said:
Is there any structural similarity at all between a glycogen body and a brain?
If not, why call it a brain?
A brain has four components: nerve cells, glial cells (cells that support the nerve cells), a fuel source and (in vertebrates) a balance organ.
A glycogen body in a chicken has all four components: nerve cells, glial cells, a fuel source and a balance organ.
The difference is that in a chicken’s glycogen body the nerve cells are not arranged in the same way as in a brain.
Balance organs in particular are very heavily conserved across hundreds of millions of years. A shark’s balance organ is almost indistinguishable from a human’s, as opposed to other organs such as the testes – a shark’s testes reside in its neck.
That makes me suspect that a glycogen body in a chicken is a vestigial brain. In the same way that the human appendix is a vestigial organ. That the glycogen body has devolved from a second brain in dinosaurs. It is still capable of making a headless chicken run, but that is not much of a useful survival trait for chickens. For larger animals with small brains in their head, such as some dinosaurs, it would be very much a survival trait.
The chicken that lived for 18 months without a head
Seventy years ago, a farmer beheaded a chicken in Colorado, and it refused to die. Mike, as the bird became known, survived for 18 months and became famous. But how did he live without a head for so long, asks Chris Stokel-Walker.
On 10 September 1945 Lloyd Olsen and his wife Clara were killing chickens, on their farm in Fruita, Colorado. Olsen would decapitate the birds, his wife would clean them up. But one of the 40 or 50 animals that went under Olsen’s hatchet that day didn’t behave like the rest.
“They got down to the end and had one who was still alive, up and walking around,” says the couple’s great-grandson, Troy Waters, himself a farmer in Fruita. The chicken kicked and ran, and didn’t stop.
It was placed in an old apple box on the farm’s screened porch for the night, and when Lloyd Olsen woke the following morning, he stepped outside to see what had happened. “The damn thing was still alive,” says Waters.
More:
https://www.bbc.com/news/magazine-34198390
Thanks. :-)
This was briefly mentioned on QI, but I hadn’t seen details. The following is a summary of my thoughts.
Dinosaurs, one brain or two?
The possibility that dinosaurs may have had two brains was first proposed by Marsh in 1877 in his description of the Stegosaurus skeleton. The stegosaurus has only a tiny brain in its head. Marsh noted that Stegosaurus has an expanded opening on the spinal column in the pelvis over 20 times as large as the brain in the skull. Since that time, this opening in the spinal column has been found in many other dinosaurs, including sauropods.
This swelling of the neural system is evolutionarily related to the glycogen body that is found in living birds and is largest in chickens and pigeons. The glycogen body is not a functioning brain, and so it is usually claimed that this means that dinosaurs only had one brain. On looking briefly at the evidence, I’ve come to the exact opposite conclusion, that at least some dinosaurs had two brains.
What is the glycogen body in living birds? On one website it is said that the glycogen body is an obvious anatomical feature with no known function. To me, it’s function is clear, it’s a vestigial brain. It’s a devolved brain, an organ that used to be a brain but no longer functions as one. The components present in a brain are there, but the nerve cells are not connected in a way that would make it a functioning brain.
A glycogen body contains nerve cells. It contains glial cells (brain cells that support nerve cells but don’t carry signals). It contains an energy source (glycogen). And in the chicken it also carries a balance organ like the balance organ associated with the brain in the head.
I want to say a few words about the balance organ here. The balance organ in the head is very strongly conserved over four hundred million years of evolution. The balance organ in the shark is almost indistinguishable from the balance organ in the human. By way of contrast, the shark keeps its testes in its neck. So I think that the balance organ in the chicken pelvis is inherited from a very long time ago.
A chicken that is decapitated can stand up and run on flat ground (as in “running around like a headless chicken”) using the balance orgin in the pelvis to maintain balance. No decapitated mammal can do this. Among extant animals it’s specifically an avian thing.
OK, the glycogen body in the chicken is a vestigial non-functioning brain, much the same as humans have a vestigial non-functioning appendix. In mammals this means that the direct ancestor of the human once had a functioning appendix, and that in some other mammals (the rabbit is the most commonly quoted example) the appendix serves a vital function.
So the ancestors of the chicken, the early dinosaurs, had a functioning second brain, and in some other dinosaurs such as the Stegosaurus that second brain had a vital function.
What is that vital function? That’s an easy question to answer. Nerve signals are slow. We know this time delay as “reaction time” when driving, the time required for signals to pass through the brain to the limbs. Reflex loops are faster, because they don’t have to travel all the way to the brain and back. The larger the animal, the slower the reaction time. For a large prey animal, such as a Stegosaurus, the difference between a reaction time of one second and a reaction time of a quarter of a second can be the difference between life and death. By having a second brain in the pelvis that controls running, swerving, tail movement, balance, pain and touch, the Stegosaurus would reduce reaction time by a factor of three or four. The brain in the head would still operate sight, hearing, smell and taste, but not running.
In case you’ve ever wondered how some dinosaurs managed with such a small brain, well, here is one answer, it wasn’t their only brain.
In addition, a sudden vertical motion of the brain in the head can cause blood to rush to or from that brain, temporarily reducing its usefulness. A brain in the pelvis has no such problems as it remains a relatively constant height above the ground so would be ready for action at all times.
