Brain Transplants

This piece, by Onno Berkan, was published on 04/15/25. The original text, by Balaban et al., was published by the International Journal of Developmental Biology on 05/01/2005.

This McGill study attempted to understand the neural circuits behind the crowing behavior elicited by chickens and quails by transplanting the part of the quail brain that causes that behavior onto a chicken embryo. This helped identify which part of the quail's brain causes that behavior and track its development compared to the rest of the chicks' brains.

Neural circuits are collections of neurons that work together to produce a behavior. How they're formed, however, is not super clear. Is it that populations develop connections within themselves and then connect, or do they grow together? These are hard questions to answer, mainly because it's hard to define neural circuits. The connections between neurons change over time, and even the neurons themselves may change their identities throughout one's lifetime.

Crowing is accompanied by a unique vocalization and neck movements in these birds, and the way they crow is entirely different, with the quail crow containing much more neck movement. The crow is consistent within members of both species (all chickens crow more or less the same way), suggesting a firm genetic basis. This begs the question: if the neurons that cause a quail's crow were to be transplanted into a chicken's brain, would the chicken exhibit the quail's crow?

The researchers removed parts of the quail premotor and motor control nuclei and implanted them inside chicken embryos. After trying out multiple different cuts, the researchers found regions within both brains that wholly integrated into the behavioral circuitry of the host brain. The crowing behavior appeared to be a distinct circuit, allowing it to be transplanted without affecting other behaviors.

Amazingly, all subjects exhibited quail-like crowing behavior. The chickens replicated exact quail behaviors from the timings to the movements, only while crowing. This begs the question: were those quails living within these chickens?

On the one hand, this study just produced 'hybrids', or chimeras. The quail part of these chickens' brains had no consciousness and would not have survived independently. On the other hand, these chickens now exhibit a defining characteristic of quails. What if this operation could be done with the brain's emotional regions? At what point are the chickens not quite chickens anymore?

The researchers also observed another, perhaps even more interesting effect. They found that the call-approach behavior, wherein a species-specific "maternal call" causes young birds to rush immediately to the call source, could also be transplanted. When sounds from several species were heard, the chimeras overwhelmingly favored quail sounds and ran towards them, indicating that the transplanted region may strongly affect what the subjects paid attention to. This could lead to studying how different perceptual predispositions are built into our brains.

This finding connects to an earlier finding that songbirds were likelier to learn their songs when playing songs from both their species and a different but related one. This suggests that songbirds are born with a preference for their own species' songs. This lends itself to a theory that neural processes that control attention at a young age shape development in a way that causes significant differences in adult behavior and cognition. 

This can extend to humans as well. What if humans are born with a mechanism that draws our attention towards language and speech? This would explain how humans can create and recall speech but otherwise have auditory systems similar to other primates.

Shout-out to Welela for sending over this wild paper.

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