Why can eels’ heads remain active, without the body?

Question

I have heard that eels heads can survive for a long time after they have been cut off. Is this true, and if so how?

Answer

Many animals have been observed moving after death, most famously chickens. Eels’ bodies are also known to move for a long time after beheading, or otherwise dispatching of the eel. However, the discarded head also continues to make movements for some time, as if breathing, and, given the opportunity, may bite the unwary.

For humans, death by beheading was thought, for a long time, to be more-or-less instantaneous, despite various historical and pseudo-historical accounts of parted heads or bodies continuing to move or speak (Saint Denis is said to have picked up his head and walked the six miles to his burial place, preaching a sermon of repentance the whole way). The only well-known case of scientific examination of human decapitation was that of Henri Languille in 1905. He was said to have been able to react to the sound of his name and focus his attention on the attending doctor for around thirty seconds after his decapitation by guillotine, before becoming unresponsive.

Accounts of eel heads moving after removal suggest that they can remain active for a lot longer than this. In part, this is due to the greater oxygen efficiency found in cold-blooded animals, which allows the eel to function for longer on the oxygenated blood left in its head at the time of decapitation. The human brain requires ~3ml of oxygen per 100g per minute in order to function. Eels, however, have an overall oxygen requirement of ~0.2ml of oxygen per 100g per minute, although this efficiency decreases at higher temperatures. The eel’s gills are also very close behind its head, and if the decapitating blow occurs behind these, and the gills are still moist, a small amount of oxygen may diffuse into the eel’s head through these, sustaining it for even longer.

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