Real Monsters – The Cockatrice


In medieval bestiaries, entries portrayed the cockatrice as a rooster with the tail of a snake. It’s said to hatch from the egg of a cockerel that’s been brooded by a snake or toad. Often interchanged with the basilisk, only a weasel (or Harry Potter) can kill it.


Wow. Crazy, right?  Maybe not so much.

Could poultry grow a snake-ish tail? Considering the evolutionary history of birds and embryonic development, yes. Birds are the last surviving descendants of dinosaurs, and early birds (evolutionarily speaking, not the worm-getting kind) had long tails. Over time, shorter-tailed members of the group prevailed as they flew unimpeded, and with better control, among brush and between branches. Vertebrae that once grew, fused as random genetic mutations delivered more successful flight. Genetically, just a few genes (possibly just one), activated early in development, control vertebral growth and tail elongation (1). So if you can stop that gene from turning on, you might just get a tail.

The second aspect of the cockatrice/basilisk story involved being born from a rooster’s egg. Now, that can’t be true – only hens lay eggs. Well, just like in humans, sex can be confusing. Primary sexual characteristics include sexual organs. In order to lay an egg, the bird would need an ovary. But most people identify hens and roosters by secondary sexual characteristics like coloration, size, and the presence of a comb and wattle. These traits develop because of hormones. If a female bird receives a dose of male hormone, she can produce rooster-like traits. This situation is actually not so rare. Because birds only use one gonad, a disease on the ovary would cause the other to develop. Since testes and ovaries develop from the same structure, the hen could get a dose of male hormone, producing rooster-like traits, and still lay an egg (2).

Brooding by a snake or toad wouldn’t help a chicken egg hatch. However, it’s not unusual to find a snake in a nest. Snakes of course eat eggs, and some birds even use snakes as partners to protect their nests from parasites or rodents.

As for death-by-weasel, the cockatrice/basilisk story is probably getting mixed with a cobra (3). The mongoose, which looks similar to a weasel, has an amazing cellular adaptation that makes them resistant to cobra venom. Many Greek and Egyptian stories (as well as the modern tale of Rikki-Tikki-Tavi) glorify the snake-killing abilities of the mongoose.

So next time you hear of a mystical beast that couldn’t possibly be real, give it a chance. And if you’d like to read more about the cockatrice and its devilish ties, visit awingandaway.


  1. Rashid DJ, Chapman SC, Larsson HCE, Organ CL, Bebin A-G, Merzdorf CS, Bradley R, and Horner JR 2014. From dinosaurs to birds: a tail of evolution. EvoDevo 2(25)
  2. Jacob J and Mather FB. 2000. Sex reversal in chickens. UFL-IFAS. FactSheet PS-53.
  3. Alexander, RM 1963. The evolution of the basilisk. Greece & Rome. 10(2) 170-181.