The basilisk is often called a cockatrice, for reasons we’ll come to later, but for now we’ll just use the name basilisk because that is the name which is used in the Swiss city of Basel, where a local medieval legend may give us a clue to the biological origin of the basilisk.
Let’s begin with one of several legends about the basilisk of Basel. During the summer of 1474 a cockerel laid an egg. The citizens of Basel were horrified and terrified because, they believed, the egg would hatch out as a basilisk. Europe was full of legends like this, and it is also the reason why the creature is also referred to as a cockatrice. In the Middle Ages the basilisk was seen as a creature of evil. It had the body of a serpent or reptile, bat’s wings and a cockerel’s head. Looking into its eyes could turn you to stone and it had a poisonous skin.
|One of the many basilisks to be seen in Basel|
By the year this legend is said to have taken place the basilisk had already been associated with the city for over 20 years. It was part of the city’s coat of arms supporting the shield. The legend is contained in a chronicle of Basel written in 1624 and may have been like one of those folk stories that turn into urban legends. Another local legend say that a basilisk lived in a cave in the city near where the present Gerber Fountain is located.
Legends of basilisks go back to Ancient Greece. The origin of the creature is lost in time and has become distorted and elaborated over the centuries, but perhaps there is a real biological reason why a cockerel could lay eggs. It still happens today.
There are two possible biological processes which may have led to the original legends. The first is that chickens can change sex. Instances of egg-laying hens becoming cockerels is more frequently reported than cockerels becoming hens but the process is known as spontaneous sex reversal. Reports in the media appear every year telling of farmers or chicken keepers noticing the sex reversal in their birds.
The second biological process is more bizarre. There are many reported incidents of birds, not just chickens, whose bodies are divided as if split down the middle – one half is male, the other half is female. This condition is called gynandromorphism (split the word into bits to get your tongue around it: gyn-andro-morph-ism). One half displays typical male plumage and physical characteristics while the other half displays typical female plumage.
The photo below is of an actual gynandromorph chicken. Its left half is a hen and its right half is a cockerel. The bird was part of a study undertaken by the Roslin Institute at the University of Edinburgh, a leading institute in animal research.
Both conditions are a result of the fact that the cells in birds have their own gender. It’s all about the chromosomes. Cells in humans contain X and/or Y chromosomes. Men have XY, and women have XX. The Y chromosome contains genes which switch on male gender production in a human embryo, otherwise nature’s “default” female gender production begins. Human embryos develop into a person which only have one set of chromosomes in every cell in their body, either XX or XY cells. Intersex and other genetic variations are rarer and are different to the process produced in birds.
In chickens the gender determination is produced differently. They have two chromosomes in each body cell as in we humans, but their chromosomes are called Z and W. Chickens have ZZ and ZW cells in the same body, irrespective of which gender the embryo develops and the chicken hatches out as.
That doesn’t really explain the half-and-half gynandromorphic chickens because, like all other chickens, their ZZ and ZW cells and distributed throughout the body. Think of these cells as 2 Lego bricks of different colours, red and yellow, representing ZZ and ZW. Generally it doesn’t matter what colour the brick is, you can use both coloured bricks to build a model of a chicken. The colours are mixed up and, roughly speaking, you’ve got an average orange chicken.
When the cells of one gender randomly congregate more onto one side of a chicken’s embryo the bird that hatches will have more male or female cells on one side of its body than the other. Again, if more red Lego bricks are randomly distributed on the left than on the right the completed Lego chicken will look more red on one side than the other. In real chickens enzymes will recognize this gender bias on one side and produce more gender-specific hormones on that side. The result is that half of the chicken looks like a hen and the other like a cockerel like in the photo above. That’s the simplified reason. There’s more to it than that, but I hope you get the basic idea.
There’s still a lot to learn about the way birds’ reproductive process creates either gynandromorphism or spontaneous sex reversal. What scientists know at the moment may go a little way to explain how the legends of basilisks and laying-egg cockerels began.