I’ve always been fascinated by the oddities and randomness of nature. I’m right in my element when I come across weird chicken stuff and am happy to post about different kinds of twins.
Cephalopagus: Two faces on opposite sides of a single, conjoined head; the upper portion of the body is fused while the bottom portions are separate. Also known as Janus, who in ancient Rome was the god of beginnings, gates, transitions, time, duality, doorways, passages and endings. He is usually depicted as having two faces, since he looks both to the future and the past.
These twins rarely hatch and generally cannot survive due to severe malformations of the brain.
Diprosopus also known as craniofacial duplication is an extremely rare congenital disorder where parts of, or all of, the face are duplicated on the head.
Although usually considered conjoined twinning, diprosopus is not normally due to the fusion or incomplete separation of two embryos, but the result of abnormal activity by the protein SHH (sonic hedgehog).
SHH and its corresponding gene play an important role in craniofacial embryonic development. Too much of it creates wide facial features and duplication of facial structures. The greater the widening, the more structures are duplicated. Lab tests that have introduced SHH protein pellets into chicken embryos result in chicks with duplicate beaks. Inadequate amounts of SHH conversely lead to cyclopia or insufficiently developed facial features.
Conjoined Twins are identical (monozygotic) twins that didn’t fully separate from one another and are still partially connected due to the incomplete division of one fertilized ovum.
Parasitic Twins occur when a twin embryo begins developing in utero, but the pair does not fully separate, and one embryo maintains dominant development at the expense of its twin. Unlike conjoined twins, one ceases development during gestation and is vestigial to a mostly fully formed, otherwise healthy individual twin. The undeveloped twin is defined as parasitic, rather than conjoined, because it is incompletely formed or wholly dependent on the body functions of the complete fetus.
This is a chick hatched by a broody hen. It’s connected to its partially formed parasitic twin via the umbilical cord, which is the latter’s source of nutrients and blood supply. Once hatched, the umbilical cord dried and the parasitic twin died and was easily removed by clipping the remnants of the cord that attached the twins together.
Vestigial Twin: When the first image came up in my newsfeed I was so tired it took me a moment to realize what was wrong (i.e. chicks don’t have four legs!).
It’s a form of parasitic twin in which one is partially absorbed by the other. The parasitic twin is usually so malformed that it usually only consists of legs and organs.
Twins: And then there are plain old twins. Double yolk eggs are created when a hen’s oviduct releases two yolks in quick succession. They travel along together and are eventually housed within the same shell. Both ova must be fertilized in order to become viable. If they do develop most often only one, or neither, make it to hatch due to lack or room and oxygen within the shell. As you can see here they often require assistance to hatch.