From the right-hand side, Dr H. E. Schaef’s
chicken looked like any normal cockerel,
with a bright red comb and a wattle. But
from the left you would think it was a hen:
its body was slighter and had plainer
Even its behaviour was decidedly confused.
The creature attempted to mount the other
hens in the yard, yet also laid small eggs
When it died, Schaef decided to prepare the
bird for his table. Once the bird had been
plucked, it was obvious that the right half of
the skeleton was much bigger than the left.
When Schaef opened the abdomen to remove
the gizzards, he found both a testis and an
ovary with a partially formed egg.
It was as if someone had cut a hen and a
cockerel in half, and merged the two bodies
seamlessly down the centre.
Keen not to waste it, Schaef proceeded to
roast and eat the chicken. But once the meat
had been stripped off the bones, he
preserved the skeleton and passed it on to
his anatomist friend Madge Thurlow
Macklin. She wrote up the story in the
Journal of Experimental Zoology in 1923 .
Today, we call these creatures “bilateral
gynandromorphs”. Unlike hermaphrodites,
whose gender-bending often begins and
ends at the genitals, these animals are split
across their whole bodies: male on one side,
female on the other.
Nearly a century after Schaef enjoyed his
strange meal, many more examples have
been found. Their odd characteristics could
explain some of the mysteries of sex, and
how our bodies develop.
Although Schaef’s account is one of the most
colourful reports, sightings of male-female
chimera date back hundreds of years.
Unsurprisingly, courtship for these
animals sometimes presents
On 7 May 1752, a Mr M Fisher of Newgate
presented the Royal Society of England with
a lobster of unique appearance , with “all the
parts of generation double”. Since then,
scientists have added crabs, silk worms,
butterflies , bees, snakes and various species
of bird to the list of animals that can develop
into bilateral gynandromorphs.
It’s impossible to say exactly how common
they are. Michael Clinton at the University
of Edinburgh in the UK estimates that 1 in
10,000 and 1 in 1,000,000 birds develop this
way. Nobody knows what the equivalent
figure would be for mammals.
Unsurprisingly, courtship for these animals
sometimes presents difficulties.
In 2008, a retired high-school teacher named
Robert Motz was looking out his back
window in Illinois when he saw a northern
cardinal whose breast was exactly half the
vibrant red of a male, half the dowdy grey of
a female. Eventually, his observation caught
the attention of ornithologist Brian Peer at
Western Illinois University in Macomb, US.
Either they are quietly shunned, or
actively attacked, by their peers
“It was an incredibly fascinating and striking
individual,” says Peer. “If you could only see
one side you would think it was male or
female. It was an almost perfect split.”
Together, they observed the bird on 40
separate occasions. Never once was it
accompanied by a mate.
Nor did it ever make an attempt to sing a
song. “Whether it was even capable of
vocalising, we just don’t know,” says Peer.
The other birds largely seemed to ignore it.
This isolation is apparently common for
gynandromorphs. Either they are quietly
shunned, or actively attacked, by their
For a long time, many assumed that the
phenomenon was down to a genetic accident
Biological gender is determined by the
combination of sex chromosomes. In
humans, men have an X and a Y
chromosome, while women have two X
chromosomes. But it works differently in
other species. In chickens, for instance, the
males have two Z chromosomes, while the
hens have a Z and a W.
Soon, the team had found another two
Crucially, a cell sometimes loses one of those
chromosomes, and that has big
consequences on the gender.
Suppose that, while a ZW chicken embryo is
developing, a single cell happens to lose the
W chromosome. That cell will be lacking the
genes that make it female, so it will develop
If that cell then replicates, all its
descendants will also be male. Meanwhile
the other cells in the embryo would still be
female – potentially leading the animal to
grow up as a gynandromorph.
At least, that was the theory. A few years
ago, Clinton received a phone call that would
cause him to reconsider this idea.
One of his colleagues had been visiting a
chicken farm, and had found a
gynandromorph that strongly resembled
Schaef’s chimera bird. “He telephoned and
asked if I was interested in getting it,” says
Clinton. “Of course, I said yes.”
The chicken was essentially formed of
two, non-identical twins, fused down
Soon, the team had found another two
gynandromorphs, all of which showed the
same, mixed characteristics.
However, when Clinton screened the
chickens’ genes, he found completely normal
sex chromosomes across the whole chicken.
Down one side they were ZW, down the
other they were ZZ.
In other words, the chicken was essentially
formed of two, non-identical twins, fused
down the centre.
That was a pretty startling result, but at first
Clinton was just disappointed at having his
idea proved wrong. “Like most scientists we
thought we knew the answer before the
experiment,” he says.
Clinton now has another idea for how
This apparent accident may actually
be a cunning evolutionary trick gone
When an egg is formed, the cell is meant to
discard half its chromosomes, in a bag of
DNA called the “polar body”. However, in
rare cases the egg may keep the polar body,
as well as its own nucleus.
If both are fertilised, and the cell starts
dividing, each side of the body will develop
with its own genome, and its own gender.
This apparent accident may actually be a
cunning evolutionary trick gone wrong.
Biologists have long known that the ratios of
males and females within a population can
switch depending on the environment.
During stressful times, mothers are more
likely to give birth to females. They tend to
be more likely to mate and pass on the
mother’s DNA, even when times are tough.
Some parrots can hatch 20 males or females
in a row , depending on the circumstances.
Now suppose one of the mother’s eggs holds
onto its polar body, and therefore has two
nuclei. If the mother allows each one to be
fertilised, she will have a half-male-half-
The mother could then somehow reject the
unwanted sex before laying the egg , neatly
controlling her offspring’s sex.
However, in the rare case that the unwanted
nucleus is not discarded, the result will be a
At the very least, Clinton’s result shows that
sex develops very differently in birds and
For mammals like us, it is the sex hormones
coursing through our blood that seem to be
most important in determining gender.
Exploring this process may be crucial
for understanding the miracle of birth
That may explain why we don’t see many
gynandromorphic mammals split down the
middle. No matter what the DNA of the cells
says, they will all be bathed in the same
hormones, and develop the same sexual
However, the fact that both sides of a bird
can develop independently, shows that it is
the bird’s cells themselves that control their
identity and growth.
This even extends to the resulting animal’s
behaviour. In one study from 2003, the right
(male) brain of a gynandromorph zebra
finch grew a thicket of neural circuits it
needed to sing courtship songs. But the left
(female) side was missing these structures,
despite the fact that both were exposed to
the same hormones .
We still don’t know whether this story
applies to every creature in this strange
In a few places, humans may have
accidentally made these creatures
Josh Jahner of the University of Nevada,
Reno studies beautiful asymmetrical
butterflies. He suspects double-fertilised
eggs may explain them, but it’s possible that
other mechanisms could contribute too.
Exploring this process may be crucial for
understanding the miracle of birth and
For example, animals’ bodies develop with
almost perfect symmetry, but how do they
manage it? Studying gynandromorphs may
hold the answer.
There is one more possible explanation for
gynandromorphs – or at least, for a few of
them. In a few places, humans may have
accidentally made these creatures more
In April 2015, Jahner reported a peculiar
coincidence. He studies American butterflies
called Lycaeides , and had never seen a
single gynandromorph before the 2011
Fukushima Daiichi nuclear disaster in Japan
– only to come across six in the 16 months
afterwards. “And I’ve never found any
since,” he says.
Researchers found a similar abundance of
gynandromorphic butterflies after the
Chernobyl disaster , suggesting that a low
dose of radiation may increase the chances
of a gynandromorph being conceived.
“There’s no way of knowing if it directly
caused it or not,” Jahner says, “but it’s a
For the moment, it is just another mystery
associated with these beautiful, almost
From the right-hand side, Dr H. E. Schaef’s