rm are weird-looking, and scientists have
done weird things with them. Some of their
accomplishments would seem to be
biologically impossible. Here are 10 such
10 Chickens That Ejaculate
Pheasant Sperm
A male can never be sure that he is the father
of his children. (Even DNA tests sometimes
fail.) There is one thing, though, that he has
always felt that he could count on. Surely his
own sperm, ejaculated from his own penis,
must be genetically his, right? Science,
however, has taken away even that small
comfort. In the lab, it’s now possible to create
what’s called a “germ-line chimera .” Such an
animal has two kinds of cells, each with its
own DNA. The first kind of cell forms most of
the animal’s body. The other kind of cell,
located in the testicles, forms the sperm.
This is pretty easy to do in birds. First, take
sperm-forming cells from one embryo. Then,
inject them into the bloodstream of a second
embryo. As the second embryo grows, the
donated cells will find their way into the
developing testicles. Later, when the bird is
mature, it will ejaculate sperm from that first
This approach enables all kinds of weird
experiments. In one, scientists transplanted
pheasant cells into chicken testicles. When
these chickens had sex with female
pheasants, some of their offspring were
completely normal pheasants. In other words,
they were genetically unrelated to the
chicken who had ejaculated the sperm.
Scientists have also transplanted chicken
sperm into ducks via the same method. Once
they transplant duck sperm into pheasants,
everything will have come full circle, or
perhaps full triangle.
9 Collecting Semen From
Dead Kangaroos
In nature, male animals usually ejaculate into
female animals. In the lab, they can
sometimes be convinced to do it in other
places . One persuasive technique is called
electroejaculation. In it, an electrical current
is applied through the animal’s rectum. The
shock causes the animal to release semen. In
1965, scientists tried it on wild kangaroos .
First, they shot the animals. After that, there
were four steps: Lay the dead animal on its
side, extend its penis by hand, insert a probe
into its anus, 23 centimeters (9 in) deep,
and . . . zap.
With most species, you don’t need to kill the
animal first. In 1975, scientists tried it on
living kangaroos . This went very badly. First,
the sessions were traumatic for the animals.
Second, they got barely any semen. If the
scientists anesthetized the kangaroos, the
results were better. Still, it didn’t work as well
as simply killing them. Weirdly enough, dead
kangaroos give the most semen.
Another disappointing factor was that under
every condition, the semen itself wasn’t very
good. It didn’t contain many sperm cells, just
seminal plasma, which is the sticky fluid that
surrounds sperm. By studying the plasma,
though, the scientists did verify an interesting
fact : The sugar profile of kangaroo semen is
different than that of more distantly related
mammals . To put that another way, kangaroo
semen probably tastes different than human
semen, though no one (we can only hope) has
ever verified that directly.
There was a happy ending to all of this
kangaroo zapping. In 1997, scientists finally
managed to get good amounts of sperm cells
by electroejaculating kangaroos that had only
been anesthetized. It was a win-win—lots of
kangaroo sperm and no dead kangaroos.
8 Goat Sperm In A Hamster
Egg (Via Pig Uterus)
Goat-hamster hybrids are tough to make. Even
the first step is a struggle. If you mix together
goat sperm and hamster eggs, not much
happens. Even stripping off the egg’s outer
layer doesn’t really help. The goat sperm still
has a hard time getting in. In 1980, though,
scientists found a solution . What really helps,
as it turns out, is to first incubate the goat
sperm inside of a pig uterus.
There is a procedure for this: First, help the
goat to ejaculate into an artificial vagina.
Collect its semen. Then, find a dead pig and
cut out a piece of its uterus. Put the sperm
inside the uterine tissue and seal both ends.
Immerse the whole thing in a saline solution
for a few hours and then flush the sperm
back out. After that, the goat sperm will be
primed, ready to fertilize hamster eggs and
make goat-hamster embryos.
There’s not much else to say. Once conceived,
the little hybrid embryos aren’t going to make
it very far. (Even goat-sheep hybrids, whose
parents are much more closely related , often
don’t do well.) But that’s another problem for
another experiment.
7 Worm Sperm Without DNA
Sperm exist to transport DNA. They start in
the testicles, and if they’re lucky, they deliver
the male’s DNA to the female’s eggs. That’s it.
It’s weird to even think about sperm without
DNA, as such a thing would seem pointless.
Scientists, however, have created just that,
using Caenorhabditis elegans, a roundworm.
In 2000, they found two mutants that could be
tricked into making DNA-less sperm, just by
raising the temperature.
Surprisingly, the lack of DNA barely mattered,
at least at the beginning. DNA-less sperm did
all of the normal things that nonmutant C.
elegans sperm do. They crawled into a sperm-
storage organ called the spermatheca. They
sought out eggs and fertilized them. Inside
each egg, a little embryo started to grow. The
one thing that DNA-less sperm can’t do is, of
course, provide DNA. Eventually, that became
critical. With only half a genome, the embryo
stopped growing while still a ball of cells.
6 Transplanting Ocelot
Sperm Into A Common Cat
The common cat is great at making babies.
They’re so great at it that populations are out
of control and we’re sterilizing them by the
millions . The cat’s wild cousins have the
opposite problem . Many species, including the
ocelot, are at risk of going extinct. In this
difference, some scientists have seen an
opportunity, namely hijacking the cat’s
reproduction. After some surgery, the cat
would go about its business—having sex,
getting pregnant, and so on. All the while,
though, it would play host to materials from
its wild cousins.
To do this, scientists have taken several
approaches. In one, they’ve implanted
embryos from wild species into a cat’s uterus.
In another, they’ve implanted sperm-forming
cells from wild species into a cat’s testicles. In
one such experiment, these sperm-forming
cells came from ocelots . To start the
experiment, the scientists blasted the cat’s
testicles with X-rays, wiping out the cat’s own
sperm. Then, they cut open the cat’s scrotum,
injected the ocelot’s cells into its testicles, and
patched it back up. Finally, after several
weeks, some of these ocelot cells became
mature sperm.
5 Monkey Cells Catch A
Virus From Rabbit Sperm
Sperm like to fuse with other cells. In a best-
case scenario, that other cell is an egg, but
sperm don’t always hold out for that.
Sometimes, any old cell will do.
Such fusions might actually be happening a
lot. After sex, when a male’s sperm are
bumping around inside his partner’s body, a
few may get a little confused. So, they’ll just
hook up with something random and leave
their DNA inside . After that, those new cells
may just sit there, perhaps forever, a
permanent part of the partner’s throat, liver,
or whatever else. Creepy. It makes condoms
sound a whole lot better, doesn’t it?
If you give sperm a little chemical nudge, they
become even less picky. In one experiment,
for example, scientists managed to get rabbit
sperm to fuse with kidney cells from an
African green monkey. Normally, monkey
kidneys have nothing to fear from rabbit
sperm. Monkeys and rabbits split ways a long
time ago, and they don’t share many diseases.
Even if a rabbit somehow ejaculated directly
onto a monkey’s kidney, the monkey probably
wouldn’t catch anything.
In this experiment, though, there was an
extra bit of weirdness. Before the fusion, the
scientists attached a monkey virus, SV40, to
the rabbit sperm. During the experiment,
many of the kidney cells caught the virus and
became sick. As strange as it might seem,
there may be a public health lesson in all of
this: Never trust rabbit sperm that’s been
handled by a scientist.
4 Is Mouse-Sheep Sex Safe?
There are many factors that might prevent a
mouse from having sex with a sheep. One
might be a fear of scrapie. Scrapie is a nasty
disease that is present in some sheep. It can
cause a loss of coordination, followed by
death. In many cases, scrapie is passed from
mother to newborn. What’s less is clear is
whether scrapie might also be passed through
sex. To find out, scientists collected semen
from scrapie-infected males .
You might think—naively—that the next step
would have involved a female sheep, but you
would be wrong. Instead, the scientists
decided to use a special strain of mice. These
mice had been engineered to contain a sheep
protein which would make them susceptible
to scrapie. Since scrapie affects the brain, the
scientists injected it directly into the mice’s
brains. The reasoning was that if the brain
wasn’t hurt by direct exposure to the semen,
it certainly wouldn’t be hurt by indirect
exposure through sex.
In the end, the mice didn’t get scrapie, surely
a great relief to any mouse considering sex
with a sheep. A few years later, scientists did
pretty much the same experiment. This time,
though, they used a different strain of
scrapie, and the results were different. After
semen injection, some of the mice were
infected. More research may be needed.
3 Frogs And Toads Are More
Than Friends
“True frogs” belong to the family Ranidae,
and “true toads” belong to the family
Bufonidae . That’s pretty clear, right? Of
course, the animals themselves don’t always
pay attention to these categories, especially
when they’re feeling amorous. In several
reports , frogs from the genus Rana have been
observed mating with toads from the genus
Bufo . Still, that’s about as far as it goes. Aside
from some disputed reports , the two
amphibians don’t seem to be able to make
babies together. To start with, Rana sperm
can’t penetrate Bufo eggs.
In the lab, however, scientists can get Rana
and Bufo to be a bit more intimate, at least on
a molecular level. In 1976, they got past the
penetration barrier by injecting the DNA-
containing sections of Rana sperm, called
nuclei, into Bufo eggs. Once inside the Bufo
egg, the Rana sperm nuclei went into action,
doing what they would usually do inside an
egg. The eggs swelled, copied their DNA, and
divided. In the end, however, no mature
Rana-Bufo hybrid resulted.
2 Zebrafish That Produce
Goldfish Sperm
In mammals, the sex of offspring is a matter
of which chromosomes are inherited. “XX”
equals female, and “XY” equals male. In
certain reptiles, it’s all about temperature . In
zebrafish, it’s very complicated. It probably
involves a bunch of genes, but the
environment is important, too. No one
completely understands it.
We know that primordial germ cells (PGCs)
are involved. The PGCs are a special set of
cells, present in the embryo, that go on to
develop into eggs or sperm. In the lab, you
can get rid of all the PGCs, either by killing
them off or by blocking a gene that’s needed
to make them. Here’s the weird part: If you
eliminate the PGCs in a zebrafish embryo, it
will develop into a sterile male that will
produce no sperm.
Getting rid of the PGCs also has another weird
effect: It turns the zebrafish into a kind of
blank slate, which can be overwritten by
transplanting PGCs from other fish. In the
most interesting experiments, the fish that
donate the cells are completely different from
the zebrafish that receive them. In one
experiment, scientists took these blank-slate
zebrafish and introduced a single PGC from a
goldfish. After these transplants, the
zebrafish’s two testicles developed very
differently. The testicle without the PGC was
thin and unnatural-looking. The testicle with
the PGC, in comparison, looked normal.
Inside, however, it was teeming with goldfish
Normally, zebrafish sperm can’t do much
with goldfish eggs. They can fertilize them,
but the resulting embryos will never hatch.
The zebrafish’s goldfish sperm didn’t have
this trouble. They fathered perfect goldfish,
which developed normally.
1 Female Sperm
Roosters make sperm. Hens don’t. That is how
it normally works, at least. In the lab, though,
this convention can be upturned. With a few
molecular tricks, you can make sperm that
started from birds that are genetically female.
There are two ways to do it. First, you can
convince a female embryo that it is male . A
little chemical persuasion, aimed at
interfering with the sex hormones, can do the
job. When the animal grows up, it will have
functional testicles that make functional
sperm. In a second approach, you can take
early germ cells from a female embryo and
transfer them to a male . Inside the male’s
testicles, these female cells will develop into
sperm, not eggs.
In birds, female sperm may be an especially
weird thing. This comes down to the way that
sex chromosomes work . Male mammals have a
Y chromosome, and females don’t. In birds,
the situation is reversed. The special
chromosome, the W chromosome, is specific
to females. The female genotype is “WZ,” and
the male genotype is “ZZ.” Under normal
circumstances, WZ sperm would never
Even in laboratory settings, a lot of WZ cells
get stuck at a late stage of development, never
becoming fully mature. A small percentage of
WZ cells do become sperm, though, and those
lucky few seem to work just fine. This has
been verified by a few rather bizarre tests
involving the eggs of other species. If injected
into quail eggs , or even into mouse eggs , WZ
cells trigger the start of an embryo, just like
normal ZZ sperm would.
It’s been suggested that female sperm might
be the start of a new world where males will
no longer be needed. Here’s what we can say
for sure, though: These experiments have
brought female chickens the closest that they
have ever come to being able to impregnate
female mice, for what that’s worth.


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