
The sparrow with four sexes - sohkamyung
http://www.nature.com/news/the-sparrow-with-four-sexes-1.21018
======
brian-armstrong
There's a diagram about halfway through that illustrates how this works. I
miunderstood the article at first and was wondering why you wouldn't just say
that there are two species, but the diagram sorted me out. The tan stripe
birds almost always mate with the white stripes, and the offspring have a
50/50 chance of having the mutation.

I wonder if the birds instinctively "know" that they have 4 sexes

~~~
vorg
> The tan stripe birds almost always mate with the white stripes

The "almost" hints at the origins of homosexuality.

~~~
astrodust
Same sex birds adopting eggs from other breeding pairs are quite common in the
avian world, especially among populations that nest in close proximity such as
penguins.

There's no "origin" to homosexuality. If anything having differentiated sexes
is an aberration, the majority of plant species have both as well as many of
the precursor species to modern mammals like snails and flatworms. In
situations where the separation of the sexes provided some kind of
evolutionary advantage it took hold. In others it never happened.

Also our preconceptions about gender goes out the window when there's four
possible expressions as in the case with these birds.

~~~
vorg
> There's no "origin" to homosexuality

Did I use the word "origin" incorrectly? Because evolution of a second sexual
system in these birds results in white-striped birds and tan-striped ones
mating only with the other _almost all_ the time instead of _always all_ the
time, homosexuality is being built into the differentiation of a species into
2 sexes, I presume because DNA processing can never be error-free. I didn't
imply it starts happening after the differentiation.

~~~
astrodust
The term doesn't even apply here since the physical expression of gender is
more complicated than the typical binary variant seen in mammals.

white/male + white/female is not "homosexual" by any conventional definition.
If anything it's "homomorphic".

Birds, like any creature, don't do anything all the time. I think you're
reading way too much into these observations.

~~~
vorg
> white/male + white/female is not "homosexual" by any conventional
> definition. If anything it's "homomorphic"

Did I use the term "homosexual" incorrectly? Perhaps "origins of
homosexuality" would be more accurate. That's what I said in the first place.

The article says "the researchers had come up with the concept that the birds
were evolving a second set of sex chromosomes" and "the inversion on
chromosome 2 doesn't include genes that determine sexual development — but it
does contain some that affect the birds' reproductive behaviour". The tan-
striped "swingers" are monogamous and fiercely protect their hatchlings from
predators, whereas the white-striped "singers" are aggressive, promiscuous,
and more cavalier about their offspring. The tan stripe birds _almost always_
mate with the white stripes, but there are exceptions. We're seeing here
firsthand (on a genetic time scale) the origins of homosexual behavior.

I appreciate your comment because when I first got the 5+ downvotes without
any comments, I thought it was homophobic attitudes from HN readers. But now I
know they could also be anyone with an overly pedantic reaction to the use of
certain words.

~~~
astrodust
> We're seeing here firsthand (on a genetic time scale) the origins of
> homosexual behavior.

Again, nope. I think you're taking a lot of flack here for implying that
homosexuality, or homosexual behaviour is somehow unnatural and needed to
evolve. That it's an aberration of some sort.

I'm asserting it has always been there, that it pre-dates the very concept of
genders. There's an evolutionary bias _in some species_ towards heterosexual
relationships, but in others there's no such pressure. Aphids don't need to
reproduce sexually, but they do if they feel it's a better deal. Snails
possess both genders and breeding gets complicated.

The only thing we're witnessing the start of here is the introduction of new
arrangements of genes that behave in a sex-like way. If there was a species
where a sex gene was being evolved away there would be just as much scientific
interest.

~~~
vorg
> I think you're taking a lot of flack here for implying that homosexuality,
> or homosexual behaviour is somehow unnatural and needed to evolve

First you dispute my use of "origin", then you dispute my use of "homosexual".
Now are you saying my use of "origin of homosexuality" to explain the _almost_
in "A _almost_ always mates with B" implies I think homosexuality is
"unnatural"? You have an overly pedantic reaction to my use of certain words
if you're inferring the opinion words "unnatural" and "aberration" from my
first one-line comment.

~~~
astrodust
I have no idea how that could be construed any other way. Whatever you're
trying to say, you're saying it in the most confusing way possible.

There's nothing even remotely "homosexual" about your "A mates with B"
statement. Horses _almost always_ mate with other horses. Sometimes they mate
with donkeys. That does not make the act homosexual in the least.

There's no origin, genesis, beginning, or start of anything homosexual with
this addition of new sex genes. It just broadens the spectrum of what gender
actually is.

~~~
vorg
> I have no idea how that could be construed any other way. Whatever you're
> trying to say, you're saying it in the most confusing way possible

I think my clarifying comments above are clear enough, certainly not "the most
confusing way possible". Your comments each argue against something I'm not
saying or implying.

If you have no idea how my original comment couldn't be construed as anything
other than implying "homosexuality is somehow unnatural and an aberration",
then I suggest perhaps you're bringing something from your personal situation
into your reading process. I wrote the comment immediately after reading the
entire article.

~~~
astrodust
You asked why you were being down-voted, so I explained how your comment was
perceived.

You can either put down the shovel or keep digging.

------
throwaway4891a
"Four sexes" sounds click-baitier than "two morphs." It's not like they have
unique procreation, other than effectively two subspecies whom cannot
interbreed (they both have males and females respectively). It's interesting a
species can diverge with a large mutation and split the gene pool suddenly.
More intriguing is how enough of a population could grow from the first
individual and thrive.

EDIT: gotta make you wonder if there were a large forest fire, volcano, human
hunters or other event that almost wipe them out, and that the survivors went
through a population bottleneck, perhaps picking up the mutation along the
way.

~~~
colanderman
No, it's very different than two subspecies. The males of one morph almost
exclusively mate with females of the _other_ morph, and produce offspring
which are one of the four sexes in roughly equal proportion.

~~~
kseistrup
Do we know that they “almost exclusively mate with females of the other morph”
or is it just because same-morph matings are infertile?

~~~
matt_morgan
The article isn't explicit on that, but they did observe the birds for 30
years, so maybe it's in the paper.

~~~
qb45
The paper says 1.5% of pairs were indeed between the same morphs. No mention
of infertility, but white/white pairs produce 1/4 of chicks with both 2nd
chromosomes mutated and the only male chick of this kind they found was 50%
underweight runt, though two such females grew up as normal white adults.

------
swsieber
The tl;dr:

The species has two chromosomes that act like sex chromosomes. That is to say,
our Y chromosome never get's to mingle with other Y chromosomes, and so it
accumulates mutations fairly quick, and thus a sex chromosome. In the bird
species, there are 2 such chromosomes, and thus 4 unique states with regards
to those unique chromosomes, as opposed to our 2 states.

They are defining sex not through sexual organs, but through the chromosomal
state.

~~~
astrodust
It's a very interesting combination of factors here, and they're also making
the observation that this could be how the original XX/XY split occurred.

Sexual genetics are actually a lot more complicated than people think. There
are YY "supermale" people, though it's more common in fish which gender-flip
depending on population balance leading to XY males breeding with XY females.
A YY male will always produce male children.

There's also XXY, XXXY and so on which is where things get even more
complicated.

------
yareally
Quite a few bird species and some other animals[2] can have chromosomes of
both sexes[1]. When it happens to northern cardinals, they end up split down
the middle in color--one half gray like a female and one half red like a
male[3].

[1] [http://www.bbc.com/earth/story/20150916-these-animals-are-
ma...](http://www.bbc.com/earth/story/20150916-these-animals-are-male-on-one-
side-and-female-on-the-other)

[2]
[https://en.wikipedia.org/wiki/Gynandromorphism](https://en.wikipedia.org/wiki/Gynandromorphism)

[3]
[http://www.remotesensingart.com/CardinalMF/index.html](http://www.remotesensingart.com/CardinalMF/index.html)

------
woliveirajr
> “This bird acts like it has four sexes," (...)

Interesting to see that one phrase was changed to make the title more
interesting, by nature.com.

~~~
unclenoriega
Sadly, that's the nature of journalism.

~~~
erroneousfunk
Were you just saying that for the pun?

Regardless, I wanted to point out that the title, while interesting, is hardly
"click bait and switch." These birds DO have four sexes, from the point of
view of their chromosomes, which results in behavioral differences between the
four sexes, and produces birds that can only mate with a quarter of the entire
population.

The biologist that said the "these birds act like..." quote was not the author
of the paper being discussed. Also, later on he says "...in a four-sex system,
each bird must work that much harder to find a mate — a white female is not
just looking for a male, she needs a tan one — so selection will favour a more
advantageous two-sex system instead."

So which is it? "A four-sex system" or something that "looks like a four sex
system"? I think the article makes it pretty clear that it's the former.

Also, from the article: "Some animals, such as reptiles, have two sexes and no
sex chromosomes, whereas the freshwater protozoan Tetrahymena thermophila has
seven sexes, each of which can mate with any type except its own. Two sexes
with one set of sex chromosomes is the most common arrangement, and has
evolved independently many times. But there's no reason a species can't have
more sex chromosomes, Wilson Sayres says. “If you've got two genes linked
together that can't cross over during meiosis, and one of them plays a role in
sex determination, suddenly you can have a new sex chromosome.”"

How is the title incorrect?

~~~
woliveirajr
Well, I would think that having a 4-sex-system would lead to all possible
combinations of those. Let's say there are A, B, C and D individuals. You
would have a breed from A x B, A x C, A x D, B x C, B x D and C x D.

But what I got is that there males and females. But some of them have a
different chromosome, to that it's transmitted for their little cute birdies.
Let's call it _n_ and _m_ versions, and keep the males (Y) and females (X)

So you have Ym, Yn, Xm and Xn. You just have Ym x Xm or Ym x Xn. OR Yn x Xm or
Yn x Xn. You don't have Ym x Yn. Nor Xm x Xn. You just have 4 possible
combinations, not the whole 6 combinations if there really were 4 different
sex.

Perhaps I'm missing something from the article, from the paper, or from
biology, but since those special mutation from the chromosome 2 doesn't
generate infertile offspring, I simply understand it as "birds with a white
hair prefer those with red hair, instead of opposite-sex with the same hair
color, generating a offspring that will have white or red hair with the same
probability."

~~~
erroneousfunk
That's not what the paper described at all. Even the article explicitly said
that the birds could only mate with 25% of the population -- the "opposite
sex" along two dimensions, instead of just one.

Those "special mutations" on chromosome 2 result in a chromosome that cannot
recombine like a normal chromosome -- a new separate sex chromosome.

------
Pitarou
There's something I don't get.

Can someone explain to me what prevents a tan-striped male mating with a tan-
striped female?

------
Ericson2314
I wonder if they could recreate the original single inversion, and check the
fertility of those.

------
countryqt30
Maybe this is just a "correlation vs. causation" error?

Think about humans. For Chinese males, it is very likely to mate up with
Chinese girls. That's not just a matter of availability (in China, there live
mostly Chinese-looking people o_o), but also a phenomenon overseas, I believe.
You have a lot more in common, similar values and similar cultural
perceptions.

PS: This case shall not be construed to address Chinese especially, I'm
convinced it works the same with all the similar ethnic groups and even
subgroups.

~~~
patall
What shall be the "correlation vs. causation" error here? I mean, if you draw
an analog here, it would mean that Chinese people could get children of any
race but those would still pair up. The birds do not know which type they are
but still pair up with the different morph, so its something intrinsic
inherited from the genotype that influences the sexual phenotype.

~~~
probablybroken
Or possibly that birds value reproductive success, and learn to mate with
partners who can produce eggs from experience.

