Do you want to hear about white-throated sparrows?!
Of course you do, they’re fantastic. They come in two models, one with tan head stripes and one with white head stripes. But the gene that controls stripe color also has a bunch of other effects! It’s a supergene!
To briefly sum up a grueling amount of fieldwork by people who were probably not getting paid nearly enough, basically the tan-stripes are nurturers and the white-stripes are fighters, across both males and females. White-stripes chase away intruders more, tan-stripes bring more food to the nest. Tan-stripe females bring more bugs to their chicks than white-stripes, white-stripe females are more aggressive and sing more.
There is a reason Jordan Peterson picked lobsters, not sparrows, to get all MRA about, because the sparrow ladies are ALL about the tan-striped males. Sexy nurturing tan-stripe males are immediately grabbed up by the more aggressive white-stripe females (who are also dead sexy if you’re a sparrow.) Then the remaining birds pair off, so you get tan and white couples reproducing in virtually all cases—nurturing male with aggressive female, hyper-aggressive male with hyper-nurturing female.*
And this is good!** Because it turns out that they can have a tough time if they don’t mate across stripes—white x white sparrows often come out undersized if they come out at all. There was some cool recent genetic sequencing and one particular chromosome is way funky, inverted, and scrambled in the white-stripes. So now every white-stripe has a funky chromosome and a normal one, and every tan-stripe has two normal ones.***
This is all really unique and means that white-throated sparrows effectively have four sexes, because they now only reproduce with a member of the opposite stripe and sex chromosome, and their offspring may be any one of the four sexes. The stripes have essentially become a second sex chromosome.
The geneticists involved think the funky chromosome probably showed up as a weird import from somebody gettin’ jiggy with another sparrow species. Presumably this created a hypersexy female whose white head stripes brought all the boys to the yard, and very unusually, that bred true.
*No word on whether there is a resulting sparrow tradwife media genre.
**Leaving aside the impact on the emotional health of the non-sexy sparrows.
**A population solely of tan-stripes can reproduce safely, they’re just not that into each other.
