If you find it difficult to meet people you can relate to, looking into the lives of white-throated sparrows (Zonotrichia albicollis) can give you new motivation. Any individual of these birds can mate with only a quarter of the species. That’s because they develop two extra sexes in addition to the two they already have.
This genetic quirk was uncovered by biologists Elaina Tuttle and Rusty Gonser in Canada, the homeland of the white-throated sparrow. They found that the bird had a genetic mutation that caused it to flip a large portion of its chromosome, resulting in four genotypes that could only breed successfully with other specific genotypes.
“This bird pretends to have four sexes,” explains Christopher Balakrishnan, an evolutionary biologist at East Carolina University in Greenville, North Carolina, in Nature. An individual can mate with only a quarter of the population. We know of very few sexual systems with more than two sexes.”
In a very broad sense, we define animals as species that can successfully reproduce to produce viable offspring, that is, babies that can grow and have babies. Strangely, the white-throated sparrow has evolved to make this breeding process a little more difficult for itself as the species’ two chromosomes morph into different subtypes that determine which birds can successfully mate with whom.
White-throated sparrows are fairly common in North America, but come in two forms: birds with white stripes on their heads and those with tan stripes. Those with white stripes have a good “singing” ability, but they are aggressive and do not display much parenting behavior. Those with tan stripes are monogamous and good parents, but bad “singers.”
Despite their differences, white stripes mate only with tan stripes (and tan ones with white stripes).
The strange division within single species can be explained by the fact that tan stripes have two identical copies of a chromosome, but in white stripes there are several reversals where parts of the genome are effectively clipped and reversed.
Tuttle and Gonser further studied these reversals and discovered that they effectively mixed genes to create the two forms. The reversals are not unique to white-throated sparrows and are actually considered to give rise to the XX and XY chromosomes in mammals that determine mammalian sex. However, as in the case of the white-throated sparrow, it is rare for it to generate two extra chromosomes to give itself four sexes.
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