If you think navigating the human dating pool is hard, it’s time to get familiar with the sex lives of white-throated sparrows (Zonotrichia albicollis). For these birds, any one individual can only mate with one-quarter of the species. Why? Because they decided to evolve two extra sexes on top of the two they already had.
The curious quirk of genetics was uncovered by Elaina Tuttle and Rusty Gonser, biologists in the white-throated sparrow’s native Canada. Together, they uncovered a genetic mutation in the species that caused it to flip a large section of the bird’s chromosome, resulting in four genotypes that could only successfully procreate with other specific genotypes.
“This bird acts like it has four sexes,” Nature reports Christopher Balakrishnan, an evolutionary biologist at East Carolina University in Greenville, North Carolina, said. He was a colleague of Tuttle and Gonser. “One individual can only mate with one-quarter of the population. There are very few sexual systems with more than two sexes.”
We define species loosely as animals that can successfully reproduce to create viable offspring, meaning their babies can have babies. Strangely, the white-throated sparrow has evolved to make this a little more difficult for itself as the species’ two chromosomes have evolved into distinct subtypes that dictate which birds can successfully mate with who.
White-throated sparrows are reasonably common in North America, but there are two morphs: birds with white stripes on their heads and others with tan stripes. The white stripes (not the band) are good at singing, but aggressive and sleep around with little in the way of parental care behaviors. Meanwhile, the tan stripes are monogamous and good parents, but lousy singers.
Despite their differences, white stripes will only mate with tan stripes and vice versa. What is it they say about opposites attract?
The strange division within the singular species can be explained by the fact that tan stripes have two identical copies of a chromosome, but in white stripes, there are several inversions where sections of the genome have effectively been snipped and reversed.
Tuttle and Gonser studied these inversions further and found that they effectively jumbled the genes to create the two morphs. Inversions aren’t unique to white-throated sparrows and are actually credited with giving rise to the XX and XY chromosomes in mammals that dictate mammalian sex, and the ZW sex-determination system that defines a bird’s sex, but the white-throated sparrow is rare in having cooked up two extra chromosomes to effectively give itself four sexes.
“Who knows,” said Gonser, “There might be many more species that have weird sex chromosomes and we've just never bothered to look.”