The ongoing debate surrounding the definition of biological sex has intensified recently, fueled by executive orders and societal discussions. This February, LGBTQ+ History Month’s theme of ‘Science & Innovation’ highlights a growing understanding: the natural world often defies the simple binary of sexes and genders. A compelling example of this complexity comes from the study of the white-throated sparrow (Zonotrichia albicollis), a common North American songbird.
Researchers have discovered that this seemingly ordinary bird exhibits not two, but four distinct sexes, challenging conventional understandings of sexual identity. This isn’t merely a behavioral difference. it’s rooted in the bird’s genetics and manifests in a unique mating system and varying life-history strategies.
Beyond Male and Female: The Four Sexes of the White-Throated Sparrow
The white-throated sparrow is easily recognizable by its plumage – either black-and-white or tan-and-brown striped heads, coupled with a bright yellow spot above each eye and a white throat patch. However, the color pattern isn’t indicative of sex; both male and female sparrows can display either coloration. The key to understanding their complex sexual system lies in a specific region of their DNA, a “supergene” located on chromosome 2.
This supergene, which emerged approximately 2 million years ago, controls the bird’s disassortative mating system – a preference for mating with individuals who are different in appearance. White-striped birds almost exclusively pair with tan-striped birds and vice versa. This preference isn’t simply a choice; it’s genetically enforced, creating four distinct behavioral categories: white-striped males, white-striped females, tan-striped males, and tan-striped females.
Each of these “sexes” exhibits different behaviors. The white-striped birds, regardless of their biological sex, tend to be more aggressive, focusing on securing mates and defending territory. They sing more frequently and are more likely to seek extra-pair copulations. Conversely, the tan-striped birds adopt a more parental strategy, prioritizing nest building, provisioning young, and guarding their mates within the territory.
This results in typical mating pairs consisting of an aggressive white-striped bird paired with a nurturing tan-striped counterpart. A white-striped male might pair with a tan-striped female who focuses on parental care, while a tan-striped male might pair with an aggressive white-striped female. Remarkably, both types of pairings demonstrate comparable reproductive success, suggesting that all four “sexes” play complementary roles rather than competing with one another.
The Genetics Behind the Complexity
The supergene’s origin lies in a chromosomal inversion – a rearrangement where a section of DNA flips and re-inserts itself. This creates two versions of chromosome 2: ZAL2 (the normal version) and ZAL2m (carrying the inversion). Tan-striped birds carry two copies of ZAL2, while white-striped birds carry one copy of each. A bird with two copies of ZAL2m, often referred to as “super white,” is extremely rare, resulting from mating between two white-striped birds.
Within this inverted region, two key genes, ESR1 and VIP, are differentially regulated depending on the chromosome version. This differential regulation accounts for the variations in aggression, mate selection, and parental behavior observed in the different morphs. The supergene operates independently of the W and Z sex chromosomes typically used for sex determination in birds, further highlighting the complexity of the white-throated sparrow’s system.
The interplay between the supergene and the W/Z chromosomes supports the concept of four distinct sexes within the species. This system isn’t without its challenges. The limited mating pool – an individual can only successfully mate with roughly one-quarter of the population – could contribute to the species’ decline. White-throated sparrow populations have decreased by 69% in the U.S. Over the past 50 years, and by 33% overall (including Canada).
A Biological Metaphor for Social Conversations
The white-throated sparrow offers a compelling biological metaphor for ongoing social conversations about sexual diversity. With approximately one in 100 individuals exhibiting intersex conditions, there’s a growing movement to normalize the concept of a spectrum of sexual identities. The sparrow’s genetic complexity demonstrates that the traditional binary of male and female is an oversimplification, and that nature often presents a more nuanced and fluid reality.
The bird’s unique system serves as a reminder that biological sex isn’t always a clear-cut distinction, and that variations can be integral to a species’ success. It’s a powerful example of how the natural world can challenge our preconceived notions and offer new perspectives on identity and diversity.
