In celebration of Pride Month, we would like to share a colorful exploration of diversity within the animal kingdom. 

A Sparrow With Four Sexes

White-throated sparrows have displayed a fascinating mating system that consists of four sexes. This species is comprised of two distinctive phenotypes: white-striped and tan-striped crown. What makes these birds so interesting is that several genes have been identified and have a direct relationship to their behaviors. White-striped crowns seek multiple partners and invest heavily in securing additional matings at the expense of paternal care. Alternatively, tan-striped males are monogamous and contribute more to parental care. The white and tan versions are genetically determined. White-striped sparrows have an inverted section of chromosome 2, which effectively acts as a second sex chromosome and leads to white-throated sparrows having four sexes. About 98 percent of observed pairings (mated or social) exclusively pair with the opposite phenotype. Because these birds have been observed to only mate with one-quarter of its species, it can be considered that they act as if they have four sexes. 

The Fluidity of Fish

Sex-determination mechanisms are particularly diverse in fish and include many species in which changing sex at least once is a normal and adaptive feature of the fish’s biology. Researchers call this “sequential hermaphroditism” and have documented fish species that change from female to male (protogyny), species that change from male to female (protandry), and species that can change back and forth repeatedly throughout their lifetimes. All these systems have adaptive advantages suited to the fish’s particular environment.

Mom’s Got it Covered

Several lizard species have evolved into all-female unisexual species that reproduce asexually through a process called parthenogenesis, which is essentially cloning. Same-sex courtship and reproductive behavior, generally referred to as “pseudocopulation” by researchers, occurs between females of the exclusively female unisexual whiptail lizard. This behavior increases their ability to reproduce by stimulating ovulation. Research on wild populations of unisexual whiptail lizards found strong evidence that same-sex pseudocopulation between females is not only beneficial to their reproductive success but is the norm for this species and practiced as regularly as copulation in sexual species. Through hormone-mediated processes, about half of the unisexual lizards in an area assume the behavioral role of “males” in these reproductive interactions, then later switch roles. 

Sources:

Tuttle E., 2016, Current Biology 26, Divergence and Functional Degradation of a Sex Chromosome-like Supergene

Hedrick  P., 2018, Negative-Assortative Mating in the White-Throated Sparrow.  Journal of Heredity, Vol. 109, No. 3

Arnold C., 2016, Nature volume 539, The Sparrow with Four Sexes

Kobayashi Y., Nagahama Y., Nakamura M., Diversity and Plasticity of Sex Determination and Differentiation in Fishes. Retrieved from https://karger.com/sxd/article-pdf/7/1-3/115/3552793/000342009.pdf

Crews D., Grassman M., Lindzey J., 1986, Proc.Natl. Acad. Sci. USA, Vol. 83, Behavioral facilitation of reproduction in sexual and unisexual whiptail lizards

Crews D., Young L., 1991, Pseudocopulation in nature in a unisexual whiptail lizard. Article in Animal Behavior

Woolley S. C., Sakata T. J., and Crews D. 2004, Tracing the Evolution of Brain and Behavior Using Two Related Species of Whiptail Lizards: Cnemidophorus uniparens and Cnemidophorus inornatus. ILAR Journal