Nov/December 2025

12 – Purebred PIGEON PIGEON BEHAVIOR by Mike McKaig WHY DO MALE PIGEONS LIVE LONGER? Wendell Levi observed that male pigeons live longer and breed longer than females. His observation is supported by reliable records of pigeon longevity. Pigeons known to have lived to advanced ages tend to be males. Levi specu- lated that cocks live longer because their bodies don’t suffer the strain of producing clutch after clutch of eggs. But there may be another reason for the sex gap in pigeon longevity. THE UNGUARDED X HYPOTHESIS In 1985 Richard Trivers developed a theory known as the Unguarded X Hypothesis to explain differences in lon- gevity between male and female animals. According to the theory, having two copies of the same sex chromosome is associated with greater longevity because the second copy provides protection against harmful genetic mutations and disease. In all mammals, including humans, females have two X chromosomes (XX) while males have an X and a Y chromosome (XY). Trivers noted that female mammals, with the added protection of a second X chromosome, tend to live longer. Males, with only one X chromosome (the unguarded X) have greater exposure to life-threatening mutations. BIRDS ARE DIFFERENT In most birds, including pigeons, it is males who have two copies of the same sex chromosome. Male birds have two Z chromosomes (ZZ) while females have a Z and a W chromosome (ZW). According to Trivers’ theory, male birds have greater longevity than females because they ben- efit from the added protection of a second Z chromosome. TESTING THE THEORY In 2020 researchers at the University of New South Wales in Australia tested the Unguarded X Hypothesis. They analyzed male and female longevity data on 229 species, including mammals, birds, fish, and insects. The analysis revealed that individuals with two of the same sex chromosomes (XX in female mammals and ZZ in male birds) live 17.6% longer on average. More specifically, the researchers found that in species where females have two of the same sex chromosomes, such as humans and all other mammals, females live on average 20.9% longer than males. In contrast, in species where males have two of the same sex chromosomes, such as birds, moths, and butter- flies, males outlive females by only 7.1% on average. In other words, the longevity sex gap is smaller in birds than in mammals. CONCLUSION The researchers in Australia concluded that the un- guarded X does influence lifespan. But they noted that longevity can be influenced by other factors as well, like predation, nutrition, and risky behavior. Wendell Levi may have been correct with his suggestion that one such factor is the strain of repeated egg laying experienced by female pigeons. REFERENCES Brengdahl, M., C.M. Kimber, J. Maguire-Baxter, and U. Friberg. 2018. Sex differences in lifespan . Evolution, 72 (3), 568-577. Connallon, T., I.J. Beasley, Y. McDonough, and F. Ruzicka. 2022. How much does the unguarded X contrib- ute to sex differences in life span? Evolution Letters, 6-4, 319-329. Levi, W. 1981. The Pigeon. Levi Publishing Company, Sumter, S.C. Trivers, R. 1985. Social Evolution. The Benjamin/ Cummings Publishing Company, Menlo Park, CA. Xirocostas, Z.A., S.E. Everingham, and A.T. Moles. 2020. The sex with the reduced sex chromosomes dies earli- er: a comparison across the tree of life . Biology Letters, 16 (3), 20190867. Mike McKaig, PhD, is Professor Emeritus of Speech Pathology at the University of Central Oklahoma. He got his first pigeons when he was six. Today he has homing pigeons, a breed he admires for its intelligence, athleticism, and stunning colors.