The international collaboration involved researchers from the Department of Evolutionary Zoology and Human Biology at the University of Debrecen, including Miranda Oscar G., Tamás Székely, and Balázs Vági, as well as researchers from Denmark, Chile, Mexico, and the UK, and experts from the Copenhagen Zoo.
"If the sex ratio at birth is significantly disrupted in a small population, it can lead to inbreeding, genetic impoverishment and, ultimately, even extinction. This is especially true for species that are already endangered in the wild," said Miranda Oscar G., the study's first author.
The researchers examined birth data for 129 bird and 324 mammal species, covering a total of more than 2.6 million individuals. They analyzed sex ratios using phylogenetic, life history-based, and statistical models to uncover the factors influencing the skew.
"The shift in sex ratios is not just a population genetics issue. In some species, if there are too many males, it can lead to increased aggression, stress, and even mating difficulties in zoos," added Balázs Vági.
In birds, the birth ratio was found to be balanced overall, although a slight overrepresentation of males was found in some groups, such as penguins, parrots, and falcons. However, skewed sex ratios were much more common among mammals: females were born more often among marsupials and ungulates, while males were born more often among primates.
It is particularly worrying that significant distortions have been observed in several species that are of particular importance from a conservation perspective. Tigers, pygmy hippopotamuses, and Père David's deer, for example, give birth to more females, while African wild dogs, ring-tailed lemurs, and Asian elephants give birth to more males.
- These species can play a key role in reintroduction programs, so it is particularly important that the sex ratio of offspring is balanced. If this is disrupted, it can jeopardize the long-term success of the programs, emphasized Professor Tamás Székely.
According to the results of the research, the mating system of mammals—that is, whether the species is monogamous or polygamous—strongly influences the sex ratio at birth. In monogamous species, males are more common, while in birds, the size of the brood and the difference in size between the sexes are more important.
The study uniquely combines large-scale data analysis, evolutionary theories, and phylogenetic comparisons. The results may help breeding programs in zoos to more consciously manage the risks arising from birth ratios.
- The sex ratio at birth is not just a demographic indicator, but a finely tuned mechanism of evolution. If we interfere with it, even unknowingly, it can upset the dynamics of the entire population. Our research draws attention to these hidden dangers, summarized Miranda Oscar G.
The study can be read here:
Oscar G. Miranda, Fernando Colchero, José O. Valdebenito et al.: Biased birth sex ratios of mammals and birds in zoos, Scientific Reports, July 1, 2025. https://doi.org/10.1038/s41598-025-05039-4
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