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The largest-ever study of over 2.6 million zoo birth records reveals skewed sex ratios in endangered species, from lemurs to elephants. The international team of scientists behind the study emphasizes that it is vital that zoos take into account the ratios of males to females born in captivity when planning conservation projects and breeding programs for endangered species to avoid the silent threat of having too many of one sex.

Biases in sex ratios of populations can dramatically increase the risk of extinction by limiting genetic diversity, increasing inbreeding and making the population more vulnerable to collapse. In captivity, a bias toward one sex can also cause problems with how to house animals safely to avoid conflicts.

Until now, there has been very little data collected on animals born in captivity. However, an international team of scientists led by the Milner Centre for Evolution at the University of Bath (UK) has published the most comprehensive study so far investigating the birth sex ratios (BSR) of animals in captivity.

Using evolutionary theory and advanced statistical methods, they found that BSR could be predicted by factors such as mating systems, sexual size dimorphism (differences in size between males and females of the same species) and brood size.

Their study, published in the journal , investigated over 450 species of birds and mammals, including 30 endangered species such as Asian elephants, tigers and pygmy hippos with significantly biased BSRs.

They found that while lions have roughly equal numbers of males and females born, cheetahs have more male offspring and tigers have a bias toward females. Scientists were surprised to find that Asian elephants displayed a male bias, which is concerning given their matriarchal system where few males reproduce.

In mammals, mating system was a significant predictor of sex ratio bias, with monogamous species tending to produce more male offspring. For example, ring-tailed lemurs—the most common zoo animal in the world—form long-term pair bonds and showed strong male biases in zoo births.

In birds, they found that clutch size and sexual size dimorphism—the difference in sizes between males and females—were more predictive of BSR bias.

"This is the largest study of birth sex ratios in zoo animals, giving us a much better understanding of the factors that shape sex ratio variation in different species," said Oscar Miranda, first author of the study and researcher at the University of Debrecen (Hungary), who will join the Milner Centre for Evolution at the University of Bath this September to begin his Ph.D.

"Our findings show that even well-managed zoos may be silently accumulating sex biases that threaten the survival of endangered species."

Professor Tamás Székely, from the Milner Centre for Evolution and Department of Life Sciences at the University of Bath (UK), said, "Our study highlights the need for zoos to collaborate more closely with each other and share data on their animals so they can manage the conservation and breeding of endangered species.

"No single zoo can prevent extinction on its own. But together, by monitoring birth sex ratios, we can turn a quiet demographic threat into a manageable challenge."