By Jean Balchin 08/03/2017

Rhinos are able to alter the sex of their offspring to avoid a gender imbalance and reduce competition for breeding, according to a new study led by a New Zealand researcher.

Twenty-four years of rhinoceros data, gathered over the course of 45 reintroductions of the animals across southern Africa, provided this first experimental evidence in the wild.

Wayne Linklater
Professor Wayne Linklater

Lead researcher, Victoria University of Wellington wildlife biologist Associate Professor Wayne Linklater, who also blogs here on Sciblogs, said the theory that uneven population sex ratios can result in a compensatory response by parents to ‘correct’ the imbalance—a homeostatic sex allocation (HSA) response—was first proposed in the 1930s by statistician Ronald Fisher.

“Almost all population models assume birth sex ratio is fixed. Our evidence indicates that this may not be the case.”

Sex-bias is particularly important in rhinoceros populations, given their critically low numbers. Javan and Sumatran species of rhino are classified as ‘Critically Endangered’, while the Asian species is ‘Vulnerable’ and a prime target for poachers. Southern white rhinos are classified as ‘Near Threatened’, and only a few Northern white rhinos remain in a Kenyan sanctuary. Although the population of Black rhinos has increased over past decades, numbers are still very low.

Parents’ corrective powers

However, the evidence of HSA alleviates this sex-bias concern somewhat, Dr Linklater said, because “parents appear able to ‘correct’ it when they breed”. Indeed, the greater the gender imbalance, the more pronounced the effect of HSA. “The further it is from an even-sex ratio, the stronger the response is by parents.”

Consequently, those rhinoceros populations where HSA is possible will be more resilient. “Their small populations will have improved establishment and greater viability,” said Dr Linklater, “such species will populate habitats faster, and be less susceptible to random demographic processes and genetic drift”.

Investigating the allocation of resources by parents between male and female offspring is a key focus in evolutionary biology. “Extreme sex ratios commonly occur,” Dr Linklater said, “so the incidence of HSA will significantly impact our understanding of a range of ecological processes including invasion biology and conservation management”. Invasion biology studies the human transport and introduction of species throughout the world.

The next step for Dr Linklater will be to conduct research into how an HSA response works in Australian brushtail possums. Future research will consider how competition to breed triggers the HSA response and at what point in the reproductive process the mother is able to control the sex of her offspring.

“Possums are ideal subjects for such a study because their offspring are born into the marsupial pouch at an extraordinarily young age—very early in development—and so can be studied in great detail,” he said. “Possums are also invasive mammals in New Zealand. Understanding their reproductive processes can provide new ways of managing population numbers.”

The research was published in the journal Nature Ecology & Evolution.

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