Rhinoceros are able to modify the sex of their offspring to keep a healthy balance of genders and limit severe breeding competition, a new study has found.

The findings bring new hope to the species, which has critically low numbers, may be more resilient than originally thought, said Victoria University associate professor Wayne LInklater, who lead the research.

Some rhino species have been driven to extinction and critical endangerment, and others are classified as vulnerable or threatened. Rhinoceros are poached for their horns, which are often ground up and used in traditional medicines.

The study was the first to provide experimental evidence in the wild that unbalanced population sex ratios can result in a compensatory response by parents to "correct" the imbalance.

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"This is called a homeostatic sex allocation (HSA) response — a biological theory first proposed in 1930," said Linklater.

The phenomenon was of being able to manipulate the sex of offspring is
The phenomenon was of being able to manipulate the sex of offspring is "mysterious" to researchers. Photo / File

"HSA has an especially strong effect when the gender imbalance is very large. In fact, the further it is from an even-sex ratio, the stronger the response is by parents."

"Because of the evidence of HSA, we need not be so concerned about that misbalance, because parents appear able to 'correct' it when they breed.

"This is good news for what we call reintroduction biology — the ability to restore populations of species at new sites in places where they have been pushed to extinction," he said.

The research team examined 24 years of rhinoceros data, gathered during the course of 45 reintroductions of the animals across southern Africa.

The gender correction theory was outlined in a book in 1930 by statistician Ronald Fisher.

"There have only ever been 14 tests of the theory, they haven't been particularly good ones," Linklater said.

The phenomenon was "mysterious" to the researchers.

"We don't understand exactly how it works, we don't understand the physiology, but there are some leading hypotheses for how it happens," he said.

One example was that the more common sex in a population would be under constraints to be able to reproduce, because of the amount of competition, and reactions would be triggered by this.

"There's quite likely a physiological mechanism in there."

Examples of physiological mechanisms that could be triggered by breeding competition could include the amount of blood sugar in the female system.

"We know that blood sugar selects for or against male or female offspring, depending on how much there is and when spikes occur."

Another idea was around the fact male embryos implant later than female, but are faster to develop.

"It might be that competition changes the readiness of the placenta to receive the embryo."

So far scientists have assumed sex ratio is fixed, and have not modelled anything different, he said.

But if animals were able to "manipulate" the sex ratio in this way it would have "far-reaching implications".

It was not yet known what species could possibly gender correct, but "it's quite possible that it's more common or easier for some species to do it than others".

"It will depend on how susceptible their reproduction is to influence."

Those populations where HSA is possible will be more resilient.

"Their small populations will have improved establishment and greater viability. Such species will populate habitats faster, and be less susceptible to random demographic processes and genetic drift."

Linklater now plans to do further research into how an HSA response works in Australian brushtail possums. This includes how competition to breed triggers the effect 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 says. "Possums are also invasive mammals in New Zealand. Understanding their reproductive processes can provide new ways of managing population numbers."

The study, published in Nature Ecology & Evolution, was co-authored by Dr Peter Law from Nelson Mandela Metropolitan University in South Africa, Pierre du Preez from Namibia's Ministry of Environment and Tourism, and former Victoria University postdoctoral researcher Dr Jay Gedir.

It was completed with funding from the United States Fish and Wildlife Service's Rhinoceros and Tiger Conservation Fund.