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Groundbreaking New Zealand-led research that could save a rare threatened rhino from extinction is giving hope to other endangered species worldwide.
New Zealand conservation biologist Wayne Linklater is leading an international project testing a theory that high sugar levels in female black rhinos are the reason disproportionate numbers of males are being born.
The theory has been borne out in his earlier research into Kaimanawa wild horses, and Dr Linklater said it could be applied to other species, particularly those, such as the kakapo, in which the males breed more prolifically than females.
Dr Linklater said extreme male-biased births in captivity were common in many species, including zebras, gorillas and giraffes.
Some species are endangered because they do not have enough females to sustain the population.
About 71 per cent of black rhinos born in captivity are male, and research suggests more males are also born after black rhinos are transferred to new game reserves, Dr Linklater said.
About 3150 black rhinos live in the wild in southern and east Africa, and 250 are in captivity around the world.
Many captive populations could be extinct in 10 to 15 years unless the sex imbalance can be fixed.
Dr Linklater said glucose levels seemed to increase in rhinos moved to zoos and game reserves.
"Glucose levels in the pregnant mothers are raised if they are stressed, fed a sugar-rich diet, and obese. This has fatal consequences, particularly for female embryos.
"It is not that more male calves are being conceived, but rather that fewer female embryos survive to be born."
Dr Linklater works at Victoria University in Wellington, but is also an adjunct researcher at Nelson Mandela University in Port Elizabeth, South Africa.
The rhino study was sparked by his research into the Kaimanawa horses, which found the sex of foals was strongly related to the mother's condition at conception.
"A mother in good condition will tend to have male foals, whereas the reverse is true for mothers in poor condition.
"Basically it's because a son, if it's a good son, will leave many more grandchildren than a daughter. But if a mother is in poor condition ... the son is likely to fail in breeding.
"A daughter, even though the mother may be in poor condition and unable to help it into adulthood, is still likely to have a few foals. So there's an evolutionary trade-off between how much a mother has to invest, and which sex she invests in."
Dr Linklater said scientific reviews had also suggested that glucose tended to kill female embryos.
"If you put the two things together, you suddenly realise this might be why females in captivity who are overfed have more male calves.
"It's one of those 'ah-ha!' moments. It's taken a lot of work in a lot of different fields for the idea to even become apparent."
Four black rhinos at Western Plains Zoo in Dubbo, Australia, are the first to be tested.
Drops of blood are being collected from the rhinos' ears or lips and glucose levels are being measured.
But it will be at least 15 months - the gestation period of a black rhino - before firm conclusions can be drawn.
Hamilton Zoo director Stephen Standley said the research would have great value.
"We don't have all the answers with species held in captivity, and many of those lessons that we do learn from captive animals are then used in the wild."
