Making use of data gathered from a 10-year field study, scientists report in Nature today that saiga antelope, which rank in the World Conservation Union's category of most endangered species, are being pushed closer to extinction because there are not enough male antelopes to mate with the females - despite the male's polygynous practice of maintaining a harem of 12-30 females.
Selective hunting of the male for its horns, for use in traditional Chinese medicine, has led to a gender bias where females outnumber males by a ratio of 100:1.
Dr E.J Milner-Gulland, lead author of the study, based in the Department of Environmental Science and Technology, Imperial College London, explains:
"Until now, in polygynous systems, it has been assumed that even when males are in short supply their ability to inseminate many females secures the viability of the population.
"Our observations indicate that if the percentage of males in the population falls below one per cent, reproductive collapse will result."
Even more striking, field observations of saiga behaviour in the year 2000 suggest that the crash in male numbers appear to have disrupted the species' in-built reproductive strategy, which is exacerbating their decline.
"Normally, a male defends his harem of females from other males," said Dr Milner-Gulland. "We found dominant females were aggressively excluding the younger females from the males and preventing them from getting pregnant.
"Once you get these indirect effects causing population decline through fecundity you can not be sure whether the population will recover.
"At present, the saiga population is halving each year and the species could soon be lost. Action must be taken now to provide a sustainable future for the antelope."
Once found roaming the grasslands of Central Asia and pre-Caspian in vast herds, the global population of saiga antelope has crashed to 50,000 over the past 10 years - five per cent of its previous size. The population decline has been driven by the collapse of the Soviet Union, which reopened its strict borders allowing the animals to be hunted for their meat and for the male's horns.
The researchers from the UK, Russia and Kazakhstan collected data on saiga population dynamics in Kalmykia, Russia between 1992-2002. They found that changes in population density or climatic variation did not account for the drop in the number of offspring. The researchers concluded that the heavily skewed sex ratio must be the driving force behind the antelopes decline.
However, according to Dr Milner-Gulland, all may not be lost. Historical data suggest a similar population crash occurred at the beginning of the 20th Century, also as a result of over hunting. The rise of the Soviet regime closed the country's borders and a strict ban was imposed on hunting, which allowed the population to recover.
"This indicates the species is very resilient. If the population has rebounded before we hope it can do it again," she said.
"Our findings have important implications for conservation of the antelope and other polygynous ungulates such as deer and wildebeest."
"There is only one viable herd in captivity and the species is difficult to breed because of their nomadic existence and diet in the wild. Efforts must be focused on involving rural people in conservation and providing resources to train and equip local law enforcement agents."
Photographs of the saiga antelope are available on request
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Notes to editors
Publication: Nature (13 March 2003)
Title: Reproductive collapse in antelope harems
Authors: E. J. Milner-Gulland (1) O. M. Bukreeva (2) T. Coulson (3) A. A. Lushchekina (4) M. V. Kholodova (4) A. N. Severtsov (4) A. B. Bekenov (5) I. A. Grachev (5)
(1) Department of Environmental Science and Technology, Imperial College London.
(2) Department of Hunting Management, Republic of Kalmykia, Russia
(3) Department of Zoology, University of Cambridge
(4) Institute of Ecology and Evolution, Moscow, Russia
(5) Institute of Zoology, Ministry of Education, Almaty, Kazakhstan
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