A common assumption is that breeding in polygnous systems is not limited by the number of males because one male can inseminate many females^{1, 2}. But here we show that reproductive collapse in the critically endangered saiga antelope (Saiga tatarica tatarica) is likely to have been caused by a catastrophic drop in the number of adult males in this harem-breeding ungulate, probably due to selective poaching for their horns. Fecundity and calf survival are known to be affected by markedly skewed sex ratios³, but in the saiga antelope the sex ratio has become so distorted as to lead to a drastic decline in the number of pregnancies — a finding that has implications both for the conservation of the species and for understanding the reproductive ecology of polygynous ungulates.

1. Department of Environmental Science and Technology, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
2. Department of Hunting Management, Elista 358000, Republic of Kalmykia, Russia
3. Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK
4. A. N. Severtsov Institute of Ecology and Evolution, Moscow 119071, Russia
5. Institute of Zoology, Ministry of Education, Almaty 480032, Kazakhstan

Correspondence to: E. J. Milner-Gulland¹ e-mail: Email: e.j.milner-gulland@imperial.ac.uk