Translocation reverses birth sex ratio bias depending on its timing during gestation: evidence for the action of two sex-allocation mechanisms
W. L. LinklaterA Present address: Centre for Biodiversity and Restoration Ecology, School of Biological Sciences, Victoria University of Wellington, PO Box 600, Wellington 140, New Zealand.
B Centre for African Conservation Ecology, Department of Zoology, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa.
C Email: wayne.linklater@vuw.ac.nz
Reproduction, Fertility and Development 19(7) 831-839 https://doi.org/10.1071/RD07027
Submitted: 11 February 2007 Accepted: 21 May 2007 Published: 8 August 2007
Abstract
Many sex allocation mechanisms are proposed but rarely have researchers considered and tested more than one at a time. Four facultative birth sex ratio (BSR) adjustment mechanisms are considered: (1) hormone-induced conception bias; (2) sex-differential embryo death from excess glucose metabolism; (3) sex-differential embryo death from embryo–uterine developmental asynchrony; and (4) pregnancy hormone suppression and resource deprivation. All mechanisms could be switched on by the corticoadrenal stress response. A total of 104 female rhinoceroses (Rhinocerotidae), translocated from 1961 to 2004 at different stages of gestation or conceived soon after arrival in captivity, were used to test for a reversal in BSR bias as evidence for the action of multiple sex-allocation mechanisms. Translocation induced a statistically significant BSR reversal between early gestation (86% male births from 0 to 0.19 gestation) and mid-gestation (38% male from 0.2 to 0.79 gestation). Captivity also induced a strongly male-biased (67% male) BSR for conceptions after arrival in captivity. The results indicate the action of at least two sex-allocation mechanisms operating in sequence, confirm the important role of sex-differential embryo death around implantation and of stress in sex allocation, and lend support to suggestions that sex-differential glucose metabolism by the preimplantation embryo likely plays a role in facultative BSR adjustment.
Additional keywords: conception, embryo, mammal, rhinoceros, stress, zoo studbook
Acknowledgements
The work described herein was supported financially by the International Rhinoceros Foundation. The author thanks and dedicates this manuscript to the late Tom Foose for facilitating the partnership. Thanks too to the many who contributed to the rhinoceros studbooks since their inceptions and Hannelore Mercado and Gabriele Hlavacek in particular for providing me the most recent versions, and Ken McNatty, Elizabeth Berkley and two anonymous reviewers for comments on an earlier version of the manuscript. The author’s work with rhinoceros in southern Africa during the development of the manuscript was also supported by the USA Fish & Wildlife Service administered Rhinoceros and Tiger Conservation Act of 1994 (grant agreement numbers 98210–2-G363, 98210–4-G920, 98210–6-G102) and a National Research Foundation of South Africa, International Scientific Liaison grant (with thanks to Graham Kerley for facilitating). Finally, the author thanks Nanz Zekala and Peter Law for their discussion on this and related topics.
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