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Population density affects sex ratio variation in red deer

Abstract

Many mammal populations show significant deviations from an equal sex ratio at birth, but these effects are notoriously inconsistent1. This may be because more than one mechanism affects the sex ratio and the action of these mechanisms depends on environmental conditions. Here we show that the adaptive relationship between maternal dominance and offspring sex ratio previously demonstrated in red deer (Cervus elaphus)2,3, where dominant females produced more males, disappeared at high population density. The proportion of males born each year declined with increasing population density and with winter rainfall, both of which are environmental variables associated with nutritional stress during pregnancy. These changes in the sex ratio corresponded to reductions in fecundity, suggesting that they were caused by differential fetal loss. In contrast, the earlier association with maternal dominance is presumed to have been generated pre-implantation. The effects of one source of variation superseded the other within about two generations. Comparison with other ungulate studies indicates that positive associations between maternal quality and the proportion of male offspring born have only been documented in populations below carrying capacity.

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Figure 1: Proportion of males born each year in relation to population density, after correcting for effect of winter rainfall.
Figure 2: Proportion of males born to dominant versus subordinate females in each year.
Figure 3: Proportion of males born each year in relation to winter rainfall (November to January, in mm), after correcting for the effect of density.

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Acknowledgements

We thank Scottish Natural Heritage for permission to work on Rum; their local staff for help and support; A. Alexander, A. Curnow, S. Morris and many others for field data collection; N.Barton, P. Brotherton, A. Cockburn, D. Elston, M. Forchhammer, H. Kruuk, J. Lindström, A. Manning and P. Meir for comments and discussion; and the NERC for funding this work.

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Correspondence to Loeske E. B. Kruuk.

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Kruuk, L., Clutton-Brock, T., Albon, S. et al. Population density affects sex ratio variation in red deer. Nature 399, 459–461 (1999). https://doi.org/10.1038/20917

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