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Sex differences in emigration and mortality affect optimal management of deer populations


Populations of red deer that are limited by food, like those of many other ungulates1,2,3, commonly include more females than males4,5,6,7. We assessed the contribution of variation in sex- and age-specific rates of mortality and emigration to density-dependent changes in the adult sex ratio, using long-term observations and demographic experiments involving the red deer population on Rum, Scotland4,5. We incorporated these effects in a stochastic model of local populations under different management regimes to show here that, when female numbers are allowed to increase to more than 60% of the ecological carrying capacity, the sustainable annual harvest of males from local deer populations will fall. Because males are typically culled by fee-paying hunters and generate more income than females5,8,9, income will decrease as the male harvest falls. Because numbers of female deer throughout much of the Highlands probably exceed the threshold at which male density starts to be affected5, many managers might be able to raise income from local deer populations by reducing female numbers, with potential benefits to the vegetation of Scottish Highland environments10.

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Figure 1: Changes in the adult sex ratio.
Figure 2: Demographic parameters contributing to the change in adult sex ratio in the deer population of the north block after 1972 (see Fig. 1c).
Figure 3: Results of a stochastic model of two contiguous deer populations subject to different culling regimes, incorporating density-dependent changes in age- and sex-specific rates of fecundity mortality, immigration and emigration, derived from the north block of Rum (see Fig. 2).

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We thank F. E. Guinness, J. M. Pemberton and S. Albon for maintaining the long-term records in the north block of Rum; C. Duck, J. Kingsley, M. Twiss and C. Covey for collecting data on deer populations throughout Rum; R. Scott for organizing the manipulations of deer numbers on Rum; the staff of the Deer Commission for Scotland, especially C. MacLean and D. Youngson, for counting and culling deer on Rum; the SNH staff on Rum, especially M. Curry and D. Reed, for support in maintaining deer research on the island; S. D. Albon, I. Gordon, L. Conradt, L. Kruuk and S. Buckland for comments or help with these analyses; J. Taylor for providing Fig. 1b; and the NERC, the Deer Commission for Scotland, SNH and the Scottish Executive for funding.

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Correspondence to T. H. Clutton-Brock.

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Clutton-Brock, T., Coulson, T., Milner-Gulland, E. et al. Sex differences in emigration and mortality affect optimal management of deer populations. Nature 415, 633–637 (2002).

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