Relationships between sex ratio, climate and density in red deer: the importance of spatial scale

Abstract

Summary 1. Recently, contrasting evidence of density‐dependent offspring sex ratio variation in red deer (Cervus elaphus L.) has been presented. Kruuk et al. (1999) reported that the proportion of male red deer born on the island of Rum, Scotland, each year declined with increasing population density and with winter rainfall, i.e. as nutritional stress in females increased. In contrast, Post et al. (1999a) reported increasing male‐biased offspring sex ratios in Hordaland, Norway, with both rising numbers of hinds and increasing values of the North Atlantic Oscillation (NAO), which they regarded an index of severe winters. 2. Compared to Post et al. (1999a), we reanalysed sex ratio variation on a greatly extended data set, including four populations of Norwegian red deer, taking into account a finer population substructure. 3. The proportion of male calves shot each autumn declined markedly as density increased in all four populations. The proportion of male calves shot each autumn increased significantly with an increasing NAO index in one of the four populations, but there was no residual effect of the NAO once the effect of snow depth (at low elevation) was controlled for (decreasingly male biased harvest with increasing snow depth). 4. At the west coast of Norway, the NAO is positively correlated with temperature and precipitation. However, since temperatures in this region often are around 0 °C during winter, the relationship between the NAO and snow depth may be difficult to predict. 5. An analysis of variation in snow depth revealed that snow depth was negatively correlated with the NAO at low altitudes (below 400 m), but this relationship was reversed at high altitudes giving a positive correlation between snow depth and the NAO. We therefore suggest that a high NAO index indicates favourable winter conditions for red deer, as red deer winter at low altitude, and since recent studies show that a high index of the NAO is generally positively correlated with body weight in red deer. 6. Female red deer thus reared fewer sons as nutritional stress increased with increasing density and severity of climate in Norway, which is consistent with the birth sex ratios observed on Rum, Scotland.