Annual cycles of body weight and condition in Scottish Red deer, Cervus elaphus



As part of a study of performance in Red deer (Cervus elaphus L.) at high population density on Scottish hill-land, the annual cycles of body weight and condition in selected classes of deer were investigated. The research was done on Rhum, a 10,600 ha island off the west coast of Scotland with an isolated population of about 1500 Red deer. Samples of deer were shot at different times of year, and various physical characteristics of the animals were assessed.

The four classes of deer chosen for study comprised: stags, hinds supporting calves (“milk” hinds), and hinds without calves (“yeld” hinds), all aged 5–10 years, and calves. These represented the three main kinds of mature full-grown deer, and immature deer at the stage of maximum growth potential. Seven sampling periods of two weeks each in July, September, October, November, February, March and April-May covered the main features of the annual life cycle. Achieving biologically meaningful results depended very much on animal selection, and on the consistency and intensity of sampling. Therefore the background and results of the shooting plan are discussed in some detail before considering the biological findings of the study.

All four classes of Red deer showed well defined trends in body weight and condition over the year, and it was clear that body weight and condition were strongly correlated within each class; the maxima and minima in body weight and condition coincided exactly. Similarities amongst the four classes were undoubtedly related to the underlying seasonal trends in environmental factors such as food and climate; deer mostly showed gains in weight and condition from spring to autumn and losses during winter.

Differences amongst the three classes of adult deer were mainly attributable to aspects of reproduction, but there were two-way relationships between these and the annual cycles of body weight and condition. Maximum body weight and condition in stags coincided with the start of the rutting season (late September to late November), but stags lost about 14–17% carcase weight and most discernible fatty tissue during the rut. Milk hinds showed lower body weights and fat reserves than yeld hinds over most of the year reflecting the effects of pregnancy and lactation. Moreover, hinds in good condition achieved a higher pregnancy rate than those in poor condition. Thus many hinds must breed successfully one year but fail the following year on account of poor condition resulting from the effects of pregnancy and lactation in a nutritionally poor environment. Calves grew actively from the calving period in late May and June until late October, but there was a severe growth check from late November until late March with most body constituents either declining or remaining constant. However, skeletal growth apparently continued slowly over the winter period. As might be expected, the best hinds had the best calves.

In adult deer the weights of livers and kidneys showed distinct annual cycles, but these were out of phase with the cycles in body weight and condition. Possible reasons are discussed briefly. In addition, the trends in kidney weights are discussed in relation to the kidney-fat-index as a measure of condition. There were distinct annual cycles also in the amounts of ingested food material in adults of both sexes. Hinds showed smaller amounts, suggesting a lower food intake, in winter than summer. The amounts in stags were very much lower during the rutting season, and much higher immediately afterwards, than at any other time of year; it is well known that stags almost cease feeding whilst rutting.