Plant phenology, migration and geographical variation in body weight of a large herbivore: the effect of a variable topography

1. A central concept in ruminant foraging ecology is that even slight changes in plant quality affect body growth substantially, because ruminants not only gain more protein and energy but also use less time for rumination when eating high-quality forage. Increased access to highly nutritious forage is thus regarded as an important driving force in the evolution of migration in large herbivores, because the temporal and spatial variation in plant quality is huge. Body weight is in turn a major determinant of reproductive performance and survival in ungulates, and therefore important for population dynamics. 2. In the temperate region, the vertical movement of cervids from a low-elevation winter range to a high-elevation summer range is the most common pattern of migration. Migration to high elevations during summer is regarded as a strategy to increase energy intake among northern temperate ungulates. 3. It has been suggested that there is increased forage quality at high elevation. This leads to the prediction that body weights are positively correlated with the proportion of high-altitude habitat. Alternatively, cervids may benefit due to prolonged access to newly emerged forage as they migrate along an altitudinal gradient. If so, it can be predicted that body weights are highest in regions with the highest diversity of different altitudes. 4. The facing of slopes, i.e. the aspect, is also regarded as an important part of the habitat for deer. Access to north-facing slopes is regarded as favourable because plant quality may be higher on north-facing slopes; access to a variety of different aspects is also favourable because this may lead to a variable snow melt and thus a longer time period with access to high-quality forage. 5. In order to test the above predictions (under points 3 and 4), we analysed the spatial variability of body weight of 8452 female and 12 474 male red deer harvested during 1975-98 from 105 municipalities in Norway. 6. Body weight of red deer was positively related to the diversity of altitudes, and negatively related to the proportion of high-altitude habitat per se. This demonstrated that the altitudinal migration of cervids is not necessarily due to increased forage quality at high elevation, but rather that migrating cervids gain from prolonged access to newly emerging forage along an altitudinal gradient as they migrate to high elevation during early summer. 7. There was also no support for the hypothesis that access to a high proportion of north-facing slopes was favourable; rather, body weight of red deer was correlated with access to a diversity of aspects. There was thus clear evidence that a variable topography, measured as different altitude levels and aspects, was positively related to body weight of red deer. 8. We discuss the possible causes of why an ideal free distribution is not reached, and conclude that phenological differences in plants related to topography can have a large impact on body weights of cervids, and therefore induce considerable spatial heterogeneity in population dynamics.