Density-dependent responses of fawn cohort body mass in two contrasting roe deer populations

Abstract

We investigated the influence of population density on juvenile body mass in two contrasting roe deer populations, in Sweden (Bogesund) and France (Chizé), in which density was monitored for >or=15 years. We investigated the effect of population density and climatic conditions on cohort performance. We predicted that: (1) body mass of growing fawns should be sensitive to environmental changes, showing marked between-year variation (i.e., cohort effects), (2) fawns in the less productive (weakly seasonal, weakly predictable summer weather) habitat of Chizé should show stronger density-dependent responses due to more severe food competition during summer than fawns in the more productive (markedly seasonal, moderately predictable summer weather) habitat of Bogesund, and (3) fawns at Bogesund should be heavier both in absolute terms and relative to their size than their conspecifics in Chizé due to a higher degree of fat accumulation in northern environments. In both study sites we found marked cohort variation and clear effects of density, with body mass varying by as much as 29% over years. While neither summer nor winter climate influenced fawn body mass at Bogesund, fawns tended to be lighter after summers with high temperatures at Chizé. In addition, fawns were heavier after acorn mast years experienced in utero at Bogesund. As expected, the strength of the density-dependent response of fawn body mass was greater at Chizé than at Bogesund. For a given density, male fawns were consistently heavier than females in both sites. Lastly, both sexes at Bogesund had higher absolute body mass and were larger for a given body size than in Chizé. Our results clearly demonstrate that absolute density is a poor predictor of roe deer performance and supports the view that habitat quality has an overwhelming importance for determining fawn body mass in roe deer populations.

Parturition date for a given female is highly repeatable within five roe deer populations

“…At Aurignac, the low value (compared with other populations) of birth synchrony could be linked to the high spatial heterogeneity of the habitat, which in turn influences female quality. The high variation of roe deer density in Storfosna probably did not influence repeatability, which was of a similar level to that at Bogesund where the density remained constant over the study period [11].…”

Environmental change and long-term body mass declines in an alpine mammal

“…Density dependence in body mass has been detected in a range of ungulate species (e.g. [27,36,66]) and even specifically in juvenile chamois [52]. At high latitudes and in alpine regions, this effect is most likely to manifest during winter, when snow cover reduces forage availability and increases forage patchiness, resulting in higher levels of agonistic interactions among individuals [67,68].…”

“…Juvenile phenotypic quality is thought to be strongly influenced by lagged environmental effects [40]. As such, lagged environmental factors [56,57], affecting the availability and accessibility of resources to mothers, as well as population density [36,58], affecting the intensity of competition for resources, can be strong predictors of juvenile body mass. To account for lagged environmental effects on juvenile mass, density, temperature and NDVI data were used not only from year y (the year a cohort was shot), but also from year y-1 (the year of a cohort's birth).…”

Population density and sex do not influence fine-scale natal dispersal in roe deer

“…Although we did not have detailed measurements to compare range quality among the three study sites, between-population differences of fawn body mass in winter can be used as a proxy for range quality in large herbivores (Saether & Heim 1993;Gaillard et al 1996). For a given density, fawn body mass was lower at Chizé than at Bogesund (Kjellander et al 2006), suggesting that dispersal rate at the population level could be inversely related to range quality. Further work will be required to investigate the determinants of natal dispersal patterns of roe deer at a fine scale.…”