Spatio‐temporal relationship between calf body mass and population productivity in Fennoscandian moose Alces alces

Abstract

Body mass is an important life history trait related to survival, mating success and fecundity in ungulates. Accordingly, we may expect that both body mass and reproductive measurements at the population level can be used as valid indices of population condition. However, several factors may modify the relationship between body mass and fecundity because of trade‐offs between maturity and early body growth, and varying mortality patterns and sex/age structure among populations. To evaluate the use of such indices for population monitoring and examine the current variation in moose Alces alces population condition in Fennoscandia, we studied the spatio‐temporal relationship between calving rate, twinning rate and average autumn calf body mass of moose in Norway and Finland. Calving rate and twinning rate were based on moose observations by hunters while body mass was the average carcass mass of harvested calves. We found a positive relationship between indices both within and among populations. Calves were on average heavier and the observed recruitment rates higher in Finland than in Norway, which is consistent with the higher moose density and presumably lower primary productivity (higher altitude) of moose ranges in Norway. We also found higher observed recruitment rates in populations and years with more even adult sex ratios (females per male) and low relative harvest rates of calves. This suggests that variation in recruitment rate is not only a matter of nutritional condition, but is also affected by varying hunting regulations and harvest structure. For monitoring purposes, we believe that twinning rate is best suited for ranking populations according to nutritional status as this index is closely related to fitness and is relatively insensitive to variation in perinatal and harvest mortality. However, variation in calf body mass may better reflect temporal variation in living conditions. This is because early body growth is sensitive to variation in food availability (and quality) and because body mass may respond more instantaneously than recruitment indices to adverse conditions. Accordingly, we found both calving rate and twinning rate to be best related to variation in mean calf body mass in the previous year.

On fitness and partial migration in a large herbivore – migratory moose have higher reproductive performance than residents

“…Like several other deer species, moose is polytocuous, and regularly produce twins and in a few cases even triplets when experiencing good nutritional conditions (Schwartz 1998, Nygrén 2003. Females of higher body mass for a given age are more likely to produce twins than are smaller females (Saether and Haagenrud 1983, 1985, Sand 1996, and high population twining-rates are typically found in areas with earlier age at maturity, higher parturition rates, lower moose browsing pressure, and higher yearling and calf body mass (Boertje et al 2007, Tiilikainen et al 2012. Because the probability of twin production varies with age (Ericsson et al 2001), we partly controlled for female age in the analyses.…”

Varied diets, including broadleaved forage, are important for a large herbivore species inhabiting highly modified landscapes

“…This result has wide implications, as body conditions early in life can affect many aspects of adulthood, creating long-lasting effects on demographics 22 . It may be such inter-generational relationships that underlie the significant correlation in our study populations between mean calf body mass and estimates of calf production by adult females as in 24 .…”

“…We link variation in moose calf BM among subpopulations to diets and food availability at the landscape level. We used calf BM as an index of subpopulation nutritional status because (i) conditions in early life have long-lasting effects on demographics 22,23 , often leading to strong correlations between calf BM and calf production in a population 24 (also tested in this study); (ii) the use of calves circumvents age effects on BM 25,26 ; and (iii) calves are less affected by hunter bias in the sex and size of individuals harvested 27 . We hypothesise that more diverse diets should be associated with populations having higher calf BM, and place our findings within the context of the potential benefits of diversifying landscapes in regions homogenized by intensive natural resource management.…”

Moose selecting for specific nutritional composition of birch places limits on food acceptability

“…Keeping these multiple nutritional factors and complexities in mind, in this study, we measured nutritional composition of foliage from a staple food source ( Betula pubescens Ehr.) available to, and used by, moose ( Alces alces ) (Figure ) in natural settings of two neighboring regions of southern Norway with contrasting animal fitness (here, indexed by body mass, which is found to capture much of the fitness variance among Fennoscandian moose populations, Tiilikainen, Solberg, Nygrén, & Pusenius, ). Long‐term research focus has not managed to fully explain the contrasts between the populations' food selection and demographic performance (Hagen, ; Hjeljord & Histøl, ; Wam, Histøl, Nybakken, Solberg, & Hjeljord, ).…”