Environmental phenology and geographical gradients in moose body mass

Intraspecific body mass in ungulates has often been shown to increase with latitude. The biological basis for such latitudinal gradients is, however, poorly known. Here we examined whether satellite-derived indices of environmental phenology, based on the normalised difference vegetation index (NDVI), as well as variables derived from meteorological stations, altitude, and population density, can explain latitudinal gradients and regional variation in body mass of Norwegian moose. The best model gave a considerably better fit than latitude alone, and included all explanatory environmental variables. Accordingly, heavy moose were found in areas with short and intense summers that were followed by long, cold winters, at low altitude relative to the tree-limit, and with low population density relative to the available plant biomass. This relationship was stronger for yearlings than for calves, except for the effect of population density. This indicates that differences in the characteristics of the vegetation quality and environmental phenology, as well as winter harshness and population density, are important factors that shape both the latitudinal and other geographical gradients in moose body mass.