The energy intake hypothesis for animal migration has gained support from ungulate research, however determining how individuals exploit the spatial gradient in energy throughout the year has found conflicting results. Here we test a series of predictions that follow from the energy intake hypothesis using GPS locations of 375 wild Norwegian reindeer and extensive maps of seasonal forage availability. The first research goal is to test whether migrations of wild reindeer are maximizing energy access (moving up the energy gradient during both seasonal movements) or minimizing seasonal deficits (only moving up the energy gradient during the season where energy is in decline). The second research goal is to test whether fragmentation of population ranges constrains the ability of individuals to exploit the digestible energy gradient. We compared the migrations and energy access of individuals from three types of population ranges: coastal ranges, interior ranges, and a large range that spans both.
Results/Conclusions
We found that individuals moved up the energy gradient toward their winter range and down it toward their summer range. Individuals would have accessed 13% more energy if they remained in the winter range year-round, suggesting that migrations departing from the winter range are driven by a factor other than energy gain. However, the seasonal deficit incurred by individuals that migrated was negligible. Individuals would have experienced larger seasonal deficits if they remained in the summer range (-417± 21 kJ/m2) or the winter range (-511 ± 21 kJ/m2).
Reindeer in coastal populations accessed the least energy year-round. For every 10 km migrated, reindeer in coastal populations compensated 216 ± 26 kJ/m2, those in interior populations compensated 26 ± 20 kJ/m2, and those in the range that spans both areas compensated 150 ± 17 kJ/m2. For every 1000 km2 in population range size, the average individual compensated 30% more digestible energy during migration to the winter range.
Our results suggest that autumn reindeer migrations support the energy intake hypothesis for animal migration, but the spring migrations do not. Reindeer move to high energy areas for the winter, but population fragmentation constrains the ability of individuals to exploit the energy gradient.
