Non-random distribution of ungulate salt licks relative to distance from North American oceanic margins

Aim: Terrestrial deposition of aerosol marine sodium declines with
distance from coastlines. Salt deprivation in vertebrate herbivores and
salt-seeking behaviors should hence increase with distance inland. We
analyze published geospatial data on ungulate-patronized salt licks to
test whether they are non-randomly distributed relative to distance from
oceans and elevation. Location: Canada, Alaska, and the contiguous United
States. Taxon: Cetartiodactyla (even-toed ungulates). Methods: We
determined the land area and median elevation of 100 km increments from
the North American coast. The null model of the expected number of licks
within each interval was determined by the ratio of the interval’s land
area to the total land area, multiplied by the total number of licks. We
asked whether the number of licks further from coastlines was
significantly higher than chance. We also assessed whether licks occur
disproportionately at higher elevations, comparing the median elevation of
observed licks to the median elevation within each interval. Results: We
found a strong positive relationship between salt lick patronage by
ungulates and distance from the coast. Licks occurred significantly less
often within, and more often beyond, 500 km inland, and at significantly
higher elevations than would be expected by chance. Main conclusions:
These findings indicate that the patronage of salt licks is constrained
geographically, and that the foraging behavior of ungulates and other
phytophagous vertebrate taxa may be influenced over large spatial scales
by sodium availability. Salt-seeking behavior varies on a wide
biogeographical scale across North America, with concomitant implications
for vertebrate herbivore behavior and ecology.