Saeideh Esmaeili, a recent graduate from UW’s Program in Ecology, tracks onagers,
also known as Asiatic wild asses, in Qatrouiyeh National Park in southern Iran. She
and other researchers documented that movements of onagers and other members of the
horse family are driven by water availability, while movements of small-bodied ruminants
are driven by forage quality. (R. Shahverdi Photo)
Research led by a University of Wyoming graduate student involving about 50 scientists
from across the globe has provided new insights into the behavior of ungulates (hoofed
animals) as it relates to forage conditions and water availability.
Saeideh Esmaeili, who graduated from UW’s Program in Ecology in December 2020 and
is now a postdoctoral assistant at Colorado State University, is the lead author of
a paper that appears in the journal Ecology Letters. It explores the foraging behavior of
30 species of ungulates across North America, Europe, Africa and the Middle East,
ranging from elk and mule deer in Wyoming to gazelles, impalas, reindeer, wildebeest,
wild asses and zebras.
“By combining GPS relocations with remotely sensed data on forage characteristics
and surface water, we quantified the effect of body size and digestive system in determining
movements of 30 populations of ungulates,” Esmaeili says.
Other UW researchers who participated are Shannon Albeke, Jeff Beck, Jake Goheen,
Matt Kauffman and Jerod Merkle, along with Hall Sawyer, of Laramie-based Western EcoSystems
Technology Inc. Goheen was Esmaeili’s Ph.D. adviser.
The research adds nuance to a long-held hypothesis that ungulates choose to forage
on grasses and other plants when the vegetation is at an intermediate stage of growth,
so as to maximize energy intake. This is called the “forage maturation hypothesis.”
Plants are most digestible at the beginning of the growing season; however, because
they are small during this time, biomass is low. As plants mature, they gain biomass,
but they become harder to digest.
The ungulates that were part of this study include both ruminants, such as deer and
elk, which acquire nutrients by fermenting plants in a specialized stomach prior to
digestion; and equids, such as horses and zebras, which don’t have such specialized
stomachs.
Most notably, the scientists found that smaller-bodied ruminants forage primarily
for the highest energy intake, while equids -- which tend to be larger -- choose to
forage in areas close to surface water, with less attention to forage condition. That’s
likely because equids don’t conserve water as efficiently as ruminants and proportionally
excrete more water. The research also confirmed that ruminants’ ability to process
higher-biomass forage increases with the species’ size.
A primary conclusion of the research is that, while the forage maturation hypothesis
largely holds true, there is some variation based upon ungulates’ body size and digestive
systems.
“The forage characteristics that influence population persistence of free-ranging
ungulates should differ according to body size and digestive system, such that access
to a combination of resource gradients is key to maintaining viable populations of
diverse free-ranging ungulates across the globe,” the scientists conclude.
Other institutions, organizations and agencies whose researchers participated in the
project include Virginia Tech University; Yale University; the U.S. Geological Survey;
Colorado State University; the University of Washington; the Wildlife Conservation
Society; France’s University of Montpellier; South Africa’s University of Pretoria;
Germany’s Senckenberg Biodiversity and Climate Research Centre; Mongolia’s Hustai
National Park Trust; the Cary Institute of Ecosystem Studies; the University of British
Columbia; the Minnesota Zoo; Iran’s Isfahan University of Technology; Japan’s Tottori
University; the Norwegian Institute for Nature Research; Austria’s University of Veterinary
Medicine; the Inland Norway University of Applied Sciences; the Mongolian Academy
of Sciences; and the Smithsonian Conservation Biology Institute.
An onager herd moves across the terrain of southern Iran. (Saeideh Esmaeili Photo)
