Kylee Wiggins
Title
Group Size and Vigilance in North American Ungulates
Introduction
An array of animals, such as the ungulates of North America, live in conspecific
assemblages, or groups of multiple individuals of the same species. One theory for this
group behavior may be in response to predation. Conspecifics possibly increase
individual protection from predators, relative to living outside of the group (Childress and
Lung 2003). According to this hypothesis, with an increasing number of individuals in a
herd, the probability of a single animal being targeted by a predator may be diluted.
These larger group sizes may allow for individuals to have reduced levels of scanning for
predators, as there are more individuals with added scanning to alert the rest of the group
of potential danger. This scanning, or being alert to danger, such as predators, is termed
as “vigilance”, and is used as a function of the animal’s perceived vulnerability. An
increasing level of predators can be viewed as an increasing level of vulnerability, which
will then likely lead to an increasing level of vigilance in the prey species. If ungulate
conspecifics are in fact a response mechanism to predation, a correlation may be present
between the numbers of individuals within a herd to the level of vigilance they exhibit.
With the historic population declines of some of North America’s most unique animals,
the ungulates, discovering information regarding their social systems may be critical in
preserving their species. Such increasing knowledge could produce positive
consequences in guiding future management and conservation practices. In order to
investigate this topic further, a study was performed at Montana’s Bison Range National
Refuge by observing the behaviors of elk, Cervus canadensis; mule deer, Odocoileus
hemionus; bison, Bison bison; pronghorn, Antilocarpa americana; whitetail deer,
Odocoileus virginianus; and bighorn sheep, Ovis canadensis. I hypothesized that in
comparing each species’ vigilance rate to their group size, there will be a decreasing
proportion of vigilant to non-vigilant behavior as group size increase. This hypothesis is
predicted to be explained by a group exhibiting a larger number of individuals, which
will lead to an increased total level of scanning performed by the entire group, thus
relieving each specific animal of an increasing fraction of essential scanning. This
decreasing need for vigilance through larger herd sizes is beneficial to the animal,
allowing for more time for other essential activities, such as foraging for food.
Study Area
The data recorded to test our ungulate hypothesis about group number and vigilance was
collected at the Bison Range National Wildlife Refuge in western Montana (Figure 1). It
is managed by the U.S. Fish and Wildlife Service and consists of 18,500 acres, protected
primarily for a bison herd of around 300 individuals. In addition to the ungulates,
mammals such as mountain cottontail, Sylvilagus nuttalli; yellow pine chipmunk, Tamias
amoenus; Columbian ground squirrel, Urocitellus columbianus; badger, Taxidea taxus;
and muskrat, Ondatra zibethica; as well as over 220 bird species also inhabit the refuge
(fws.gov). Common predators include coyotes, Canis latrans; grizzly bears, Ursus
arctos; and grey wolves, Canis lupus (Childress and Lung 2003). The refuge habitat
consists of prairie hills and mountains, with relatively more minor areas consisting of
pine forests and wetlands.
Methods
During the weekend of September 20-22, 2013, students from the University of Idaho
conducted scan-sampling at 5-minute intervals to document vigilance behavior of the
ungulate species at the Bison Range. Students were divided into groups of 3-4, and each
group observed an ungulate herd for at least 20 minutes. Herd size and the number of
individuals per herd performing vigilant behavior was recorded for each scan. Vigilance
was defined as the individual showing alertness to danger, and not performing other
tasks, such as lying down, feeding, or walking. Observations were performed using
binoculars and spotting scopes to allow scanning from a far distance, so as not to disturb
the animals (which could possibly skew data, causing extra vigilant time to be recorded
from the herds).
In order to test the hypothesis for group size and vigilance correlation, each ungulate
species’ data was analyzed separately. For each group size per ungulate species, the
number of vigilant observations was divided by the number of observations of the animal
performing any other behavior, which provided the vigilance rate (expressed as a
percentage).
Results
The results consisted of vigilance rates for pronghorn, bison, elk, whitetail deer, mule
deer, and bighorn sheep. In order to perform accurate comparisons between each species
of ungulate, only group sizes between 1 and 37 were analyzed. The vigilance rate results
were compared using a stacked line graph (Figure 2). When analyzing the line graph,
there is no distinct slope for any of the species’ line graphs, concluding that there appears
to be no significant correlation between vigilance and group size according to this
analysis.
Discussion
The hypothesis that in comparing each species’ vigilance rate to their group sizes, there
will be a decreasing proportion of vigilant to non-vigilant behavior as group size
increases would require each species slope on the line graphs to have a distinct negative
slope. Since they did not, we must reject the hypothesis. This may be due to the use of
multiple groups of students recording ungulate behavior differently. Interpretations of
different behaviors will have different effects on the type of data recorded, skewing data
reliability. Equally important may have been due to the random levels of data per group
size analyzed. For some group sizes, dozens of groups were recorded, allowing for a
more accurate group size sample. On the other hand, some group sizes were only
recorded a single time. If each group size were recorded and analyzed at the same rate,
this might have provided a more accurate analysis. Other varying factors for group size
and vigilance may include season, presence of juveniles, and breeding stage of each
animal. Although this data may not have provided accurate results, increasing knowledge
of these ungulates will be important for future conservation of these species.
Literature Cited
Childress, M.J., & Lung, M.A. 2003. Predation risk, gender and the group size effect: does
elk vigilance depend upon the behavior of conspecifics? Animal Behaviour 66:389-398.
U.S. Fish & Wildlife Service National Bison Range. 10 Nov 2013.
<http://www.fws.gov/refuge/National_Bison_Range/wildlife_and_habitat/index.html>
Figure 1. Map of the area of the National Bison Range in Montana, also displaying surrounding
roads. <http://www.fws.gov/refuge/National_Bison_Range/map.html>. Accessed 10 Nov
2013.
120
Vigilance Rates of Pronghorn, Bison,
Elk, Whitetail Deer, Mule Deer, and
Bighorn Sheep
100
80
Pronghorn
Bison
Vigilance
Rate (%)
Elk
60
Whitetail Deer
Mule Deer
40
Bighorn Sheep
20
0
1
3
5
7
9 11 13 15 17 19 21 23 25 27 29 31 33 35 37
Group Size
Figure 2. Results of pronghorn, bison, elk, whitetail deer, mule deer, and bighorn sheep vigilance
rates.