Research Article
Evaluating Spatial Overlap and Relatedness of White-tailed Deer in a
Chronic Wasting Disease Management Zone
Seth B. Magle mail, * E-mail: SMagle@lpzoo.org
Affiliation: Urban Wildlife Institute, Lincoln Park Zoo, Chicago, Illinois,
United States of America
X Michael D. Samuel, Affiliation: U.S. Geological Survey, Wisconsin
Cooperative Wildlife Research Unit, University of Wisconsin, Madison, Wisconsin,
United States of America
X Timothy R. Van Deelen, Affiliation: Department of Forest and Wildlife
Ecology, University of Wisconsin, Madison, Wisconsin, United States of America
X Stacie J. Robinson, Affiliation: Department of Forest and Wildlife
Ecology, University of Wisconsin, Madison, Wisconsin, United States of America
X Nancy E. Mathews
Abstract
Wildlife disease transmission, at a local scale, can occur from
interactions between infected and susceptible conspecifics or from a
contaminated environment. Thus, the degree of spatial overlap and rate of
contact among deer is likely to impact both direct and indirect transmission of
infectious diseases such chronic wasting disease (CWD) or bovine tuberculosis.
We identified a strong relationship between degree of spatial overlap (volume of
intersection) and genetic relatedness for female white-tailed deer in
Wisconsin’s area of highest CWD prevalence. We used volume of intersection as a
surrogate for contact rates between deer and concluded that related deer are
more likely to have contact, which may drive disease transmission dynamics. In
addition, we found that age of deer influences overlap, with fawns exhibiting
the highest degree of overlap with other deer. Our results further support the
finding that female social groups have higher contact among related deer which
can result in transmission of infectious diseases. We suggest that control of
large social groups comprised of closely related deer may be an effective
strategy in slowing the transmission of infectious pathogens, and CWD in
particular.
snip...
Discussion
Social interactions, as well as group membership, may influence
transmission of wildlife diseases [1], [2] and relatedness may be more important
for transmission than simple proximity [21]. However, proximity can be a poor
surrogate for relatedness [14] or group membership [6]. While related female
white-tailed deer form social clusters on the landscape [9], [21], [44], social
groups may overlap in space, but not in time. Thus, proximity of deer alone is
not enough to discern relatedness, and by extension, the likelihood of
transmission of infectious diseases [21]. Even adult females and fawns trapped
in the same location are not always mother-offspring pairs [45]. The mechanisms
by which related deer transmit infectious disease to one another are unclear,
however. Because volume of intersection is a useful predictor of both direct and
indirect contact rates in deer [6], it appears that related deer are more likely
to come into contact, and therefore drive the dynamics of infectious diseases
[21]. We identified a clear relationship between overlap (as measured by a
volume of intersection) and relatedness for white-tailed deer in south-central
Wisconsin. In addition, we found that age of deer influenced degree of overlap,
with adult-fawn, yearling-fawn, and fawn-fawn pairs overlapping more strongly,
whereas adult-adult pairs, and adult-yearling and yearling-yearling pairs
exhibited lower overlap. Kinship categories were stronger predictors than
continuous Rxy values, suggesting that deer beyond a certain degree of
relatedness exhibit higher amounts of overlap. Even within kinship
classifications, such as half-siblings or parent and offspring, there is
variation in the proportion of shared DNA, and thus degree of relatedness may be
less important than the nature of the social relationship between individual
deer (e.g., parent offspring vs. cousins).
We found that first order kin had 32.5 times as much overlap as unrelated
deer. This value is somewhat larger than a previous finding that deer in
Illinois had 5.0–22.1 times greater odds of direct contact when they belonged to
the same social group, as estimated by proximity [6]. However, our results may
be closer to a separate study in Wisconsin indicating that deer were >100
times more likely to become infected with CWD when a highly related infected
female was in close proximity, with much lower effects from proximal unrelated
animals [21]. This indicates that probability of CWD infection is likely higher
among closely related deer, because they have much higher contact rates, as
opposed to unrelated deer that simply share space, but have lower contact rates.
In addition, a higher probability of transmission may occur because of the more
intense nature of contacts among related deer [46], [47]. Previous observational
studies indicate that parent-offspring pairs engage in significant contact
during the first year of life [46]. Studies that investigate the spatial
dynamics of disease transmission in wild populations should include direct
observation of deer behavior to more thoroughly address the heterogeneous
disease transmission that result from the social structure of deer [22].
Our finding that adult-fawn pairs had higher overlap is not surprising
given patterns of maternal care in white-tailed deer [8]. The average VI of
probable parent-offspring pairs (adult-fawn pairs with Rxy values >0.5) were
very high (0.64 in datasetadult, 0.55 in datasetcapgroup), compared to the
overall mean (0.22 in datasetadult, 0.20 in datasetcapgroup). Adult-fawn pairs
with moderate relatedness value (0.26< Rxy <0 .5="" 0.22="" 0.36="" adult="" and="" approximating="" between="" breed="" datasetadult="" datasetcapgroup="" deer.="" disperse="" div="" establish="" exhibited="" explain="" females="" from="" had="" help="" home="" however="" in="" low="" lower="" may="" mean="" mother="" natal="" of="" on="" once="" overall="" overlap="" pairs="" periphery="" range="" ranges="" rarely="" reduced="" relatedness="" respectively="" s="" slightly="" sometimes="" system="" the="" their="" they="" this="" those="" values="" vi="" which="" with="" xy="" yearling="">
Female white-tailed deer are highly philopatric, characterized by stable
home ranges with a high degree of overlap among individuals within social groups
[6], [9], [44], [46]. However, social structure of deer is less typical where
rates of harvest are high and age structure is biased towards young animals
[14], [48]. Nonetheless, we found that overlap (as measured by VI) closely
associated with degree of relatedness, providing evidence for social structure
at a local scale in spite of heavy harvest pressure. While ongoing disease
eradication efforts may have temporarily increased deer harvest, this population
has been subjected to ongoing harvest for many years, and CWD control efforts
are unlikely to have produced the patterns observed. The strong matriarchal
social structure of female white-tailed deer likely prevents homogenous mixing
of individuals [6], [9] and homogeneous CWD transmission among members of
different social groups [21].
The degree to which deer contact each another varies seasonally [6, 56].
However, to ensure sufficient observations, our analyses were based on annual
data and provide no insight into seasonal patterns. In addition to relatedness,
hotspots of activity such as scrapes, rubs, feeding/baiting sites, and mineral
licks also likely play a role in contact rates of cervids and potential disease
transmission [49], [50], [51]. We did not identify such features in our study
and have no basis to evaluate the contribution of these behavioral hotspots to
potential transmission of disease. Deer may also be more likely to overlap in
agricultural areas due to concentrated food sources [3], [16], [25], [52]. In
fragmented systems, deer would likely congregate closely in areas of remaining
resources, particularly in seasons when food is limited [52]. Unfortunately,
accuracy of the spatial locations in this study was insufficient to investigate
the effects of habitat use, given that the study area is a complex mosaic of
forested and agricultural land [25]. Our study focused on female deer because
they are most often targeted for population control and, unlike males, rarely
engage in long-distance movements [9], [21], [25]. However, males are more
frequently CWD positive than females (Grear et al. 2006), and long-distance
movements by males may be important in the geographic spread of CWD.
CWD can be transmitted both directly (by deer-to-deer contact) and
indirectly (via contamination of the environment), though the importance of
these modes of transmission in the wild are unknown [17], [19], [21]. VI
provides a metric for both direct and indirect contact, though the
spatial-temporal resolution of our data is insufficient to differentiate these
specific events. While it is possible for two deer who overlap in space to avoid
direct contact, indirect contact is virtually guaranteed, particularly given the
likely occurrence of congregation points such as scrapes, rubs, feeding/baiting
sites, and mineral licks. However, given previous findings that contact rates
vary predictably with VI [6], we believe our findings likely apply for both
direct and indirect transmission scenarios. Our findings support previous
research that suggest CWD should spread more rapidly among related deer [21]. As
such, control of large related social groups may be an effective strategy in
slowing pathogen transmission, particularly given that there is little evidence
that female harvest impacts movement behavior [16]. We also found limited
overlap among unrelated deer, suggesting that disease spread among social
groups, which is needed to sustain disease, may occur between neighboring social
groups. We believe the rate and mechanisms of disease transmission between
adjacent social groups is an important area for future research.
We provide an important step in understanding the mechanisms underlying
observed patterns of CWD transmission, namely, that related individuals are more
likely to come into close proximity on the landscape, where disease transmission
may occur either directly or indirectly. Further progress in understanding the
specifics of disease spread will be necessary to devise practical strategies for
deer management.
Citation: Magle SB, Samuel MD, Van Deelen TR, Robinson SJ, Mathews NE
(2013) Evaluating Spatial Overlap and Relatedness of White-tailed Deer in a
Chronic Wasting Disease Management Zone. PLoS ONE 8(2): e56568.
doi:10.1371/journal.pone.0056568
Editor: Justin David Brown, University of Georgia, United States of America
Received: October 5, 2012; Accepted: January 14, 2013; Published: February
20, 2013
Copyright: © 2013 Magle et al. This is an open-access article distributed
under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the
original author and source are credited.
Funding: This project was supported by the Wisconsin Department of Natural
Resources, Whitetails Unlimited, the University of Wisconsin, the National Beef
and Cattleman’s Association, and the North Central Agricultural Experiment
Station Hatch Program. Thanks to the University of Wisconsin’s Department of
Forest & Wildlife Ecology and the Nelson Institute for Environmental Studies
for assistance with publication costs. Note that any use of trade, product or
firm names is for descriptive purposes, and does not imply endorsement by the
United States Government. The funders had no role in study design, data
collection and analysis, decision to publish, or preparation of the
manuscript.
Competing interests: The authors have declared that no competing interests
exist.
Saturday, February 04, 2012
Wisconsin 16 age limit on testing dead deer Game Farm CWD Testing Protocol
Needs To Be Revised
Friday, February 08, 2013
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