A Low-Cost GPS GSM/GPRS Telemetry System: Performance in Stationary Field Tests and Preliminary Data on Wild Otters (Lutra lutra)
Boitani L (2012) A Low-Cost GPS GSM/GPRS Telemetry System: Performance in Stationary Field Tests and
Preliminary Data on Wild Otters (Lutra lutra). PLoS ONE 7(1): e29235. doi:10.1371/journal.pone.0029235
A Low-Cost GPS GSM/GPRS Telemetry System: Performance in Stationary Field Tests and Preliminary Data on Wild Otters (Lutra lutra )
Lorenzo Quaglietta 0 1
Bruno Herlander Martins 0 1
Addy de Jongh 0 1
Anto nio Mira 0 1
Luigi Boitani 0 1
Rohan H. Clarke, Monash University, Australia
0 Current address: Centro de Investigac a o em Biodiversidade e Recursos Gene ticos - University of E vora , E vora , Portugal
1 1 Department of Biology and Biotechnology ''Charles Darwin'', University of Roma 'La Sapienza' , Roma , Italy , 2 CIBIO - Centro de Investigac a o em Biodiversidade e Recursos Gene ticos , Porto, Portugal, 3 Dutch Otterstation Foundation, Leeuwarden , The Netherlands , 4 ICAAM - Mediterranean Agricultural and Environmental Sciences Institute - University of E vora , E vora , Portugal
Background: Despite the increasing worldwide use of global positioning system (GPS) telemetry in wildlife research, it has never been tested on any freshwater diving animal or in the peculiar conditions of the riparian habitat, despite this latter being one of the most important habitat types for many animal taxa. Moreover, in most cases, the GPS devices used have been commercial and expensive, limiting their use in low-budget projects. Methodology/Principal Findings: We have developed a low-cost, easily constructed GPS GSM/GPRS (Global System for Mobile Communications/General Packet Radio Service) and examined its performance in stationary tests, by assessing the influence of different habitat types, including the riparian, as well as water submersion and certain climatic and environmental variables on GPS fix-success rate and accuracy. We then tested the GPS on wild diving animals, applying it, for the first time, to an otter species (Lutra lutra). The rate of locations acquired during the stationary tests reached 63.2%, with an average location error of 8.94 m (SD = 8.55). GPS performance in riparian habitats was principally affected by water submersion and secondarily by GPS inclination and position within the riverbed. Temporal and spatial correlations of location estimates accounted for some variation in the data sets. GPS-tagged otters also provided accurate locations and an even higher GPS fix-success rate (68.2%). Conclusions/Significance: Our results suggest that GPS telemetry is reliably applicable to riparian and even diving freshwater animals. They also highlight the need, in GPS wildlife studies, for performing site-specific pilot studies on GPS functioning as well as for taking into account eventual spatial and temporal correlation of location estimates. The limited price, small dimensions, and high performance of the device presented here make it a useful and cost-effective tool for studies on otters and other aquatic or terrestrial medium-to-large-sized animals.
Funding: This work was integrated in LQs PhD project on otter ecology and behaviour that was carried out independently (although under formal supervision of
LB) in southern Portugal. A small amount of funding was provided by AM through the Fundacao Luis de Molina of University of Evoraby to help with field
expenses. The Dutch Otterstation Foundation also provided a small discount on the GPS used for this study. The funders had no role in study design, data
collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: AdJ is the Director of the Dutch Otterstation Foundation and the developer of the GPS system and Python script. Although this could be
seen as a competing interest, the Dutch Otterstation Foundation is a non-profit NGO (Dutch Representative of IUCN/SSC OSG), and it has developed this system
for its own purposes. Now that it is successful it is also sold in low volumes to others, but the small profit that is made goes straight back to their environmental
work, as they may confirm. Moreover, neither AdJ nor the Dutch Otterstation Foundation had any role in study design, data collection and analysis, decision to
publish, or preparation of the manuscript, with AdJ contributing only by correcting English mistakes and providing some details on the used material. This does
not alter the authors adherence to all the PLoS ONE policies on sharing data and materials.
Continuous improvements in component technologies, combined
with price reductions for Global Positioning System (GPS) devices is
resulting in their increase use for animal tracking [1,2] and often
preferred to traditional very high frequency (VHF) telemetry .
GPS technologies indeed improve the efficiency and accuracy of
animal locations [4,5], allowing for more flexibility in sampling
design and, often, greater cost-effectiveness in data acquisition [1,5].
Careful field testing to determine accuracy and potential
location biases has always been considered essential for every
form of wildlife telemetry  and GPS telemetry is no exception
. Researchers have performed several studies with the aim
of assessing the effects of habitat type, topography, canopy closure,
vegetation structure, cloud cover, day period, fix time interval, and
other variables on GPS location acquisition and errors .
Nevertheless, as recent as 2010, understanding the causes of GPS
errors has been considered a critical need .
It is therefore surprising that, to date, the performance of GPS
devices used in wildlife telemetry has never been tested specifically
in a riparian habitat, despite it being one of the habitat types most
commonly used by many animal taxa [11,12]. The existence of
such gap is still more noticeable considering that intense canopy
closure, one of the basic constituents of riparian vegetation
galleries, has been identified as one of the major causes of GPS
failure [4,5,13]. Thus, inferences from studies on habitat selection
by riparian animals (or animals that intensively use such a habitat
type) tagged via GPS devices may yield biased results. Such biases
could be even more pronounced in the case of diving animals, as
repeated water submersion could interfere with proper GPS
functioning. Before they can be applied confidently to diving or
riparian animals, GPS technologies should be tested in such a
habitat, and preferably under water.
Eurasian otters (Lutra lutra) are diving mammals that live almost
exclusively in riparian habitats , often selecting dense
vegetation cover [15,16]. They seem, therefore, a suitable species
on which to test the reliability of GPS tracking systems in riparian
habitats, on diving animals, and in dense canopy closure
conditions. In addition, this species provides an opportunity to
test GPS performance on a medium-sized carnivore, with most
previous studies considering larger mammals [17,18].
Most of the GPS wildlife studies that have been performed so far
utilized commercial GPS devices; and, although costs associated
with such devices have decreased over the last few years, they
typically remain very expensive, limiting their use in projects with
budget limitations [10,17]. Consequently, we developed a low-cost
easily constructed GPS GSM/GPRS system (H Dutch Otterstation
Foundation, Netherlands) and examined its functioning, under
field conditions, in a series of stationary tests performed in
The main objective of this study was to test the performance of
our GPS device in a riparian habitat and at different depths under
water. We, therefore, assessed the effects of three habitat types
(including the riparian), canopy closure, and water submersion, as
well as the influence of certain climatic variables on the rate of
successful localization and accuracy of our GPS system, while also
investigating for any potential temporal or spatial correlations in
data. In addition, we fitted the device on wild otters (Lutra lutra) for
the first time to date. This preliminary study allowed us to evaluate
our devices performance on free-ranging animals within the
habitats they typically use.
We conducted our study from April 2009 to May 2010 in
Alentejo, in Southern Portugal. Portugal is a country where otters
are widely distributed . The climate in the study area is
typically Mediterranean , with an average annual rainfall of
600700 mm, and rainy days occurring mostly from October to
April (http://www.cge.uevora.pt/). Relief is mostly plain with
smooth elevations, ranging from 200 to 370 m. The dominant
land use is the traditional Mediterranean woodland, designated
montado, consisting of cork oak (Quercus suber) and/or holm oak
(Quercus ilex) stands combined with extensive agriculture, forestry
and livestock grazing . Streams, ponds, and small dams are
common, providing an almost continuous water network. The
riparian vegetation is dominated by alders (Alnus glutinosa), poplars
(Populus nigra), willows (Salix atrocinerea), and brambles (Rubus spp.),
which provide refuge for otters [22,23]. Human settlements are
concentrated in cities and small villages, with a few farmhouses
scattered throughout the landscape.
As a GPS receiver, we used a GE863-GPS (H Telit, Italy), the
smallest (41.4631.463.6 mm) combined GPS Global System for
Mobile Communications/General Packet Radio Service (GSM/
GPRS) module available in the market at the time of the study,
integrating full 20-channel GPS functionality mounted onto a
Printed Circuit Board (PCB) (Video S1).
To power the device, we chose a VARTA LiPo battery with
2500 mAh capacity and an estimated average life of 42 days at 4
location records per day (Video S1). To save power, the unit made
use of awakedeep sleep cycles  on which at each attempt, the
unit recorded a location during a 24380 second period
(average = 145.2 seconds; SD = 112.1) and then turned to a low
consumption operation mode for the rest of the time until next
To the module we connected an encapsulatable GPS antenna
for marine and submarine applications (Wi-Sys Communication
Inc.) and a PCB GSM antenna (Video S1). Overall dimensions
reached approximately 65 mm length645 mm width628 mm
thickness, for a total weight of 84 g and a price of 630,25 J
We coated all components with polyethylene foam and encased
them in an enclosure made of a shrinkable tube of Thermoplastic
Polyolefin sealed through heat to prevent water penetration (Video
S1). We then covered the tube with a thin layer (about 5 mm) of
Epoxy Adhesive Super Glue for stronger protection from water,
bites, and scrapes against rocks or bushes.
GPS devices usually are programmed by researchers to acquire
locations according to a requested sampling schedule and battery
limits . The Telit module has an onboard PythonTM application
script interpreter. A GPS data-reporting script using GPRS was
produced (H Dutch Otterstation Foundation), with a remote online
potential for changing reporting times. We retrieved location data
through a combined GSM/GPRS service from a pre-paid SIM
card and stored them in an online MySQL database, allowing for
visualization of locations directly on Google maps (Fig. 1). The
software had a data logger, capable of storing (up to 100) locations
whenever the GSM network was not available and sending them
during the subsequent reporting attempt with good GSM
coverage. We tested two different versions of the GPS software.
The second version differed from the first in having a 10-second
longer time period for location logging.
Each successful GPS attempt provided the following
information: geographic coordinates, date, time, battery state, location
status (1D, 2D, and 3D fixes, based upon the number of satellites
detected) and the time employed by the unit to retrieve, given a
From July to December 2009, we performed a series of field
tests. Test sessions lasted, on average, 5.54 h, with 1 location
scheduled every 10 or 20 minutes, for a total of 390 attempts. In
each session, we placed GPS harnesses at test sites above the
ground at roughly the same level as an otters height. We selected
test sites with a purposive convenience sampling criterion ,
stratified on the basis of the different habitat types (Table 1) used
by otters monitored (by VHF radio-telemetry) within the
framework of a project on the ecology and behaviour of this
species at the study site (L. Quaglietta, unpublished data). Test
sites located in the middle of dams (on the water surface) were
pooled with those located in open woodland, as the former were of
limited sample size (N = 24) and both shared almost no vegetation
Figure 1. Locations of a wild-free-ranging otter accessed on the Internet in real time. They concerns the GPS harness (H3) (male otter) and
are projected on a Google map. Yellow points represent each location; the red point is the capture (and release) site.
Besides habitat type, during each session we recorded other
environmental variables (Table 1) and, specifically for the riparian
habitat, GPS inclination towards the sky, GPS position within the
riverbed, and underwater depth at which the GPS was submerged
(at a mean depth of 0.4960.26 m, based upon information on otter
swimming behavior in the study area L. Quaglietta, unpublished
data). A meteorological station located at the core of the study area
furnished some fine temporal-scale climatic data (Table 1).
We defined two types of success rate: the GSM Fix-Success Rate
(GSM SR), computed as the proportion of scheduled locations
transmitted by the GSM device even if it resulted in no
coordinates; and the GPS Fix-Success Rate (GPS SR),
corresponding to the proportion of scheduled attempts that resulted in
successful location acquisition.
We gave location accuracy, hereafter GPS Location Error (GPS
LE), as the Euclidean distance in meters between each individual
GPS software version: (1) older version; (2) newer version
Habitat types: (1) human settlements; (2) open woodland; (3) riparian
Canopy closure: (1),60%; (2) 61100%
GPS submersion or emersion: (1) Underwater; (2) Out of water
Classes of depth at which the GPS was submerged: (1) Out; (2) 030 cm; (3) 3150 cm; (4) 51150 cm
Daily period: (1) Night (2) Day
Number code identifying GPS locality
Date of the field test
Orientation of the GPS: (1) 90uL; (2) 45u; (3) 90uR; (4) 180u
GPS position relative to the river bed: (1) Near river bank; (2) Center of the river bed
Solar radiation (W/m2)
Scheduled attempts resulting in successful (1) or unsuccessful acquisition (0)
Error value of GPS locations comparing with their actual true value (m)
The fine temporal scale (10610 minutes) climatic data were provided by the Geophysics Centre of the University of Evora (http://www.cge.uevora.pt/).
test location and the true position , established by a portable
GPS receiver (Garmin eTrexH H with high-sensitivity
WAASenabled). The use of a portable GPS, which is also prone to error,
could lead to low accuracy. Unfortunately, we did not have access
to a reference station, which could have allowed us to limit such
problems by performing a differential correction of the
coordinates, as done in other studies [25,26]. However, we think that our
solution may be regarded as acceptable, considering the scope of
this study (which was more focused on identifying which
environmental variables influence the performance of our GPS
system than exactly quantifying the associated errors), the high
accuracy displayed by the portable GPS (3.93 m; SD = 1.12;
range: 27; N = 305) and the very high percentage of 3 D fixes
the most accurate [25,27,28,29] - gathered both in field tests and
by wild marked otters (see Results section).
We modeled the influence of canopy density (fCanopy2), daily
period (fLight), habitat type (fHabtype), GPS submersion
(fWatSub), software version (fSftw), solar radiation (SR), wind velocity
(W), precipitation (Rain) (Table 1) and the interactions between
SR and W on GPS SR through a generalized linear mixed-model
(GLMM) with a binomial error distribution. We named this model
the Overall acquisition model.
We evaluated the effects of the same explanatory variables on
GPS LE through a Gaussian GLMM, named Overall accuracy
model. In order to approach normality and stabilize the
variances, we transformed (log x + 1) the response variable.
As a second step, we wanted to test the device only in the
riparian habitat and using only the last GPS version (the one that,
so far, performed better, had the data-logger and that was going to
be used later on wild otters). We, therefore, developed two similar
models, only including data taken in this habitat type, naming
them Riparian acquisition model and Riparian accuracy
model. Besides the original variables, we included three more
specific variables: GPS inclination, GPS position, and Depth
(Table 1). We transformed (log x + 1) the response variable in the
Riparian accuracy model. Moreover, we performed a T-test (equal
variances not assumed) on the GPS LE average values taken while
the GPS was under and out of water.
We based model selection on the Information Theoretic
Approach and performed it through Akaikes Information
Criterion (AIC) , following the protocol suggested by Zuur
et al. . Consequently, we further validated the best model by
graphic inspection [31,32].
The resulting temporal and spatial correlation in the data, that
is, the influence of the day and site of the trial on GPS acquisition
and accuracy, has been seldom addressed in previous studies (but
see Discussion section). To reduce the former, we used the date of
field test trials as a random term (Table 1). As for the spatial
correlation, we divided the hydrographic network into
evenlyspaced (about 2 km) stretches, assigned each GPS test site to a
stretch, named fLocal (Table 1), and fitted it also as a random
term. Moreover, we used the variable GPS as random term, in
order to take into account the variability between each individual
GPS used in the field tests. The Riparian accuracy model was
more difficult to run without convergence problems, so it was
necessary to simplify it to avoid interaction terms .
All analyses were performed using R software, version 2.11 (R
Development Core Team 2009 - http://www.R-project.org).
Since abdominal implantation of a GPS device is not possible,
due to the requirement for an external antenna, implementing this
technology on otters must overcome methodological constraints. A
standard collar has already been deemed impractical or even risky
for otters and it is not recommended for these or other mustelids,
because of the similar diameter of their neck and head [14,3437].
Harnesses seem to be the most reliable alternative for the
application of GPS on otters, to date. In fact, they have been
already used on otters, and, although some concerns regarding
animal welfare and possible risks exist [14,35,38], no significant
events are reported in the cited studies.
We fitted harnesses with the second software version of the
GPS-GPRS device onto six wild otters live-trapped during the
already-cited ecological project. Otter trapping and handling
procedures followed [38,39], being in accordance with the
guidelines approved by the American Society of Mammalogists
for the use of wild mammals in research . The protocol was
approved by the Portuguese Institute for Nature and Biodiversity
Conservation (license permits Nu 104/2009 and 105/2010).
Before fitting wild animals, we evaluated the impact of the
GPSharness on a wild-born female otter in captivity, at the Santo
Andre Wildlife Recovery Center (Video S1).
We attached the previously-described case to a thin leather
harness (Video S1). For mounting the harnesses on otters, we
followed the procedures of two previous studies which deployed
VHF-harnesses on the same species [35,38]. We partly constructed
harnesses beforehand, so as to only activate and fit GPSs in the
field. This avoided longer animal-handling periods, saved battery
power, and allowed for the necessary initial good reception of
satellites under open sky.
We added a small VHF radio transmitter to the GPS unit to
allow for device retrieval after it dropped off the animal. Drop off
was expected to be provoked by wear and tear of the leathers
straps. Each harness weighed approximately 220 g, accounting for
4.8% of the weight of the smallest fitted otter and costing
approximately 40 J in materials (i.e. leather, tube, glue, tapes). We
collected data at a cost of one J per day, independent of the
programmed schedule. We scheduled global positioning devices
(GPDs) to record locations during nocturnal hours, coinciding with
the period of major activity by otters [14,22], especially in the
study area (L. Quaglietta, unpublished data).
When harnesses apparently had dropped off, we made an
attempt to recover them, in order to reuse the GPDs, as well as to
compute location accuracy in real ottery places (Euclidean
distance in meters between the true ottery position, measured by
the portable GPS, and the location recorded by the GPD). We
used data from marked otters to compute the overall GSM and
GPS SR, as an indication of GPS performance on free-ranging
Equipment development and field tests
The final average price of a single GPS-GPRS device, materials,
and assembling labor was roughly 790 J.
The wild born otter in captivity appeared to move and behave
naturally, paying almost no attention to the GPS harness on its
back after the first hour (Video S1). After releasing the animal into
the wild, the GPS worked very satisfactorily, yielding a GSM SR
of 88% and a GPS SR of 71% (N = 66).
Stationary field-test harnesses successfully acquired locations at
the different test sites. Out of the 390 attempts, 263 locations were
successfully retrieved by the GSM service, yielding a GSM SR of
67.4%. Satellites were successfully acquired on 239 occasions,
giving a GPS SR of 61.2%; within these successful locations,
98.8% were 3D locations and 1.2% 2D. The GPS LE varied
widely, considering both software versions used, from 0 to 400 m
(mean = 19 m). However, the observed variability in the second
version (the one used later on wild otters) was smaller, generating
an average GPS LE of 8.94 m (SD = 8.55; range: 041 m, with
50% of locations within 6 m and 95% within 27 m; N = 193),
much lower than the 60.38 m (SD = 92.23; range: 0400; N = 45)
of the first. GPS LE computed only in the riparian habitat was
According to the Overall GPS acquisition model, the GPS SR
was much higher in human settlements than in the other 2 habitat
types (P = 0.015 and 0.008, respectively, for open woodland and
riparian). Software version played a very important role (having
the highest estimate between the coefficients), with the third
version of the software performing significantly better (P = 0.007).
Surprisingly, the percentage of canopy had a positive effect on the
GPS SR (P = 0.003). Higher solar radiation values were positively
associated with the GPS SR (P = 0.009) (Table 2). The model with
two random effects (fDate and fLocal) was selected, with fDate
accounting for the majority of variability in these data (SD: 1.94),
while both AIC values and likelihood ratio test showed no
meaning of using GPS as random term. No effects were noticed
related to the period of day, wind or precipitation.
The Overall accuracy model (Table 3) revealed a reverse
pattern, in terms of habitat type: the GPS LE was lower in open
woodland (P = 0.007) and in the riparian zones (P = 0.046) than in
human settlements (although, in the latter case, the relationship
was only marginally significant). Performance, in terms of
accuracy, therefore seemed to be higher in these two habitat
types, contrary to what happened with the GPS SR model. The
third software update version improved GPS accuracy (P#0.023).
Solar radiation, wind and precipitation did not exert any
influence, while nightly hours were positively associated with the
GPS LE (P = 0.025). Spatial correlation was limited, having a
standard deviation of 0.43. Models with observations nested by
individual GPS were selected, with a GPS standard deviation of
0.40. In the Riparian acquisition model, GPS SR was mostly
influenced by GPS inclination (whatever the angle was), water
submersion (independent of the depth), and, secondarily, by
position in the riverbed (Table 4). Indeed, attempts with the device
under water, oriented in any position other than horizontal, and
located near the river bank significantly worsened GPS SR results.
Moreover, high levels of solar radiation seemed to be associated
with higher GPS SR (P#0.001). Also in this case, the model with
two random effects (fDate and fLocal) was selected and fDate
accounted for most of the variation (SD = 3.68).
As with the Riparian accuracy model, the GPS LE primarily
was negatively affected by water submersion and location within
the river bank (Table 5). Both AIC values and likelihood ratio tests
of the nested models indicated that the best model was the one
with fLocal as the only random term (SD = 0.44).
Average GPS LE underwater was significantly bigger
(mean = 13.37 m; SE = 1.35; N = 41) than the one out of water
(mean = 6.58 m; SE = 0.55; N = 96) (t = 24.64; df = 53.56;
P = 0.00).
GPSs mounted on wild otters
We fitted six wild free-ranging otters with GPS-GPRS harnesses
(Table 6) (Video S1). All harnesses gathered data, corresponding to
a total of 711 locations, for overall GSM and GPS SR of 86.5%
and 68.2%, respectively.
Four harnesses worked well (H1, H2, H3 and H6), obtaining all
581 location programmed, giving a GSM SR of 100%. Within the
retrieved locations, the positional information was successfully
acquired 445 times, yielding an average GPS SR of 78.0%, with
84.7% in 3D and 15.3% in 2D (Table 6). The remaining 2
harnesses (H4 and H5), despite having retrieved 130 of the 203
programmed locations (average GPS SR = 48.6%; 83.9% were 3D
and 16.1% 2D), did not transmit at the requested frequency, but
instead at irregular times, because of software malfunctioning.
These locations, however, still seemed highly accurate, as points
were never located far from watercourses (Fig. 1).
The monitoring period for each animal averaged 9 days,
ranging from 4 to 15 (Table 6). Four harnesses (H1, H3, H5 and
H6) were recovered from the field. Their accuracy was, on
average, within 4.3 m. Recovered harnesses revealed good
condition of the transmitter case (only a few minor scratches)
with ventral and dorsal straps weakened by water and abrasion,
suggesting easy release by the otters. Harness H2 was recovered
with the carcass of the animal, killed by humans as revealed by
necropsy (L. Quaglietta, unpublished data). The stomach was full
of food and the general condition of the animal was good,
suggesting that the harness did not have an impact upon its death.
Overall acquisition model
The harness was still in good condition and well-deployed on the
otters body; consequently, it likely would have remained attached
for a few more days.
The female marked with harness 5 was recaptured after she
dropped off the device. Because of this, it was possible to
document that her body showed no signs of previous injuries and
that she was lactating. Based upon the scant existing information
on wild otter gestation periods  and on the time passed
between captures, we estimated that this animal reproduced while
still with the harness or immediately after it fell off. Animals fitted
with GPS harnesses (Fig. 2) showed space use patterns similar to
those of otters monitored by VHF telemetry in the same project
and area (L. Quaglietta, unpublished data) and were observed
while swimming and moving with no apparent restrictions on their
movements (pers. obs.).
The wide applicability and benefits of GPS technologies in
wildlife telemetry are increasingly cited in the literature [2,9].
However, most GPS functioning tests have examined devices
produced industrially and designed for large mammals .
Moreover, GPS functioning has never been previously tested on
diving freshwater animals or in a riparian habitat; and, to our
knowledge, only a few studies have evaluated the potential effects
of climatic variables (see Introduction).
Here, we have presented a manufactured GPS-GPRS device
that is affordably priced (similar to another low-cost GPS system
recently used on pampas deer - ); and we provide information
on its performance in riparian as well as other habitat types, and
on wild otters.
Overall accuracy model
Riparian acquisition model
categories even exerted a positive effect upon GPS SR (but see text
below for a discussion of the possible negative effects of dense
Several authors have reported that habitat characteristics may
significantly reduce location acquisition [13,27,47]. In our study,
open woodlands and riparian habitats were associated with more
accurate locations than human settlements (although the p-value
concerning the relationship with riparian habitat only approached
significance), which is a very unexpected and reassuring result for a
GPS that has to be used on riparian animals. A possible
explanation is that human settlements may be more susceptible
to different types of interference, and these may have played a role
in the process of location logging, thereby lowering accuracy. On
the other hand, urban areas exhibited higher acquisition rates than
the other two habitat types, possibly because, in the former
habitat, GPDs remained in more flat and stable conditions, which
may have had a positive influence upon satellite acquisition. The
high values of solar radiation, which were associated with higher
GPS SR, are normally related to clear skies which, in turn, could
have allowed for better satellite availability. In this regard, a
positive relationship between sky availability and GPS functioning
has been documented already [26,42]. However, the very small
coefficient estimate value of the solar radiation suggests care in the
interpretation of the effect of this variable.
Riparian accuracy model
The GSM and GPS Fix-Success Rates of our GPS-GPRS
system are well within the range reported by other authors who
have used commercial GPS radio-collars [1,7,41].
The higher amount of logging time provided with the second
software update was very effective, leading to an average GPS LE
of 8.9 m. This result is in accordance with previous studies that
identified a positive relationship between logging duration and
GPS performance . Importantly, GPS accuracy was high
even if computed only in the riparian habitat. Such error values
are lower than the mean of most of those reported in GPS
wildlife-tracking studies, which is roughly 1030 m
[4,5,13,25,26,43,44]. The remarkable frequency of 3D locations,
which are normally related to higher accuracy [13,25,27,29],
seems to confirm the high reliability and accuracy of our GPD. In
addition, a visual examination of the locations collected in the
riparian habitat, both in field tests and by tagged otters, revealed
that these were always located within the narrow (10 m) band of
riparian vegetation of the sampled rivers, thereby appearing quite
accurate (Fig. 1).
Unlike other studies, where canopy density has been reported to
significantly reduce the rate of successful location acquisition
attempts [45,46], we failed to detect any sound influence of this
variable; and in our Overall acquisition model, denser canopy
1 Oct 09
Underlined data refer to the harnesses that had limitations due to software malfunction. TWL = total of the harnesses without limitations. NPL = number of programmed
locations. NRL = number of retrieved locations. NSL = number of successfully acquired locations. MP = monitoring period.
Figure 2. Wild free-ranging female otter (Lutra lutra) fitted with a GPS harness after the release.
As concerns riparian models, GPS SR and LE largely were
affected by water submersion (independent of depth) and,
secondarily, by GPS position within the riverbed. The negative
effect of water submersion, both on the probability of localization
and location accuracy, is not surprising. So far, and to the best of
our knowledge, no previous study has tested GPS devices under
freshwater conditions, so no comparisons are available. However,
since otters are not completely submerged most of the time (they
mainly plunge during predatory activities)  (pers. obs.), the
error due to submersion may not be such a strong limitation as is
often noted for marine mammal GPS tracking [48,49]. This
conjecture is further supported by the high success rate achieved
when the GPS was mounted on wild free-ranging otters (see next
section). The explanation regarding the position within the
riverbed may strengthen the call for caution concerning the
potential negative effects of very dense canopies. Indeed, when
GPDs were placed on the river bank, they were between roots, or
under dense shrubs and rocks, simulating usual otter resting
habitats . Consequently, these elements may have constituted
a more robust barrier to satellite availability than the taller
arboreal riparian vegetation, which did not exhibit a negative
effect in the Overall Accuracy model and even exerted a positive
effect in the Overall Acquisition model.
The inclination of the GPS, whatever the angle, promoted a
lower GPS SR, in accordance with what was reported by  and
identified in other studies . This can lead to missing locations,
since GPS inclination may occur frequently during an animals
normal activities . However, it is worth noting that GPS LE
was not influenced by this variable and that the GPS SR of wild
otters was even higher than that obtained in the field tests (see
Results and the next section), contrary to what occurred in other
studies [8,33]. Solar radiation also, in this case, seemed to have a
positive effect on gathering the signal, possibly confirming the
importance of clear weather.
Thus, constraints in tracking otters or other aquatic mammals
with the presented GPS device mainly seem related to the amount
of time animals spend under water and, secondarily, to the
frequency with which they are near the river bank, instead of at
the center of the riverbed, or burrowed in dens. On this regard, no
locations were acquired by a GPD fitted on a wild free-ranging
otter (H2) when the latter was inside a hole in the ground (pers.
obs.). It is therefore highly suggested to avoid localization trials
during the period of otter inactivity.
Random effects accounted for some variability in the datasets,
in every model, suggesting that the particular day (and,
secondarily, site) in which the tests were done may have played
a role and that there was some slight variability in the performance
between the individual GPDs. Ignoring temporal and spatial
correlations can, therefore, lead to biased results. Hence, we
suggest that researchers take these factors into account when they
evaluate GPSs in stationary tests, as only rarely done (but see ),
adapting to what instead is frequently done in habitat selection
GPS devices mounted on wild otters
GPS harnesses on wild otters performed very well, as overall
GPS SR was indeed higher than in field tests and a very high
frequency of 3D locations (84.4%) was obtained. These results are
surprising, given that GPS SRs from free-ranging animals
traditionally have been lower than those obtained during
stationary tests [5,25,33] and that otters used the same habitats
that we used for the stationary tests.
Other authors who previously used harnesses on otters have
expressed some concerns because of potential deleterious effects
upon the animals themselves, both related to friction from the
harness material and the risk of the harness becoming entangled
under water [35,38]. In our case, no injuries related to the
equipment were noted for any monitored otter. On the contrary,
we documented one female otter actually lactating shortly after the
monitoring period with the harness. Moreover, the fur of this
recaptured female revealed no signs of abrasion from the harness.
At this stage, the applicability of our GPS telemetry system may
be primarily limited by the harness weight/dimensions and
retention times. Indeed, although the equipment was lighter than
most previously-used devices  and weighed less than 5% of the
fitted otters body mass, as generally recommended , the
harnesses still were not appropriate for cubs/young otters or other
animals weighing less than 4 kg. Its application on smaller animals
(up to 23 kg) remains possible, however, albeit with modifications
(e.g. collars, which are smaller and lighter than the harnesses). Our
retention times were quite inferior to those obtained in previous
studies of harnessed otters [35,38], in which retention times
averaged 3040 days and extended as long as 98 days. Such a big
difference may be explained partially by a difference in harness
dimensions (our GPS harness was larger and this could have
produced more friction and consequent wear), and our probably
exaggerated concern about securing the harnesses tightly.
However, an ongoing study in Ireland is showing that obtaining
longer durations is possible for otters using GPS harnesses, as
otters marked with GPS harnesses very similar to ours already
have furnished data up to 40 days ( O Neill and De Jongh, personal
Based upon the results of this study, we propose a reliable and
low-cost method of GPS telemetry that seems to be reliable in
riparian habitats and diving animals. Demonstrating a low average
error, the tool seems to be suitable for a wide range of studies
[2,5,45], especially in short-term research for fine temporal-spatial
scale resource use, monthly home-ranges, activity patterns, and
social interactions, among others. With a very convenient trade-off
between price and quality, it represents a useful and cost-effective
alternative to available commercial GPS devices for research on
otters and many other medium to large sized terrestrial and diving
mammals (as well as on smaller animals through the use of collars).
Moreover, GPS biases (affecting both accuracy and success rate)
appear to be affected by many other factors, which are too
numerous to identify at this time . This highlights the need to
undertake more site-specific pilot studies , aimed at addressing
and identifying the main factors affecting GPS performance in
specific study areas, habitat types and animals, in order to properly
correct for GPS errors and missing locations in posterior habitat
selection studies .
Video S1 Videos of GPS assembling and fitting on an otter in
captivity and a GPS-harnessed wild free-ranging otter.
Some of the GPS devices were partially sponsored by the Dutch
Otterstation Foundation and Fundacao Lus de Molina. The aid of B.
Wroblewski was indispensable in the development of the python software.
We thank L. Lerone, who helped in the field tests data collection; D.
Cardador and M. Fernandes, for their cooperation concerning the
wildborn otter in captivity; Tj. de Jong who helped with otter trapping, and two
anonymous referees whose suggestions considerably improved the article.
Conceived and designed the experiments: LQ. Performed the experiments:
BHM LQ. Analyzed the data: LQ. Contributed reagents/materials/
analysis tools: AM AdJ LQ. Wrote the paper: BHM LQ AdJ LB AM.
1. Rodgers AR ( 2000 ) Recent Telemetry Technology . In: Boitani L, Fuller TK , eds. Research techniques in animal ecology. Controversies and consequences . New York : Columbia University Press . pp 79 - 121 .
2. Tomkiewicz SM , Fuller MR , Kie JG , Bates KK ( 2010 ) Global positioning system and associated technologies in animal behavior and ecological research . Philos T Roy Soc B 365 : 2163 - 2176 .
3. Pellerin M , Sad S , Gaillard JM ( 2008 ) Roe deer Capreolus capreolus home-range sizes estimated from VHF and GPS data . Wildlife Biol 14 : 101 - 110 .
4. Hulbert I , French J ( 2001 ) The accuracy of GPS for wildlife telemetry and habitat mapping . J Appl Ecol 38 : 869 - 878 .
5. Frair JL , Fieberg J , Hebblewhite M , Cagnacci F , DeCesare NJ , et al. ( 2010 ) Resolving issues of imprecise and habitat-biased locations in ecological analyses using GPS telemetry data . Philos T Roy Soc B 365 : 2187 - 2200 .
6. Withey JC , Bloxton TD , Marzluff JM ( 2001 ) Effects of Tagging and Location Error in Wildlife Radiotelemetry Studies . In: Millspaugh JJ, Marzluff JM, eds. Radio Tracking and Animal Populations . San Diego: Academic Press. pp 45 - 75 .
7. Gau RJ , Mulder R , Ciarniello LJ , Heard DC , Chetkiewicz CLB , et al. ( 2004 ) Uncontrolled field performance of Televilt GPS-Simplex TM collar on grizzly bears in western and northern Canada . Wildlife Soc B 33 : 693 - 701 .
8. Graves TA , Waller JS ( 2006 ) Understanding the causes of missed global positioning system telemetry fixes . J Wildlife Manage 70 : 844 - 851 .
9. Cagnacci F , Boitani L , Powell RA , Boyce MS ( 2010 ) Animal ecology meets GPS-based radiotelemetry: a perfect storm of opportunities and challenges . Philos T Roy Soc B 365 : 2157 - 2162 .
10. Hebblewhite M , Haydon DT ( 2010 ) Distinguishing technology from biology: a critical review of the use of GPS telemetry data in ecology . Philos T Roy Soc B 365 : 2303 - 2312 .
11. Rondinini C , Ercoli V , Boitani L ( 2006 ) Habitat use and preference by polecats (Mustela putorius L.) in a Mediterranean agricultural landscape . J Zool 269 : 213 - 219 .
12. Matos HM , Santos MJ , Palomares F , Santos-Reis M ( 2009 ) Does riparian habitat condition influence mammalian carnivore abundance in Mediterranean ecosystems? Biodivers Conserv 18 : 373 - 386 .
13. Di Orio AP , Callas R , Schaefer RJ ( 2003 ) Performance of two GPS telemetry collars under different habitat conditions . Wildlife Soc B 31 : 372 - 379 .
14. Kruuk HA ( 2006 ) Otters: ecology , behaviour, and conservation Kruuk HA, ed. New York : Oxford University Press . 265 p.
15. Green J , Green R , Jefferies DJ ( 1984 ) A radio-tracking survey of otters Lutra lutra on a Pertshire river system . Lutra 27 : 85 - 145 .
16. Durbin LS ( 1998 ) Habitat selection by five otters Lutra lutra in rivers of northern Scotland . J Zool 245 : 85 - 92 .
17. Zucco CA , Mourao G ( 2009 ) Low-Cost Global Positioning System Harness for Pampas Deer . J Wildlife Manage 73 : 452 - 457 .
18. Mattisson J , Andren H , Persson J , Segerstrom P ( 2010 ) Effects of Species Behavior on Global Positioning System Collar Fix Rates . J Wildlife Manage 74 : 557 - 563 .
19. Trindade A , Farinha N , Florencio E ( 1998 ) A distribuicao da Lontra Lutra lutra em Portugal - situacao em 1995 . Estudos de Biologia e Conservacao da Natureza , 28 . Lisbon: Instituto da Conservacao da Natureza. 139 p. (in Portuguese) .
20. Carmel Y , Flather CH ( 2004 ) Comparing landscape scale vegetation dynamics following recent disturbance in climatically similar sites in California and the Mediterranean basin . Landscape Ecol 19 : 573 - 590 .
21. Pinto-Correia T , Ribeiro N , Sa-Sousa P ( 2011 ) Introducing the montado, the cork and holm oak agroforestry system of Southern Portugal . Agroforest Syst 82 : 99 - 104 .
22. Beja P ( 1996 ) Temporal and spatial patterns of rest-site use by four female otters Lutra lutra along the south-west coast of Portugal . J Zool 239 : 741 - 753 .
23. Mason CF , Macdonald SM ( 1986 ) Otters: ecology and conservation . Cambridge : Cambridge University Press . 236 p.
24. Thompson WL , White GC , Gowan C ( 1998 ) Monitoring Vertebrate Populations . San Diego: Academic Press, Inc. 365 p.
25. Cargnelutti B , Coulon A , Hewison AJM , Goulard M , Angibault JM , et al. ( 2007 ) Testing Global Positioning System performance for wildlife monitoring using mobile collars and known reference points . J Wildlife Manage 71 : 1380 - 1387 .
26. Sager-Fradkin KA , Jenkins KJ , Hoffman RA , Happe PJ , Beecham JJ , et al. ( 2007 ) Fix Success and Accuracy of Global Positioning System Collars in OldGrowth Temperate Coniferous Forests . J Wildlife Manage 71 : 1298 - 1308 .
27. Moen R , Pastor J , Cohen Y , Schwartz CC ( 1996 ) Effects of moose movement and habitat use on GPS collar performance . J Wildlife Manage 60 : 659 - 668 .
28. Dussault C , Courtois R , Ouellet JP , Huot J ( 2001 ) Influence of satellite geometry and differential correction on GPS location accuracy . Wildlife Soc B 29 : 171 - 179 .
29. Rempel RS , Rodgers AR ( 1997 ) Effects of differential correction on accuracy of a GPS animal location system . J Wildlife Manage 61 : 525 - 530 .
30. Burnham KP , Anderson DR ( 2002 ) Model selection and multimodel inference: a practical information-theoretic approach . Second Edition Burnam KP, Anderson DR, eds. New York : Springer Verlag. 488 p.
31. Zuur AF , Ieno EN , Walker N , Saveliev AA , Smith GM ( 2009 ) Mixed effects models and extensions in ecology with R Gail M, Krickeberg K , Samet J , Tsiatis A , Wong W, eds. New York : Springer science. 575 p.
32. Zuur AF , Ieno EN , Smith GM ( 2007 ) Analysing Ecological Data Gail M, Krickeberg K , Samet J , Tsiatis A , Wong W, eds. New York : Springer. 672 p.
33. Schwartz CC , Podruzny S , Cain SL , Cherry S ( 2009 ) Performance of Spread Spectrum Global Positioning System Collars on Grizzly and Black Bears . J Wildlife Manage 73 : 1174 - 1183 .
34. Melquist WE , Hornocker MG ( 1979 ) Methods and techniques for studying and censuring river otter populations . Forest, Wildlife, and Range Experiment Station , Technical Report 8 , Moscow: University of Idaho.
35. Mitchell-Jones AJ , Jefferies DJ , Twelves J , Green J , Green R ( 1984 ) A practical system of tracking otters using radiotelemetry and Zn65 . Lutra 27 : 71 - 74 .
36. Zschille J , Stier N , Mechthild R ( 2008 ) Radio tagging American Mink (Mustela vison) - experience with collar and intraperitoneal implanted transmitters . Eur J Wildlife Res 54 : 263 - 268 .
37. Melquist WE , Hornocker MG ( 1983 ) Ecology of river otters in west central Idaho . Wildlife Monogr 83 : 1 - 60 .
38. O Neill L, Wilson P , de Jongh AWJJ , de Jong Tj , Rochford J ( 2008 ) Field techniques for handling, anaesthetising and fitting radio-transmiters to Eurasian otters (Lutra lutra) . Eur J Wildlife Res 54 : 681 - 687 .
39. O Neill L, de Jongh AWJJ , Ozolins J , de Jong Tj , Rochford J ( 2007 ) Minimizing Leg-hold Trapping Trauma for Otters with Mobile Phone Technology . J Wildlife Manage 71 : 2776 - 2780 .
40. Gannon WL , Sikes RS , and the Animal Care and Use Committee of the American Society of Mammalogists (2007) Guidelines of the american society of mammalogists for the use of wild mammals in research . J Mammal 88 : 809 - 823 .
41. D'Eon RG , Serrouya R ( 2005 ) Mule deer seasonal movements and multiscale resource selection using global positioning system radiotelemetry . J Mammal 86 : 736 - 744 .
42. Hansen MC , Riggs RA ( 2008 ) Accuracy, Precision, and Observation Rates of Global Positioning System Telemetry Collars. J Wildlife Manage 72 : 518 - 526 .
43. Lewis JS , Rachlow JL , Garton EO , Vierling LA ( 2007 ) Effects of habitat on GPS collar performance: using data screening to reduce location error . J Appl Ecol 44 : 663 - 671 .
44. D'Eon RG , Delparte D ( 2005 ) Effect of radio-collar position and orientation on GPS radio-collar performance, and the implications of PDOP in data screening . J Appl Ecol 42 : 383 - 388 .
45. Frair JL , Nielson SE , Merrill EH , Lele SR , Boyce MS , et al. ( 2004 ) Removing GPS collar bias in habitat selection studies . J Appl Ecol 41 : 201 - 212 .
46. Heard DC , Ciarniello LM , Seip DR ( 2008 ) Grizzly Bear Behavior and Global Positioning System Collar Fix Rates . J Wildlife Manage 72 : 596 - 602 .
47. Rempel RS , Rodgers AR , Abraham KF ( 1995 ) Performance of a GPS animal location system under boreal forest canopy . J Wildlife Manage 59 : 543 - 551 .
48. Sisak M ( 1998 ) Animal-borne GPS and the deployment of a GPS based archiving datalogger on Hawaiian Monk Seal (Monachus schauinslandi) . Mar Technol Soc J 32 : 30 - 36 .
49. Jay CV , Garner GW ( 2002 ) Performance of a satellite-linked GPS on Pacific walruses (Odobenus rosmarus divergens) . Polar Biol 25 : 235 - 237 .
50. Dennis TE , Chen WC , Koefoed IM , Lacoursiere CJ , Walker MM , et al. ( 2010 ) Performance Characteristics of Small Global-Positioning-System TrackingCollars for Terrestrial Animals . Wildl Biol Pract 6 : 14 - 31 .
51. Fieberg J , Matthiopoulos J , Hebblewhite M , Boyce MS , Frair JL ( 2010 ) Correlation and studies of habitat selection: problem, red herring or opportunity ? Philos T Roy Soc B 365 : 2233 - 2244 .
52. Aldridge HDJN , Brigham RM ( 1988 ) Load carrying and maneuverability in an insectivorous bat: A test of the 5% ''rule'' of radio-telemetry . J Mammal 69 : 379 - 382 .