Movements of Western Alligator Snapping Turtles, Biotaxa
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Herpetology Notes, volume 14: 985-994 (2021) (published online on 28 July 2021)
Movements of Western Alligator Snapping Turtles,
Macrochelys temminckii (Testudines, Chelydridae), in an urban
ecosystem: Buffalo Bayou, Houston, Texas
Eric Munscher1,2, Valeria Gladkaya1, Jeff Stein1,2, Brian P. Butterfield3, Rachel Adams4, Jordan
Gray1, Arron Tuggle1,2, Andrew S. Weber1,5, Kelly Norrid6, and Andrew Walde1
Abstract. Little is known about many life history traits of the Western Alligator Snapping Turtle, Macrochelys temminckii,
including the species’ ecology within urban ecosystems. Our study population inhabits waterways of Houston, Texas, the
third-largest city in the United States. We conducted a telemetry study on 19.5 km of the Buffalo Bayou that laterally bisects
inner metropolitan Houston. Ten radio transmitters were attached to six male and four female M. temminckii. Turtle movements
were monitored biweekly between November 2018 and May 2020, except when the area experienced excessive water fluctua-
tions and flooding. We recorded a total of 242 turtle locations. Turtles were almost always associated with structure (fallen logs,
vegetative banks, cut banks) and showed strong preference for areas of the bayou exhibiting greater tree canopy/in-channel
debris. Many observations documented turtles at the same locations throughout the study period, at times with multiple individ-
uals sharing the same location. Pooled mean linear range for all turtles was 575.4 m. Males and females moved for 284–2285
m and 686–4226 m, respectively, with females averaging both greater movement distances as well as total observed range.
Turtle movement increased in early spring, peaked in late spring, and decreased to little or no movement in the summer and
winter months. Compared to most previously published data on the species, this population showed more limited movement.
This could be due to physical barriers, including man-made structures (dams, bridges, and other flood control structures). These
data could help with future conservation efforts for this species in the Houston area and other urbanized habitats, including
minimizing habitat alteration/destruction throughout their associated waterways, and aiding in the preservation of undervalued
urbanized ecosystems.
Keywords. Urban ecology, home range, radio-tracking, bayou, conservation
Introduction needs including searching for food, mates, nesting
or nursery habitat, or to flee from predation and other
Understanding animal movement patterns within the
unfavourable environmental conditions (Mueller and
ecosystems they inhabit has for decades been a core
Fagan, 2008; Bodie and Semlitsch, 2000).
tenet of ecology and conservation biology (Mueller
Specific habitat usage by animals within an ecosystem
and Fagan, 2008; Millar and Blouin-Demers, 2011).
is of great interest to ecologists and conservation
Animals move within ecosystems to meet life history
biologists because it can provide valuable insight into the
specific requirements of a species within that ecosystem
(Harless et al., 2010), and what the cost/benefits are
1
Turtle Survival Alliance, 1030 Jenkins Road, Suite D, of those movements. By tracking species movements
Charleston, South Carolina 29407, USA. over time, essential habitat types can be identified
2
SWCA Environmental Consultants, 10245 West Little York and management plans for these core/critical habitats
Road, Suite 600, Houston, Texas 77040, USA. can be developed (Millar and Blouin-Demers, 2011).
3
Freed-Hardeman University, 158 East Main Street, Henderson, However, studying movement patterns of many species
Tennessee 38340, USA.
is problematic because these studies are expensive, time
4
Houston Zoo, 6200 Hermann Park Drive, Houston, Texas
77030, USA.
consuming, labour intensive, and can be in difficult-to-
5
National Park Service, 274 River Road, Beach Lake, access study areas. Therefore, movement patterns for
Pennsylvania 18405, USA. many species are not well known and represent gaps in
6
Texas Parks and Wildlife Department, 14320 Garrett Road, our understanding of their basic life histories.
Houston, Texas 77044, USA. Movement patterns and habitat use are not well
*
Corresponding author. E-mail: emunscher@swca.com known for many freshwater turtle species. However,
© 2021 by Herpetology Notes. Open Access by CC BY-NC-ND 4.0. these data are important because many turtle species
986 Eric Munscher et al. worldwide are experiencing population declines (e.g., on media such as iNaturalist, Vertnet, and Facebook, Gibbons et al., 2000; Lovich et al., 2018). Of the 359 with no long-term studies establishing population recognized extant turtle species, 187 (approximately demographics (Munscher et al., 2020). Alligator 52%) are considered Threatened according to criteria of snapping turtles are generally described as ranging as the International Union for the Conservation of Nature far west as the Trinity River watershed (Dixon, 2013) (IUCN), and 127 (approximately 35%) are considered but were recently discovered in Harris and Montgomery Endangered or Critically Endangered (Stanford et al., Counties (Munscher et al., 2020). The State of Texas 2020). These declines are a result of, but not limited to, added the Western Alligator Snapping Turtle to the state habitat loss and degradation, climate change, pressure list of protected species in 1987 (Texas Secretary of from illegal harvest, mesopredators, and overcollection State, 1987), and in 2018 elevated its status to Imperiled for the pet and meat trades, as well as for traditional (R2 rank; Munscher et al., 2020). However, unlike medicine trades (Turtle Taxonomy Working Group, other hard-shelled turtles, M. temminckii rarely bask, 2017; Lovich et al., 2018; Rhodin et al., 2018; Stanford are generally nocturnal, and typically spend most of the et al., 2020). For conservation efforts to be successful time submerged, rarely coming on to land (Pritchard, for species, key aspects of their life history, such as 1989; Hibbitts and Hibbits, 2016). These behavioural movement and habitat use, must be identified, and characteristics have led to a potential perceived decline monitored. Habitat and species management plans can in some parts of their range, though trapping efforts only be successful if all critical habitats and life history suggest that some populations may be more numerous stages are considered. than originally believed (Hibbitts and Hibbitts, 2016; One notable group thought to be in decline, for which Munscher et al., 2020). Recent surveys, in combination a comprehensive understanding of spatial movements with verified observations, have documented the species is lacking, includes members of the North American in 37 Texas counties, with unverified records from genus Macrochelys. This genus has two recognized another four counties (Rudolph et al., 2002; Dixon, species (Thomas et al., 2014), the Suwannee Alligator 2013; Munscher et al., 2020). Snapping Turtle (M. suwanniensis) and the Western Data on the movement of alligator snapping turtles, Alligator Snapping Turtle (M. temminckii). The while one of the more studied aspects of these species, Suwannee Alligator Snapping Turtle ranges from is still limited to a few studies across a large range northwestern peninsular Florida into southern Georgia, (Sloan and Taylor, 1987). Anecdotal observation data whereas the Western Alligator Snapping Turtle ranges and more robust studies do occur outside of Texas, but from the Florida Panhandle to eastern Texas and north as of the time of this writing no such data exist for M. into eastern Missouri and southern Illinois (Pritchard, temminckii within the state of Texas. Pritchard (1989) 1989; Thomas et al., 2014; Enge et al., 2014). Both stated that alligator snapping turtles were “habitual species have been documented as living in a variety upstream wanderers,” and that “certain individuals of freshwater habitats including lakes, rivers, canals, may wander upstream for decades.” In addition, Ernst swamps, and bayous (Sloan and Taylor, 1987; Pritchard, and Barbour (1972) stated that this species seems to be 1989; Munscher et al., 2020). Large freshwater turtle sedentary and seldom moved long distances (Sloan and species, including alligator snapping turtles, exhibit low Taylor, 1987). Shipman (1991) observed individuals recruitment, slow growth, and long generation times, remaining inactive and sedentary for up to eight days life history traits which make localized populations at a time. More detailed studies have been conducted on vulnerable to exploitation (Gibbons, 1987; Iverson, this species’ movement patterns in Louisiana, Missouri, 1991; Ernst and Lovich, 2009). Both alligator snapping Kansas, Oklahoma, and Florida (Riedle et al., 2006; turtle species have been heavily impacted by legal Enge et al., 2014). These studies showed that, in contrast collection and illegal poaching (Pritchard, 1989; to what was originally believed regarding its movement Huntzinger et al., 2019; Munscher et al., 2020). patterns, this species is capable of extensive movements Until recently M. temminckii was not known to inhabit throughout its aquatic habitats (Riedle et al., 2006). urban ecosystems. However, this species was recently The objective of this study was to describe the documented from the urban flood system (bayou) movement patterns of the Western Alligator Snapping waterways of Houston, Texas (Munscher et al., 2020). In Turtle in the Houston waterway area. These data will Texas, much of what is known about the species’ range be compared to other studies in habitats void of urban is based on museum records and public observations influences in order to understand how this species is
Movements of Western Alligator Snapping Turtles; urban ecosystem 987
using this urban ecosystem. Our results will provide Basic morphology. For all turtles, various
agencies tasked with protecting the Western Alligator measurements were recorded, including straight mid-
Snapping Turtle information on movement patterns and line carapace length (SCL) from the precentral (nuchal)
habitat use by the species, in an urban setting. to postcentral marginal scutes to the nearest mm using
700-mm aluminium tree callipers (Haglof Inc., Madison,
Materials and Methods Mississippi, USA). Sex of turtles was determined by
visual inspection of sexually dimorphic characters,
Field site. We conducted our study in Buffalo Bayou
including distance from the cloaca to the posterior edge
in Harris County, Texas, USA. Buffalo Bayou has
of the plastron, proportional head width, and overall size
a length of ca. 85 km, from Brookshire to Galveston
(Ernst and Lovich, 2009). Male alligator snapping turtles
Bay, flowing through urban and suburban residential
have more distally positioned cloacae, larger heads, and
neighbourhoods, parkland, recreation areas, and
reach larger sizes than females (Ernst and Lovich, 2009).
industrial/commercial real estate, including Houston’s
We weighed all turtles using a 130-kg hanging scale
city centre (Munscher et al., 2020). The bayou is
(Cabela’s Inc., Sidney, Nebraska, USA). Turtles were
characterized by turbid water, a benthic sedimentary
marked using a variation of the shell notching technique
layer, abundant debris, revetments, and water levels
described by Cagle (1939) and Riedle et al. (2016) and
that fluctuate both rapidly and drastically with heavy
passive integrated transponder (PIT) tags were injected
precipitation and releases from upstream dams
into the muscle and connective tissue between the pelvis
(Munscher et al., 2020). Furthermore, the easternmost
and the plastron, just lateral to the midline of the turtle
3.6 river-km of Buffalo Bayou are tidally influenced and
(Buhlmann and Tuberville, 1998; Munscher et al. 2015).
brackish (Bosquez, 2010; Munscher et al., 2020).
We released turtles back into the bayou at their capture
We tracked turtles biweekly within a 19.5-km section
location immediately after processing.
of the bayou that includes Memorial Park, Buffalo Bayou
Data collection and tracking. Each turtle was
Park, mixed residential/commercial property on its north
outfitted with Holohil’s AI-2F radio transmitter, that
bank, and a private golf course and mixed residential/
was safely bolted by drilling 6.3 mm holes in the
commercial property on its south bank (Fig. 1). While
posterior carapace and attaching the unit with nuts and
Memorial Park has been kept in a relatively natural
bolts. Each unit was then covered and secured with
state, the residential and commercial properties and the
epoxy in effort to protect the animal and the unit from
municipal parkland have a variety of mitigated structures
damage (Fig. 2; Sloan and Taylor, 1987). The frequency
to curtail bank erosion, including metal and concrete
of each transmitter was set and checked and adjusted to
walls, riprap, concrete revetments, and coir netting.
165–166 MHz before releasing each turtle at the point
Capture method. This study was carried out as part
of capture after approximately one hour. The capture
of a planned ten-year population monitoring study
location was marked via global positioning system
(Munscher et al., 2020). For this effort, turtles (n = 10)
(GPS) so that it could be used as a reference point for
were selected at random (six males, four females), at two
turtles moving up and down the bayou.
study sites along the Buffalo Bayou approximately 6.4
A Turtle Survival Alliance - North American Freshwater
river-km apart (Fig. 1). Turtles were trapped using five
Turtle Research Group (TSA-NAFTRG) intern and two
single-throated hoop nets (1.2 m diameter, 3.6 m long;
volunteers carried out telemetry surveys by kayaking a
Louisiana Sports Net, Baton Rouge, Louisiana, USA).
19.5 km segment of the bayou from November 2018–
All traps were baited with an assortment of freshly cut
May 2020 (the study is ongoing; Fig. 3). We tracked
fish, including Nile Tilapia (Oreochromis niloticus) and
each individual biweekly or as often as safely possible
Buffalo Carp (Ictiobus cyprinellus). Traps were set with
(dependent upon weather and bayou levels) by kayaking
a portion of the trap above water to provide trapped
the total distance of the bayou segment. Telemetry was
turtles with access to air and adjusted to accommodate
carried out with a R-1000 Telemetry Receiver and
the rise and fall of water levels to ensure that any trapped
individuals were located using a three-element Yagi
turtles would not drown due to an overnight influx of
antenna (later replaced with Telonics RA-23K VHF
rainfall and/or dam release (Munscher et al., 2020). Bait
antenna) attached to a handheld receiver. Fixes were
was held in place by metal wire looped through the trap
made from a kayak, which made it possible to get
and through the fish. Traps were checked regularly over
directly above the turtle in order to determine the turtle’s
the next 40 h and were never left unchecked for more
precise location. GPS points were taken to document the
than 16 h (Munscher et al., 2020).988 Eric Munscher et al.
Figure 1. Map of the western portion of Houston, Texas, that includes the section of Buffalo Bayou (shown in blue) surveyed for
Macrochelys temminckii. The water-line distance between the beginning and end points of our study area is 19.5 km, and the end
is ca. 3.2 km west of Houston City Hall. Insets show the project’s location within Houston (red dot, left inset) and within the State
of Texas (red dot, right inset). The right inset also shows the scale of the map.
location. The GPS point taken also included the exact Data analysis. Buffalo Bayou has relatively uniform
date and time of the observation. width across its length. No overland movement was
After data collection was completed, the GPS locations recorded during the 17-month survey, so range length
were imported into ArcGIS software (version 10.7.1, was used rather than area. Home range length was
Environmental Systems Research Institute, Redlands, calculated by adding the farthest upstream movement
California, USA). In ArcGIS, GPS coordinates were to the farthest downstream movement (Harrell et al.,
converted into point feature classes and overlaid on the 1996). We tested the hypothesis that the mean distance
national hydrography dataset (NHD) for Buffalo Bayou. between observations would be greater for females
To normalize the data and account for GPS error, any than males with a one-tailed t-test. We also tested the
GPS points per specimen within a 5-m vicinity were hypothesis that mean home range length would be
aggregated into a single, averaged location. GPS greater for females than males with a one-tailed t-test.
locations were then linearly referenced along the
bayou GIS line feature class. This gave each location Results
a referenced distance that was used to analyse the
We monitored the movements of ten Western Alligator
movement distances and direction.
Snapping Turtles (six males and four females) for a
duration of 17 mo within an urban ecosystem. We recordedMovements of Western Alligator Snapping Turtles; urban ecosystem 989 Figure 2. (A) Telemetry device mounted on the rear carapace of a Western Alligator Snapping Turtle, Macrochelys temminckii. (B) Adult Female Western Alligator Snapping Turtle captured in Buffalo Bayou, Harris County, Texas. Figure 3. Turtle Survival Alliance volunteer Rachel Adams kayaking on Buffalo Bayou while tracking Western Alligator Snapping Turtles.
990 Eric Munscher et al.
a total of 242 locations ranging from 10–33 fixes per turtle Discussion
(Table 1). Total observed range for males was 284–2285
While movement of M. temminckii is one of the better-
m and for females 686–4226 m. Mean distance between
studied aspects of its natural history, there are still gaps
observations (Table 1) was not significantly different (t =
in our knowledge of this species across its range. No
1.04, P = 0.1643) for males (237.1 ± 191.8 m) or females
studies of movement have been conducted in the state
(442.9 ± 434.9 m), and neither was mean linear home
of Texas on the species and, to our knowledge, no study
range length (1613 ± 1132.5 m in males, 2301.3 ± 1992.4
has looked at movements within urban ecosystems.
m in females; Table 1). Monthly movement patterns
The mean linear home range for our turtles was 1613
showed an expected trend, with females showing more
m for males and 1992 m for females. In an Oklahoma
movement during mating and egg-laying months (Fig. 4).
Table 1. Data for selected individual Western Alligator Snapping Turtles (Macrochelys temminckii) radio-tracked in Buffalo
Bayou, Houston, Texas, USA. Included are length of observation (days), number of observations, mean distance between
observations (in m), and total linear range observed (m).
Identification Sex Length Number Mean Distance Total Range
Individual 3 Male 132 10 130.9 284
Individual 5 Male 421 33 77.7 470
Individual 29 Female 490 33 505.6 3812
Individual 32 Male 862 29 171.8 2101
Individual 41 Male 211 17 612.2 2825
Individual 48 Female 588 33 143.2 686
Individual 53 Male 762 33 189.6 1182
Individual 54 Male 462 16 240.2 2816
Individual 55 Female 391 20 89.3 481
Individual 400 Female 273 18 1033.4 4226
Figure 4. Bar graph showing the monthly movement (in m) of radio-tracked Macrochelys temminckii in Buffalo Bayou, Harris
County, Texas.Movements of Western Alligator Snapping Turtles; urban ecosystem 991 population, Riedle (2001) calculated a mean linear Spring Branch, outside of our study area. We speculate home range of 778 m for both sexes combined and that he likely found his way back into Buffalo Bayou noted that females tended to make longer movements during another flood event. Since that time, this turtle than males, but the differences were not significant. has been found in approximately the same area (near the The reason for the small calculated home range for the Spring Branch confluence) during every tracking event. Oklahoma animals is thought to be the high quality Another male (Individual 3) disappeared from the study of available habitat. The Oklahoma study occurred area sometime during August 2019 and has not been in small streams that had an abundance of structural located since. We suspect that this turtle either perished habitat, including fallen down trees, logs, root jams, and washed downstream and away from our study area and cut banks (Riedle, 2001). In Arkansas, Trauth et (we are aware of alligator snapping turtles perishing from al. (1998) reported a mean travelled distance of 191 ingesting fishing hooks in the bayou) or the transmitter m for nine trapped alligator snapping turtles in two malfunctioned and is no longer providing a signal. freshwater creek systems. It must be noted that the We suspect another major reason for the differences in distances calculated from this study were distances from movement in this study compared to other studies is due to trap locations. The animals were not radio-tagged and the Buffalo Bayou system itself. Buffalo Bayou has been monitored. In contrast, Buffalo Bayou is a major urban modified over time to function as the major floodwater waterway that lacks definitive riparian zone across its conveyance for the Greater Houston Metropolitan area length. There are also areas of relatively intact habitat (Harris County Flood Control District, 2021). The habitat where trapping has been more successful and where our floods on a regular basis and the amount of water that movement study has taken place. passes through this system results in regular bank and Our results show truncated movements around what channel bottom changes (Munscher et al., 2020). While we consider core habitat areas within Buffalo Bayou. sections of the bayou may offer adequate riparian zone Females moved more often and greater distances than habitat, the within-channel habitat is subject to change males from March–August, although the difference was and, as a result, Western Alligator Snapping Turtles may not statistically significant. The difference is most likely be forced to adjust their localized “core habitats” on a related to mating and nesting seasons and the need for routine basis. Extreme temperatures, either hot or cold, females to leave the bayou to find adequate terrestrial have been noted in suppressing alligator snapping turtle nesting habitat. Our greatest documented movement was activity and movement (Riedle et al., 2006; Fitzgerald by a female that moved a distance of 4226 m within a and Nelson, 2011; Munscher et al., 2020). With cities time span of 24 d. In addition, this same turtle moved documented to experience a phenomenon referred to as 4049 m a month later, this time within a span of 26 d. “urban heat island” (Streutker, 2003), the amount of heat- In contrast, other studies have shown that the species is absorbing materials, such as concrete, asphalt, and metal capable of much more extensive movements throughout cover in downtown Houston and within the vicinity their aquatic habitats. For example, Wickham (1922) of the Buffalo Bayou ecosystem could result in higher documented the movement of a single individual up to localized water and air temperatures. These variables 30 km upstream from where it was captured and released. could facilitate decreased or increased movement of the In Arkansas, Trauth et al. (1998) reported a young female species (Streutker, 2003; Fitzgerald and Nelson, 2011). moving 1.8 km between trapping sites. Riedle (2001) Understanding how and why animals move within reported that several individuals disappeared from his their ecosystem is extremely important for both species study site, suggesting possible permanent dispersal away conservation and habitat conservation. Urban habitats are from the site. One individual was reported moving 16 under anthropogenic stressors such as roadside / runoff km in a 2-mo time span (Riedle, 2001). We also had two pollution and construction activities. In a recent study individuals that disappeared from our study site. One analysing turtle home ranges across the entire group, male (Individual 54) disappeared for a 5-mo time span Slavenko et al. (2016) found that home range sizes for (March–August 2019). The turtle was later found again turtles are dictated less by species trait and more by at the confluence of Spring Branch and Buffalo Bayou. extrinsic factors, such as environmental productivity, The confluence of these two bayous is an elevated cement topography, and the size of the overall available habitat. and pipe culvert and is surrounded by residential homes Species home range size can be indicative of habitat making access problematic. We believe the turtle entered resource distribution and abundance within an ecosystem the culvert during a flood event and found his way into which may impact population density (Gehrt et al.,
992 Eric Munscher et al.
2009). Species that exhibit smaller than expected and Acknowledgments. We would like to thank the Texas Parks
or previously observed home ranges in rural habitats and Wildlife Department for managing this resource, providing
support, and making this research possible. We thank the
may indicate a higher population density (Gehrt et al.,
Hess Corporation for providing much needed funding for this
2009). The ability to move through habitat is integral to
project. We are grateful to TSA volunteer Jamie Kittle for all
an individual’s and even a species’ success (Nathan et al., of her assistance with the project. Additionally, we would like
2008). Therefore, understanding organismal movement to express our gratitude to the Memorial Park Conservancy,
within a habitat is imperative for ecosystem management specifically Carolyn White and Shellye Arnold, for their support.
and restoration (Nathan et al., 2008), particularly when We also thank all the Turtle Survival Alliance – North American
you consider the role that a particular species may Freshwater Turtle Research Group members and volunteers who
assisted. We thank Hinrich Kaiser and an anonymous reviewer
provide for integral ecosystem functions and services
for their reviews and edits that improved this manuscript. Finally,
(Kremen et al., 2007; Allen and Singh, 2016).
we are grateful to SWCA Environmental Consultants for their
Urbanization is one of the leading causes of species constant support. Texas Parks and Wildlife Department approved
extirpation and extinction globally and in the United all capture and handling protocols, and all methods conformed
States (McKinney, 2006), where it has been suggested to the ASIH animal use guidelines (ASIH, 2004). We conducted
that urbanization is the primary cause of the decline the study under a Texas Parks and Wildlife Department permit (#
in more than half of the species listed on the U.S. SPR-0212-019).
Endangered Species Act (Miller and Hobbs, 2002).
Three states, California, Florida, and Texas contain References
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