A Review and Comparison of Seasonal Movement Distances and Home Ranges of Bog Turtles (Glyptemys muhlenbergii)

by Gary Leschinsky

Abstract:

Bog turtles (Glyptemys muhlenbergii) are a critically endangered species endemic to the United States. This paper details, compares, and draws conclusions from 76 scientific papers regarding male and female seasonal movement distances and home ranges for future conservation efforts and scientific reports. The literature shows that bog turtles are most active during spring, summer, and early fall, and decrease activity to brumate. There is no clear agreement about which season bog turtles are most active in though: many studies conducted in the Northeast indicate spring to be the most active season, whereas studies in the Southeast indicate summer is when bog turtle movement peaks. The Northeast is more frigid than the Southeast throughout the year, and spring is colder than summer, so this difference ought to be further researched to determine why such a discrepancy exists. Very few studies have been conducted comparing bog turtle home range throughout the seasons, and such an analysis could yield interesting results, given that the few studies that have documented differing bog turtle home ranges across the seasons seem to share the same discrepancy found for bog turtle movement distances (largest home ranges in spring in the Northeast, and summer corresponding to the season with the greatest home ranges in the Southeast). Overall, there are a few key gaps and interesting patterns found in this literature review, most notably the mentioned discrepancy between the Northeast and Southeast, and further research ought to be conducted to better understand bog turtles.

Introduction

Bog Turtles (Glyptemys muhlenbergii) are a critically endangered species endemic to the eastern United States (Hulse, Censky, and McCoy, 2001). They live in bogs and fens, sometimes occupying meadows and spring-fed, shallow water with soft mud (Chase et al., 1989; Rosenbaum & Nelson, 2010; Buhlmann et al., 1997; Somers, 1999). Bog turtles often linger near woodland streams with seepage, spring-fed meadows, and marshes within deciduous forests, small ponds, and bog woodlands (Arndt, 1977).

Over the past century, Bog Turtle populations have decreased by as much as 90% (IUCN, 2021). Estimates for the current population range between 2,500 and 10,000 (Endangered Species Coalition, n.d.; Tryon & Herman 1991). The causes of the population decline include invasive plants and habitat destruction. Plant succession alters vegetation composition, removing one of the main food sources of Bog Turtles, and reduces open areas, limiting the area Bog Turtles have to bask and build nests (Morrow et al., 2001). Examples of exotic plant species that frequently occupy Bog Turtle wetlands include purple loosestrife (Lythrum salicaria), multiflora rose (Rose multiflora), and Japanese honeysuckle (Lonicera japonica). In addition, urban developments, such as the construction of shopping malls, parks, roads, sewage disposal plants, and landfills, destroy the habitats of Bog Turtles (Nemuras, 1967). Other human activities like bog destruction for muck, draining moist pastures, and quarry mining have also contributed to the decrease in the Bog Turtle population. Bog Turtles require specific habitats to survive, so the destruction of their habitat greatly lowers bog turtle survival. As Bog Turtles do not typically travel long distances, their odds of discovering a new suitable habitat are reduced even more. (Carter et al., 2000).

Due to their low and rapidly diminishing population, Bog Turtle conservation is critical. As the knowledge about Bog Turtle behavior is limited, summarizing what is already known and identifying gaps in Bog Turtle movement, especially in a seasonal scope, is essential for future conservation efforts and scientific studies. Such data can help determine study periods when Bog Turtles are most active relevant to the aims of the conservation or research and can be used to further explain other Bog Turtle activity. This research paper reviews the literature on Bog Turtle movement and home range with a particular emphasis on the lack of clarity regarding comparisons and differences in findings.

Methods

Google Scholar, JSTOR, PubMED, and the University of Reno, Nevada (UNR) Library were used to find literature related to Bog Turtles related to movement, home range, and factors impacting movement and home range. Research papers not related to Bog Turtle movement, but rather genetics or other topics were excluded due to their lack of relevance to the aims of this paper. Additionally, research papers published prior to 1950 were mostly excluded in this review since most rarely went into detail on relevant information to this review.

Results

26 original papers that studied Bog Turtle movement distances and 14 that studied home range were found, and are documented in Tables 1 and 2 respectively. One important observation is the low distances and small home ranges of these turtles regardless of the season, which can be attributed to their specialized needs. Relevant literature on bog habitats shows that Bog Turtles typically inhabit bogs, wet sedge meadows with nearby rivulets or brooks with organic substrates, spring-fed calcareous fens, which are wetlands with non-acidic water (Mitsch and Gosselink 1986), and sometimes abandoned beaver ponds (Whitlock 2002; Carter et al., 1999; Smith & Cherry 2016; Carter et al., 2000; Ernst et al., 1994; Pitts, 1978; Bury, 1979). Bog turtles also typically gravitate to drainage basins with shallow water, soft mud, areas with low grasses and sedges, and an area with both wet and dry locations given their propensity for terrestrial and shallow water meandering (Chase et al., 1989; Barton & Price 1955). Typical vegetation in Bog Turtle habitats include sedges (Carex sp.), sphagnum mosses (Sphagnum spp.), rushes (Juncus spp.), swamp magnolia (Magnolia Virginia), alder (Alnus sp.), swamp orchids (Arethusa bulbosa, Nymphaea tuberosa, Pogonia ophioglossoides), and sometimes even woody vegetation like multiflora rose (Rosa multiflora) (Barton & Price 1955; Ernst et al., 1994; Myers & Gibbs 2013; Tyron & Herman 1991; Kiviat 1978; McCoy et al., 2021; Carter et al., 2000; Morrow et al., 2001; Torocco et al., 2002). Given the repeated finding of this vegetation in different geographic areas, it is likely that Bog Turtles are at least somewhat limited to areas with these types of vegetation. The natural resources of Bog Turtle habitats are also different than surrounding areas: Carter et al. (1999) find that Bog Turtles inhabit areas with significantly higher water and mud depth and greater vegetation height than surrounding areas, and Arndt (1977) found that Bog Turtle habitat water pH ranges from 5.5 to 7.4 (with a mean closer to 6.4) consisting of almost no salinity and that water temperature can fluctuate from 2.0° to 28.0° (C) throughout the year across three sites in Delaware, although further testing ought to be done to corroborate these results. Overall, the literature indicates that habitat, vegetation, and natural resource abundance restrictions could be a cause of the lower net movement frequencies and home ranges of Bog Turtles, especially compared to similar turtle genera.

Discussion: Movement Distances

Already limited by their environment, Bog Turtle movements can further vary greatly each season. Table 1 includes data on original literature that identified a most active season for Bog Turtle movement, but finding a clear most active season for the Bog Turtle ought to be further investigated due to several considerations. Studies in Table 1 published before Lovich et al (1992) mostly showed that Bog Turtles are most active in spring and have a resurgence of activity in fall to indicate a bimodal activity pattern. More specifically, these studies found that, during spring, Bog Turtles may leave areas of flowing water and travel upstream to more shallow water for more food as well as warmer air and water. As opposed to the other seasons, spring’s climate is ideal for Bog Turtles to be active, so they accumulate food to prepare for summer when finding food is more thermally stressful. Ernst (1977) explained that Bog Turtles seem to require air temperatures of 18.5°C to 21.3°C to remain active. These temperatures are most frequent in open areas during spring in the Northeastern United States. These papers also find that Bog Turtles reduce movements in response to the warm summer temperatures and will sometimes aestivate during the summer, resurface during the early months of fall as basking keeps them warm, and then brumate during winter (Ernst, 1977; Arndt, 1977; Ernst & Barbour, 1972; Nemuras, 1967).

Many of the studies after Lovich et al. (1992) found that summer was the most active season for Bog Turtle movement, or presumed it to be (Lovich et al., 1992; Carter et al., 2000; Smith &; Cherry 2016; McCoy et al., 2021). One difference between these study groups is papers before Lovich et al. (1992) measured a most active season by measuring time and distance intervals between hand captures of Bog Turtles and divided net movement by the time duration for m/day values. However, such techniques are inaccurate for daily calculations, as shown by Carter et al (2000), Eckler et al. (1990), and Lovich et al. (1992). Instead, nearly all studies that measured Bog Turtle movement (m/day) values from Lovich et al. (1992) and onward used radio telemetry which is significantly more accurate than hand capture methods to calculate movement (m/day) for longer time and distance intervals.

The data before Lovich et al. (1992) probably should not be used for getting an accurate sense of Bog Turtle daily movement, but can it be used to try and determine the movement patterns of Bog Turtles on a relative season scale, a main aim of this review? The answer to such a question is unclear, especially since hand captures assume that a small movement area is indicative of little movement, an assumption proven invalid by Chase et al. (1989). Given these method factors, the seasonal results of these earlier studies should probably be weighed less compared to radio telemetry studies. Still, the bimodal activity (spring and fall) observations, or at least the idea of Bog Turtles being more active in spring may not be completely erroneous since these pre-telemetry studies were all conducted in the Northeast US, and four of five studies with telemetry located in the Northeast (Morrow et al., 2001; Whitlock et al., 2002; Farrel et al., 2002; Byer et al., 2017) showed that Bog Turtles are most active in spring, compared to the other telemetry studies located in the Southeast states of North Carolina and Virginia, which indicated Bog Turtles are most active in summer. The described habitats of these studies were similar, but one obvious difference between these two different geographic and active season findings is that summer in the Southeast is much warmer than spring in the Northeast, both because summer is warmer than spring, and the Southeast is typically warmer than the Northeast. Unfortunately, very little testing has been conducted on both environmental and cloacal Bog Turtle temperatures in these regions, and as such further studies in the North, or ideally studies with multiple sites across the US, ought to be conducted to confirm if turtles in the Northeast really move more in spring compared to Southeast turtles who move more in the summer. Such testing is particularly important since it might shed new light on movement-related behavior like mating, possibly redefining the current knowledge of the seasonal behavior of Bog Turtles.

Another important consideration for future research analyzed in Table 1 is sample size. Many articles (n=12), mostly quite old and quite recent articles, have total sample sizes of less than 10 total turtles studied. Such a sample size is not only problematic for statistical and accuracy purposes, but also discourages aims where larger groups are needed, as was the case for McCoy et al. (2021). There are few bog turtles left, particularly in known areas since most papers do not disclose exact locations for conservation efforts, but higher sample sizes are critical for further research.

Table 1 also documents movement distance data comparing male and female Bog Turtles, which can help further understand other Bog Turtle seasonal behavior, and results on the matter are unclear. The most prominent studies on how the genders compare in regards to movement distances are Lovich et al. (1992), Morrow et al. (2001), and Carter et al. (2000), given that they tracked specific movement and their data is quantitative rather than qualitative. Lovich et al. (1992) found that the difference between male and female movement was statistically different (P=0.05), with males traveling 2.1 m/d and females 1.1 m/d using radio telemetry from June to September. Morrow et al. (2001) found that while male distances were generally greater than female distances, the difference was not significant (P=0.65 through year, P=0.38 for mating season, P=0.26 during post-mating season). Carter et al. (2000) found that females travel more than males, but not significantly so (P=0.65 from May to November). There is no consistent trend regarding the comparative movements of male and female Bog Turtles during the mating season (spring) nor the post-mating season (summer and early fall). Lovich et al. (1992) found the most movement data from June to September, with very little data in spring. Morrow et al. (2001) had sufficient data in both the mating and post-mating season, and found that males move more than females during the mating season, but the reverse occurs during the post-mating season, and Carter et al. (2000) only studied movement during the post-mating season. From these results, it could be hypothesized that males travel more than females during spring due to the mating season, where males often search for females for courtship, but little clarity can be found for the post-mating season, since Lovich et al. (1992), Carter et al. (2000), and Morrow et al. (2001) have contradictory results. Lovich et al. (1992) is somewhat limited in that only 5 turtles were studied with radio telemetry, whereas Carter et al. (2000) and Morrow et al. (2001) studied nearly 50 turtles each. Summer is characterized as the post-mating season, and from known behavior, females often move to sunnier and drier areas to construct nests during the post-mating season, and a decrease in male movement distances is logical given a lack of mating, which is the primary factor in movement during spring. Basking is also common during spring and summer, but Pittman and Dorcas (2009) identified no significant difference in basking frequencies, with each gender basking ∼ 2.1 times a day. It is also important to note that radio-telemetry, while much more accurate than hand capture methods, is not extremely accurate, especially for non-leveled planes, which are the main environment of Bog Turtles. This lack of accuracy may explain differences found in movement between the genders, especially since distances throughout the active seasons of spring to early fall were not too different between the genders (only a few feet per day). Ultimately, further research is needed to confirm movement differences between the genders during the post-mating season, although such a study would take less precedence than a seasonal comparison study mentioned earlier since movement distances between the genders are not significantly different.

Discussion: Movement Home Range

A Bog Turtle’s home range is the area it frequently inhabits, and there are several ways to calculate home range. The classical method to calculate home range is by minimum convex polygon (MCP) analysis, which is a parametric method by which a polygon with exterior angles (180 degrees) is constructed using coordinates of an organism’s tracked location. (Burgman & Fox, 2003). MCP analysis often overestimates home range because the constructed polygon often takes into account areas that were never reached (Jennrich & Turner 1969; Kenward 1987; Worton 1987; Burgman & Fox, 2003; Row and Blouin-Demers 2006). Also, MCP analysis is subject to data bias, because more data points of tracking often result in a greater polygon (Jennrich & Turner 1969). Most papers that use MCP analysis use 95% MCP to try and subvert these flaws. A more recent method that is gaining popularity is kernel estimation (KE), a nonparametric method to estimate home range while emphasizing areas that Bog Turtles use. Different kernel bandwidths, which are calculated by the radius of a circle overlapping kernels using reference bandwidths and least squares cross-validation (LSCV), have different biases, often overestimation, but not to as great of an extent as MCP analysis. 50% and 95% KEs are often used to prevent such overestimation bias. Two rarer methods to calculate home range are cluster analysis and harmonic mean analysis but are too dependent on a factor to be effective, and as such are quite rare (Worton 1987). In his authoritative study on different methods for estimating home range, Byer et al. (2017) found that 95% MCP and 95% KE are significantly different, which was also found by many other studies, but 95% MCP and 50% KE are more comparable.

Bog turtles have relatively small home ranges—almost all studies found that Bog Turtles have home ranges of less than 1 hectare using different estimation techniques—compared to other turtles, and as seen in Table 2, home range estimates are highly variable, not only between experiments but often within studies themselves (as indicated by the high relative standard deviations). It is also important to note that many studies did not pay particular emphasis to home range differences between seasons, and instead only listed a most active season, but without much quantitative data. Quantitative home range data by season is critical to better understand Bog Turtle seasonal behavior and activities, and ought to be done more. Of the studies that reported a most active month, spring and summer were the two months were Bog Turtles had the greatest home ranges, and there is a similar, peculiar, trend to the movement data which indicates that Northeast Bog Turtles had the greatest home ranges in spring, and Southeast Bog Turtles in summer: of the six studies in the Northeast, four indicated spring to be the most active season, and of the four studies in the Southeast, all four indicated summer to be the most active season. As mentioned, this spring-Northeast and summer-Southeast connection is surprising given the two-fold heat disparity, and since it has been observed in both the movement distances and home ranges, it really ought to be further investigated, and the same recommendations of a study with a large sample size and most ideally, including both regions, apply.

There seems to be general agreement in the literature regarding the genders when it comes to home ranges during spring and summer. Several studies have found that males have greater home ranges than females during the mating season, which is logical since studies have shown that males are active in finding females during mating, and females have slightly larger home ranges during summer when the post-mating season occurs and females construct nests, which frequently involves traveling to higher parts of bogs to build their nests due to the dry and sunny conditions of these locations compared to their previous living areas (Harding, 2002). Nearly none of the studies in Table 2 documented Bog Turtle home range during later fall and winter, when they brumate, which is logical since brumation typically involves less movement. Still, a comprehensive study comparing the sexes throughout the entire year is yet to be conducted and could reveal new information on their seasonal behavior and activity.

Conclusion

This review has covered the movement distances and home ranges of Bog Turtles, comparing and synthesizing relevant literature on the topic. A fascinating trend found, both within movement distance and home range data, is that spring in the Northeast is when Bog Turtles move the most and cover the greatest area, whereas Summer is when Bog Turtles move the most and cover the greatest area in the Southeast. Further research ought to be conducted to explain this trend among the data. Additionally, further research ought to be conducted to compare male and female movements and the season when Bog Turtles’ home ranges are the greatest—both of these topics are understudied, and studying them can lead to further knowledge about general Bog Turtle seasonal behavior.

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