The magnitude of river turtle movements varies depending on life history requirements related to foraging, reproduction, and survival [15
]. Turtles in temperate regions typically have temporal patterns in movement rates [25
]. Turtles were rarely inactive during intensive tracking from April through October, and our regular tracking did not extend late enough in the year to adequately document a cease in activity. Thus, the active season for A. mutica
in south-central Illinois is at least April through October and possibly longer. In Kansas, A. mutica
was most active from May through September, with some activity occurring in April and October [30
]. Thus, our population had an active season at least as long as populations from Kansas. The closely related spiny softshell turtle (Apalone spinifera
) also has an active season of April through October in most of its range [25
] and potentially as long as March through mid-November [67
Long distance movements spanning many river kilometers are noteworthy because of their potential relevance to nesting and migration. Such large-scale movements were often reflected in large linear ranges in our study. Some upstream females made long-term, possibly permanent movements after being captured (36.23 km upriver, 22.97 km upriver, 13.17 km upriver, and 11.31 km downriver). It is possible extensive movements were related to nesting, as many turtles are known to make migrations to nesting areas [68
]. Some long-distance movements by A. mutica
in Kansas coincided with the nesting season, and females did not return to previously inhabited areas [17
]. An alternative explanation for the long movements of some females is an escape response brought on by the stress of being captured [17
], although other turtles captured in the same areas did not leave. Some downstream males had long linear ranges because they moved large distances south, then moved back north to previously used areas. The timing of some large movements suggested overwintering migrations, whereas others were quick downstream and back movements in spring/early summer that were probably exploratory or for foraging. Both A. mutica
and A. spinifera
made exploratory movements in other studies [17
The mixed-effects model showed movement rate decreased with day of the year for both sexes during the active season of A. mutica
. Increased feeding to restore depleted energy reserves after overwintering, seeking mates, and nesting are possible causes of higher movements earlier in the active season [42
]. Thus, this is consistent with the reproductive strategies hypothesis, which predicts higher movements early in the year for males associated with mate searching and females associated with nesting forays [69
]. There was a larger change in male movement rate (started higher and ended lower) with day of the year, so perhaps males are particularly active in the early spring seeking mates. The interaction of water temperature and the day of the year showed higher water temperatures resulting in increased movement earlier in the season. Thus, A. mutica
responds quickly to warming temperatures in the spring, likely related to reproduction and foraging, and it responds progressively less to warmer temperatures as the season progresses.
Movement rate increased steadily with increasing water temperature until it peaked in the mid-20 s (°C) and then declined at higher temperatures. Movement rate was similar for both sexes at low water temperatures. Female movement rate was slightly higher than males as water temperatures increased. As temperatures rise, the metabolic rate of turtles increases, and, thus, they increase in activity [70
]. The pattern of increased movement at intermediate temperatures is found in many turtles [25
]. An eventual decrease in movement as temperatures became hotter was also noted for A. mutica
in Kansas [30
]. Overall, the effect of water temperature on movement rate in our study is consistent with expected patterns for turtles.
Male movement rate was slightly higher at very low gage heights. However, female movement rate increased steadily with increasing gage height, whereas male movement rate decreased. Similar results were found in a Kansas study, whereby females were more likely to move away from the shoreline during floods than males [17
]. There is evidence larger northern map turtles (Graptemys geographica
) have higher swimming speeds and can handle strong currents better [71
]. It may be the larger body size of adult female A. mutica
affords them the strength to move more effectively in stronger currents associated with high flows.
Our analysis showed summary movement rates of A. mutica can be divided into two principal components. The relationship between the mean and maximum movement rate was positive; thus, turtles with high mean movement rates tended to exhibit higher maximum movement rates (vagility). The positive relationship indicates some turtles tended to make long movements and others generally moved shorter distances. The relationship between the frequency of movement and minimum movement rate indicated turtles with smaller minimum movement rates moved less frequently (sedentary behavior). Interestingly, the separation of principal components suggests that highly vagile turtles do not necessarily move more frequently. For instance, a turtle may show sedentary behavior more often than normal, but when active, still move large distances. One caveat is that minimum movement rates and movement frequency did not vary much between turtles, which is possibly why they loaded together. Thus, mean and maximum movement represent more relevant biological variation between turtles.
The top models for vagility and sedentary behavior both contained only site; however, the sedentary behavior confidence intervals indicated the effect of site was not strong. Thus, there was no discernable explanation for variation in minimum movement and frequency of movement, possibly because the variables were similar for most turtles. For vagility, it indicated higher movement downstream than upstream. The mixed model for movement rate also indicated site was a predictor. The narrower 6th order upstream channel may have an effect of restricting lateral movement compared to the 7th order downstream channel. The larger downstream channel may also necessitate greater movements between core areas. The movement of fish is also known to be higher in larger rivers [44
]. Thus, water body size can play an important role in determining the overall magnitude of movement for riverine organisms. Heterogeneity in resource availability can also result in different movement patterns [4
]. Perhaps resources were more densely concentrated upstream. Overall, variation in movement between sites is likely due to a combination of stream size and resource availability.
The mean movement rate was slightly higher for females than males, though not to the degree seen in a Kansas study, where females (165 m/day) had significantly higher rates than males (61 m/day) [17
]. Males in Kansas had similar maximum movement rates (2–3 km/day) in our study, whereas females in Kansas sometimes moved 3–4 km/day [17
]. Other river turtle species have also shown sexual differences in movement. Male false map turtles (Graptemys pseudogeographica
) and sliders (Trachemys scripta
) in the Missouri River and its backwaters had significantly lower mean and maximum movement than females [43
]. However, the mean movement rate of A. spinifera
males (141 m/day) and females (122 m/day) in Arkansas was not significantly different [67
]. A possible explanation when movement differs is sexual size dimorphism because of size-specific influences in turtles [15
Our study indicates that the magnitude of sexual differences in movement rate vary with environmental conditions and time of year. Thus, simple comparisons of mean movement rate between sexes may result in an incomplete understanding of behavior. The interaction terms in our model indicate female movement is higher in most but not all conditions. For instance, at low water temperatures or gage heights, the movement of A. mutica
males and females is similar. Female movement becomes greater at higher temperatures and flows. Ultimately, we need to consider interactions to understand such nuances. For instance, a study in the Mississippi River found that females moved larger average distances than males, but this study did not track turtles during spring months [32
], a time where males in our study had higher movement rates. Thus, the Mississippi River study may have missed some of the larger male movements. Understanding turtle movements is largely about weighing the potential risks and benefits of moving [67
]. Movement decisions may depend on environmental factors, the spatial context of resources used by each sex, the availability of nesting habitat for females, and mate searching in males. Overall, sexual differences in movement rates appear to vary among different river turtle species and possibly within a species based on environmental factors and geographic location.
While the overall trend was for higher movement downstream, there were noteworthy exceptions contributing to the high variability in mean and maximum movement rates. For instance, one downstream female had a very high mean movement rate of 537.4 m/day, whereas some moved <100 m/day. Upstream, one male moved only 56.6 m/day whereas another moved 126.5 m/day. Meandering rivers provide habitat heterogeneity [73
], and heterogeneous landscapes may result in an increased variability in movement [74
]. Furthermore, the shifting success of resource acquisition strategies may result in individual variation in movements [67
]. It is possible some turtles in our study employed a strategy of reduced movement while exploiting modest but spatially consistent resources, whereas others had equal success moving longer distances between seasonally abundant resources or searching out less predictable resources (nomadism) [4
4.2. Home Range
We found linear ranges of A. mutica
varied greatly by individual. A few stayed in short reaches only 1–2 km even when tracked for well over a year, whereas other ranges spanned over 10 km. In the Kansas River, the average length of ranges using point clusters was 474 m and 1228 m for male and female A. mutica
, respectively, and variation between individuals was high [17
]. The Kansas study considered long movements to be the establishment of new home ranges, and, thus, the linear range for all radio-locations regularly spanned several km [17
]. Our results are in agreement that linear range varies and can be large. The linear range of closely related A. spinifera
in Vermont and Quebec was 13.7 km for females and 3.9 km for males [19
]. Thus, most males in our study had larger linear ranges and females smaller linear ranges than A. spinifera
. Large linear ranges may highlight important movement corridors between different core areas of highly aquatic river species or may simply result from exploratory movements to find new resources or mates. However, linear range does not describe frequency or size of space use.
Our study estimated two-dimensional space use for A. mutica
by calculating home range area. Estimates of home range area given by MCP typically included large areas of terrestrial habitat not used or even accessible to A. mutica
under normal conditions, particularly for turtles with long linear ranges. The inclusion of unavailable space using MCPs has been noted in other studies, and sometimes the estimates are clipped to the river boundary [8
]. For our study, the meandering of the river coupled with large distances between locations would include seemingly random areas of the river and exclude others. Thus, MCP is not a meaningful indicator of home range size for organisms constrained to meandering rivers, and clipping does not completely address the issue.
Kernel density estimates (KDE) also include some areas of terrestrial habitat not used—but to a much lesser degree than MCP. Furthermore, the density estimates are only in areas of point locations. Clipping the KDE to the river boundary provides a more reasonable estimate of space use within the confines of the river [60
]. KDE using the CVh smoothing were larger than estimates using LSCV smoothing. LSCV has a tendency to under-smooth [56
], potentially resulting in unrealistically fragmented home ranges [55
]. The undersmoothing may result in numerous disjunct areas of home range around individual points [75
], as was the case in our study. CVh also resulted in disjunct areas, but these were mostly clusters of points in different river reaches. Given there is no consensus on which smoothing parameter is best, it is helpful to keep in mind the biological relevance of any KDE in regards to the study animal. The extreme undersmoothing of LSCV did not provide reasonable areas of activity for a highly vagile species. Thus, we conclude that clipped 95% KDE using CVh smoothing give a meaningful home range estimate for A. mutica
, with clipped 50% KDE (CVh) considered core areas [5
Serial autocorrelation occurs when radio-locations are not independent of one another (location is partly dependent on the previous location) and can potentially render home range estimates invalid [76
]. The effect is strongest when locations are taken at very small time intervals that do not afford animals the opportunity to move to a different area. In our study, autocorrelation was minimal because radio-locations were never taken on the same day (intervals were often multiple days) and A. mutica
are capable of quickly moving large distances. Furthermore, simulation studies show that even if a moderate level of autocorrelation existed, it is not problematic when calculating home ranges using KDE [77
]. Our sampling frequency could have introduced some bias, because we typically tracked for five days every other week at each site. Clustered samples could potentially overestimate space use in certain areas [77
]. This would manifest as areas of high use defined by four-to-five radio-locations, which was not apparent for most turtles. It was occasionally noticeable for turtles found to be more nomadic or having smaller samples with home range sizes not asymptoting in the bootstrap analysis.
With enough samples, home range size attains an asymptote for animals that do not wander excessively or behave nomadically [11
]. Most home ranges in our study asymptoted. Small sample size is a possible explanation for why four turtle home ranges with <40 locations did not asymptote. One female turtle home range that did not asymptote with 66 radio-locations over one year was possibly nomadic, and some of the upstream turtles with long linear ranges which could not be tracked sufficiently to calculate a home range may have been nomadic as well. Some turtles made occasional sizeable exploratory movements but returned to their home ranges afterward. Overall, the majority of A. mutica
in the Kaskaskia River have well-defined home ranges, and a small minority behave nomadically. The Kansas study concluded that A. mutica
continually redefined its home range [17
], suggesting that home range size for A. mutica
does not typically asymptote. However, their definition of home ranges as shifting is possibly analogous to movement between core areas in our study, so actual differences may not be that great. Another complicating factor for comparison is the short period of tracking in the Kansas study (technological limitations). If turtles in Kansas could have been tracked over a longer time, they might have moved back to previously inhabited areas.
Some A. mutica
home ranges contained few core areas whereas others had several. Roughly a third of all turtles had only one core area, the majority had between two and five, and a few had greater than eight. The variation is not surprising considering the large differences observed in movement rates, and it is not uncommon for animals to have multimodal home ranges [2
]. Space use often varies along a continuum from sedentary to nomadic [4
], and our study indicates there is variation among individual A. mutica
. In Kansas, A. mutica
did not restrict space use to single areas [17
], which was true for some turtles in our study but not all. The number of radio-locations needed to determine home range size averaged 56 overall, though it was higher downstream. Thus, as a general guide, we recommend that future studies use a minimum of 56 radio-locations to determine A. mutica
home range size, preferably obtained over multiple seasons. Obtaining the locations over multiple seasons will ensure areas of home range are not overlooked; for instance, if different core areas are used throughout the year.
For our top models of home range area, mean movement rate was a predictor in both. The effect of site is reasonable given it was also in the mixed model for movement rate. The top two models had an effect for the number of radio-locations, which is known to influence home range size [64
]. Sex was in the second model, indicating a small effect. Body size is often considered a strong predictor of home range size [65
], but it was removed from our analysis because it was strongly correlated with sex. Our home range estimates only include adults so body size may still be an important factor when considering a larger ontogenetic series of individuals.
Movement rate was a predictor in both top models and had an effect of increasing home range size. The relationship is not surprising, as home ranges reflect the distances traveled by an animal [44
]. Optimal foraging theory suggests home range size should be positively related to movement rate, with animals exploiting scarcer resources requiring increased movement throughout larger areas [80
]. Perhaps some individuals maintained smaller home ranges because they were better at exploiting a concentrated resource. If all animals were using resources similarly, one would expect less variation in home range size. The specialization of individual animals can occur if there are trade-offs between using different resources [81
]. Overall, turtles with higher movement rates will have larger home ranges, possibly because they have specialized foraging strategies.
The effect of site was strong, with home range size larger downstream. The difference may be due to the kernels being clipped to the smaller channel width upstream. However, the physical boundary typically limits lateral movement, thus A. mutica
living in smaller streams should have more confined space use. The only instance where turtles may move inland is when females move to the higher areas of sand bars to nest [29
]. We did not observe nesting during our study, so our home range estimates do not encompass nesting grounds. Our results are in agreement with previous findings where aquatic turtle home range sizes increase with water body size [66
]. Water body size is also a good predictor of home range size for riverine fish [44
]. Lower movement rates could be driving the smaller home ranges upstream. It also suggests A. mutica
does not increase home range size longitudinally simply because lateral movement is restricted.
The top two models showed that, as the number of radio-locations increased, so did home range size. The positive relationship is because home range size did not reach an asymptote for some turtles, particularly ones with fewer radio-locations. It has been shown when using KDE with a sample size <50, the number of radio-locations can influence home range estimates [64
]. It also provides further evidence that a minority of A. mutica
display nomadic behavior and continually expand their home range (or change home range depending on definition). However, some individuals with many radio-locations had only one or two core areas and most home ranges asymptoted, so nomadism appears to be uncommon. In Mississippi, the number of radio-locations was not correlated with home range area for yellow-blotched map turtles (Graptemys flavimaculata
]. Perhaps softshell turtles are more likely to display nomadic behavior than hard shelled river turtles. Overall, the number of radio-locations had an effect resulting from small sample size for some turtles and the possible nomadism of a few others.
Sex was in the second best model, with males having slightly larger home range size than females. The effect was not strong, and confidence intervals overlapped. In contrast, females had significantly longer linear ranges and more lateral movement in Kansas, so they were considered to have larger home ranges than males [17
]. Female A. spinifera
in Vermont and Quebec have much larger home ranges than males [19
]. The smaller sexual differences in our study are likely due to site-specific variability and differing home range calculations. Male and female A. mutica
are known to have different dietary preferences also [28
]. Thus, sexual differences in home range size at different sites may be due to the relative distribution and availability of preferred resources.