Home Range, Site Fidelity, and Movements of the Timber Rattlesnake (Crotalus horridus) in West-Central Illinois

doi:10.21203/rs.3.rs-3193050/v1

Understanding the home range of reptiles is important to their conservation and recovery efforts. Despite numerous home range studies for the imperiled timber rattlesnake (Crotalus horridus), only a single study has been undertaken in the central midwestern United States. We report on the home range size, site fidelity, and movements of this species in west-central Illinois. Using VHF telemetry, we located 29 C. horridus (13 female, 16 male) every 1.3 days over a 5-year period for 51 years ofrecords of the species' locations and movements. We calculated annual home ranges for each snake per year using 99%, 95%, and 50% isopleths derived from Brownian Bridge utilization distributions, and we also report 100% minimum convex polygons for comparison with other studies. We examined the effects of sex, mass, SVL, and year on home range sizes and reportedon movement metrics as well as home range fidelity using both UDOI and BA statistics. The homerange sizes for male and non-gravid C. horridus were 56.91 Ha (CI = 41.04-72.77) and 20.22 (CI = 3.81-36.62) for 95% BBMM; 7.56 Ha (CI = 5.47-9.65) and 2.36 Ha (CI = 0.19-4.53) for 50% BBMM; and 80.03 Ha (CI = 50.95-109.11) and 30.80 Ha (CI = 1.55-60.06) for MCP. The estimated daily distance traveled was significantly greater for males (mean = 57.25 m/day, CI=49.06-65.43) than females (mean = 27.55 m/day, CI = 18.99-36.12). Similarly, maximum displacement distances from hibernacula were significantly greater for males (mean = 2.03 km, CI = 1.57-2.48) than females (mean = 1.29 km, CI = 0.85-1.73], and on average, males were located further from their hibernacula throughout the entirety of their active season. We calculated habitat fidelity using 11 snakes that were tracked over multiple years. The mean BBMM overlap using Bhattacharyya's Affinity (BA) for all snakes at the 99%, 95%, and 50% isopleths was 0.46 (SE=0.03), 0.38 (SE=0.03), and 0.08 (SE=0.02), respectively. The mean BBMM overlap for all snakes using the Utilization Distribution Overlap Index (UDOI) at the 99%, 95%, and 50% isopleths was 0.78 (SE=0.13), 0.41 (SE=0.03), and 0.09 (0.02), respectively. Our results are largely consistent with those of other studies in terms of home range sizes and theinfluence of sex on those home ranges and distances traveled. The species also exhibits strong site fidelity with snakes generally using the same habitats each summer. Particularly interesting was the tendency for snakes to disperse from specific hibernacula in the same general direction to the same general areas. We propose some possible reasons for this dispersal pattern.

timber rattlesnake

Crotalus

home range

home range fidelity

utilization distribution overlap index

Bhattacharyya’s affinity

Brownian Bridge utilization distributions.

The home range of an animal was defined by [1] as the geographic space “traversed by the individual in its normal activities of food gathering, mating, and caring for young”. This definition has been well accepted since it was first defined and today is frequently characterized as the area encompassed by 95% of the animals use of a geographic space (its “utilization distribution” or UD) [2]. While home range estimation usually approximates an area surrounding observed or reported (e.g., through telemetry) locations, it also often involves some assumption (or estimation) of the pathway between these discrete locations. Methods used to connect these locations into pathways have evolved significantly since the home range was first defined [3, 4]. At its most simple, home range is defined by the use of the Minimum Convex Polygon (MCP) [5], where all the outer locations are connected to form a polygon and the home range is expressed as an area. Because MCP does not include information on the intensity of use within the polygon, it assumes equal use of the entire enclosed area, which can be an unrealistic appraisal of home range. However, due to the ease of calculating the MCP [2] and because it conservatively encompasses the entire range of an individual, it is still reported in the home range literature. Methods to calculate utilization distributions that include intensity-of-use within the range (e.g. Kernel Density Estimators) and probability-structured pathways based on knowledge of movement behavior (e.g., Brownian Bridge movement models) have become more common and are usually considered a more accurate representation in visualizing space use or location choice by animals, despite increased computational complexity [6–8].

Evaluation of the home range of an animal can provide valuable insights into the ecological niche of a species, particularly when using utilization distributions (UD) because each UD delineates intensity-of-use areas that can be used to infer the habitat preference of individuals and provide a ready means to establish boundaries for microhabitat sampling efforts by using the assumption that increased presence is correlated with preference (e.g., [9–17]. Delineation of such high-use areas is particularly valuable when trying to understand critical habitats for protection or enhancement with imperiled species. [18–20, 20]. It can also be valuable in understanding habitat selection by a species [21–23], how habitats might change annually or seasonally [24–26] and in describing the site fidelity of individuals [27, 28].

While the original definition of home range did not include a temporal component, one is usually provided [2], e.g., annual home range [29], seasonal home range [30], and internesting home range [31]. Designating a temporal limit to the designation of home range provides the opportunity to compare animal habitat choice over time or under varying environmental conditions.

The imperiled timber rattlesnake, Crotalus horridus, is found throughout much of eastern North America, although their range has been reduced from its original distribution, and many local populations are extinct or in decline [32, 33]. Crotalus horridus inhabits a wide diversity of habitats, including upland hardwood forests, mixed hardwood/pine forests, pine barrens and savannas, and coastal plain habitats, which include forest or forest/savanna mixes (Appendix Table 1). Studies of C. horridus habitat use, home range size, and annual or daily movements are relatively common, particularly from the northeastern United States (Fig. 1; Appendices Table 1), although many are unpublished graduate theses. Such studies commonly report that males have larger home ranges than females (e.g., [34–41] and travel greater daily distances during their active period [34–36, 41] and that gravid females have highly restricted home ranges that are much smaller than those of non-gravid females [34, 35, 37, 38, 40, 41]. The larger male home range and daily movements are usually attributed to mate searching during mid-summer breeding periods.

Table 1

—Summary statistics for 51 Timber Rattlesnake (*Crotalus horridus*) snake-year combinations from 31 individuals (13 Female, 16 Male) tracked between 2015–2019 at Principia College and surrounding properties in Jersey County, Illinois. Columns represent: snake ID (ID); year of track (Year); sex and reproductive condition (Sex; M = male, F = female), G = gravid); Hibernacula (Den ID); snake mass (Mass); snout-vent length (SVL); start (Track Start) and end (Track End) date of track; track duration (Track Length); number of GPS coordinates (# Fixes); average number of days between fixes (Days / fixes); minimum number of fixes required for home range asymptote (Min. Fixes); proportion of home range (Prop. HR); mean daily distance (Daily Dist.); maximum distance/displacement from hibernacula (Max. Disp.); total planimetric distance moved between fixes (Total Dist.); first (Sig1) and second (Sig2) smoothing parameter for Brownian bridge Movement Model; 99% (99% BBMM), 95% (95% BBMM), and 50% (50% BBMM) isopleth home range area using Brownian Bridge Movement Model; and Minimum Convex Polygon home range size (MCP). Snake-year combinations below the black dashed line were comprised of partial tracks or failed to reach home range asymptote and were removed from home range and movement analysis (See text).
ID	Year	Sex	Den ID	Mass (g)	SVL (cm)	Track Start	Track End	Track Length (Days)	# Fixes	Days / fixes	Min. Fixes	Prop. HR	Daily Dist. (m)	Max. Disp. (km)	Total Dist. (km)	Sig1	Sig2	99% BBMM (Ha)	95% BBMM (Ha)	50% BBMM (Ha)	MCP (Ha)
1113A	2015	M	2	430	79	5/13	10/2	143	95	1.51	32	1.01	21.31	1.77	4.55	0.16	3.00	44.76	28.61	2.33	41.47
1113A	2016	M	2	430	79	5/6	9/2	120	115	1.04	40	1.02	57.83	1.90	7.07	0.44	3.00	126.13	75.87	11.00	121.71
3150A	2016	M	1	2200	126	5/4	10/11	161	145	1.11	113	1.00	90.82	3.49	13.31	0.41	3.00	191.43	128.77	14.70	243.51
65636	2017	M	2	1700	130	4/30	10/9	163	128	1.27	72	1.02	77.50	1.51	9.60	0.35	3.00	99.23	68.22	9.20	73.91
65636	2018	M	2	1700	130	5/16	9/14	122	87	1.40	53	1.02	47.19	1.29	4.43	0.19	3.00	39.31	26.50	3.89	25.62
65808	2017	M	2	1700	137	4/30	10/9	163	122	1.34	67	1.03	67.89	1.90	9.75	0.39	3.00	158.12	107.27	15.16	106.54
65808	2018	M	2	1700	137	5/15	8/30	107	92	1.16	57	1.00	57.03	1.86	5.32	0.34	3.00	78.23	46.82	6.54	46.72
66176	2019	M	1	1230	110	7/1	10/11	103	102	1.01	52	1.04	47.28	2.78	4.74	0.20	3.00	35.66	22.95	2.82	48.25
66248	2019	M	1	1000	100	5/22	10/11	143	136	1.05	126	0.99	46.68	2.78	6.21	0.21	3.00	36.73	19.89	2.43	26.83
66406	2018	M	2	530	92	5/16	10/12	150	80	1.88	41	1.02	49.35	2.13	5.96	0.20	3.00	60.21	34.23	3.80	89.34
66406	2019	M	2	530	92	5/22	10/13	145	136	1.07	83	1.01	42.64	1.96	6.65	0.23	3.00	50.42	26.83	3.52	78.02
7346A	2015	M	3	1680	124	5/13	10/15	156	83	1.88	56	0.99	79.63	1.70	7.93	0.33	3.00	152.18	97.17	16.27	85.34
7346A	2016	M	3	1680	124	5/5	10/10	159	122	1.30	52	1.02	42.11	1.96	9.37	0.23	3.00	106.80	78.46	8.90	103.92
7346A	2018	M	3	1680	124	6/6	10/24	141	131	1.08	106	1.00	72.55	1.86	9.27	0.26	3.00	82.34	55.40	6.27	93.93
7346A	2019	M	3	1680	124	5/22	9/29	131	116	1.13	98	0.98	43.48	1.85	4.97	0.35	3.00	81.40	44.67	6.61	51.16
9263A	2015	M	2	370	80	6/16	9/21	98	73	1.34	37	1.00	41.19	1.11	3.39	0.32	3.00	53.74	33.41	4.74	35.90
9730A	2015	M	2	1450	115	5/13	10/2	143	98	1.46	45	1.02	60.16	1.22	6.50	0.35	3.00	90.72	59.23	8.98	40.92
9730A	2016	M	2	1450	115	5/6	10/8	156	138	1.13	112	1.00	75.03	1.18	11.86	0.36	3.00	86.16	56.92	7.03	97.31
4125A	2015	F	3	790	107	6/10	10/15	128	66	1.94	45	1.02	20.19	1.45	1.99	0.10	3.00	17.97	11.19	1.46	19.59
4125A	2016	F(G)	3	790	107	5/9	10/10	155	128	1.21	82	0.99	10.44	0.85	0.58	0.07	3.00	6.37	3.43	0.15	7.52
62804	2019	F	2	400	73	6/4	10/11	130	127	1.02	80	1.03	21.77	1.33	2.87	0.19	3.00	16.74	10.20	1.66	14.27
62997	2019	F	1	500	81	6/4	10/10	129	127	1.02	50	1.05	22.63	0.99	2.82	0.15	3.00	11.94	6.60	0.85	15.98
63086	2019	F	1	600	91	5/22	9/19	121	117	1.03	64	1.01	16.00	0.61	1.87	0.07	3.00	3.65	2.43	0.32	4.58
65557	2017	F	1	400	94	4/17	9/27	164	131	1.25	110	1.01	28.24	1.42	3.46	0.12	3.00	17.55	11.31	1.18	17.79
65557	2018	F	1	400	94	5/16	10/14	152	94	1.62	48	1.04	33.23	1.91	3.56	0.20	3.00	62.74	37.89	4.28	13.55
65685	2017	F	1	500	91	4/17	7/19	94	79	1.19	48	1.02	46.74	1.97	3.92	0.17	3.00	37.49	25.56	2.97	51.88
65962	2018	F	1	580	84	5/16	10/12	150	142	1.06	100	1.00	14.12	0.47	2.01	0.07	3.00	3.40	2.17	0.26	3.19
66462	2018	F(G)	1	850	97	5/16	10/14	152	139	1.09	45	1.04	20.04	1.15	0.89	0.09	3.00	13.63	7.57	0.45	25.04
66462	2019	F	1	850	97	5/22	10/11	143	130	1.10	38	1.02	42.82	2.56	6.69	0.21	3.00	64.75	41.13	4.03	120.44
66953	2019	F	1	450	85	5/22	10/11	143	140	1.02	70	1.02	15.04	0.49	2.00	0.05	3.00	3.27	2.12	0.24	4.36

Home range size, daily movement distances, and distance traveled from hibernacula vary considerably between studies (see Appendix Table 1). Ecological factors such as habitat types, food resources, and habitat fragmentation can contribute to such variation (as summarized in [42]). However, some of this variation might also be attributed to data collection techniques or analytical methods. Petersen et al. [41]) showed that movement distances and the distance traveled from hibernacula are influenced by the number of locations recorded for individual snakes.

Only one study of the home range of C. horridus has been conducted in the central west part of the species range (Fig. 1), and no studies have been conducted in Illinois, where the species is listed as Threatened (https://dnr.illinois.gov/content/dam/soi/en/web/dnr/espb/documents/et-list-review-and-revision/illinoisendangeredandthreatenedspecies.pdf). Anderson [38] tracked 23 (12 males, 11 females) individual timber rattlesnakes for at least one active season in St. Louis County, Missouri and estimated the minimum and maximum “activity area” (which we interpret as a seasonal home range) for each snake using Minimum Convex Polygons (MCP). We calculated the mean home range for male, nongravid and gravid female C. horridus using the data tables provided: on average males had larger home ranges (x = 97.5 ha, SE = 24.8, n = 21) than non-gravid females (x = 12.1 ha, SE = 1.7, n = 22) and gravid females (x = 7.3, SE = 1.6, n = 11). Anderson also reported that males moved greater daily distances during the mating season.

In this study, we provide a second evaluation of home range and movement patterns for C. horridus in the west-central part of the species' range, and we also provide an assessment of site fidelity and annual overlap of high-use areas.

Study Site and Data Collection. — Our study was conducted at Principia College and surrounding properties in Jersey County, west-central Illinois (Fig. 1). The surrounding landscape is dominated by upland Oak-Hickory Forest interspersed with occasional fields and human development. The site is bounded to the south by 75 m high limestone bluffs that run adjacent to the Mississippi River and are covered with a vegetational matrix of remnant hill prairies and woodlands. Crevices, scree slopes, and holes along the bluff front serve as hibernacula for C. horridus throughout the winter. Knowledge of these dens and their locations date back to the acquisition of the land for the College in the 1930s.

We captured C. horridus along ~ 3 km of bluff front during the species’ fall ingress (September – October) and spring egress (April – May) periods or opportunistically throughout the active season (May – October). Captured snakes were weighed, measured (snout-vent-length (SVL); tail-length (TL); subcaudal scale, and rattle segment counts) and sexed via cloacal probing, and a passive integrated transponder (PIT) tag (Avid Microchip ID Systems or BioMark PIT tag) was implanted subcutaneously in the distal 1/3 of the snakes’ body. A subset of captured individuals was chosen for radiotelemetry based on size (i.e., ability to support a transmitter), sex (attempting to achieve an equal sex ratio), and reproductive status (gravid females were not implanted to prevent undue stress). Following the methods of Reinert and Cundall [43], temperature-sensitive radio transmitters (Holohil SI-2T, Carp, Ontario) were surgically implanted in the peritoneal cavities of snakes under inhalation anesthesia (isoflurane) by veterinarians at the Saint Louis Zoo. The transmitter weight was < 5% of a snake’s body mass. Snakes were released at their original capture locations, usually within one week after surgery. We located C. horridus daily throughout their active season from their date of release, or spring egress in subsequent years, to fall ingress into hibernacula using a receiver (Advanced Telemetry Systems (ATS) model R410) and a three-element folding yagi antenna. At each location, we recorded the snake’s geographic position using a handheld GPS device (Garmin eTrex 10 (2015–2019) or blue-tooth GPS (Garmin Glo) and Apple Ipad mini combination (2019), average accuracy of both systems ~ 3 m).

Home Range Estimation. — All analyses were conducted using R Version 3.6.0 [44]. Maps were created in ArcMap 10.7 [45]. We calculated annual home ranges for each snake per year using 99%, 95%, and 50% isopleths derived from Brownian Bridge utilization distributions (BBMM;[46]. We also report 100% minimum convex polygons (MCP [5]) for comparison with other studies. BBMM requires two smoothing parameters, sig1 (related to the speed of an animal as calculated via its movement trajectory) and sig2 (related to the precision of GPS locations). Following the approach of Horne et al. [46], we calculated the maximum likelihood estimation of sig1 for each snake-year combination (using the ‘liker’ function in the R package ‘adehabitat’), and we used the average GPS accuracy for all locations collected in the field (3 m) for sig2. We used area-asymptote curves to determine if enough fixes had been obtained for each snake-year combination to estimate annual home range sizes [47, 48]. The area-asymptote curve was estimated by removing all partial tracks (resulting from snake mortality or loss) from the data set and bootstrapping the MCP home range area of the remaining snake-year combinations by increments of 5 radiolocations (beginning at 5) for 500 iterations. We determined the point of asymptote by generating a line of best fit through the bootstrapped home ranges using nonlinear regression with a monomolecular growth function [49, 50]. We considered the number of fixes to be adequate if the home range size was > 95% of the asymptote. Snakes with non-asymptotic home ranges were removed from further analysis (but see methods on movement metrics below).

We examined the effects of sex, mass, SVL, and year on home range sizes independently for each of the four home range methods (99%, 95%, 50% BBMM isopleths, and 100% MCP) using linear mixed effects models (LMM; R package ‘lme4’) [51], with individual snake and year of tracking as random effects in all models. Although we had previously accounted for the effect of radio telemetry fixes by removing non-asymptotic home ranges in this analysis, we also included the number of fixes as a covariate in all models to control for varying sampling efforts. We ranked candidate models, including a global (fully-additive) and null (intercept-only) model, using Akaike’s information criterion adjusted for small sample sizes (AIC_c; R package ‘AICcmodavg’ [52]). Parameters with 95% confidence intervals (CI) not overlapping zero were considered significant influences of annual home range size. The top model(s) in each candidate set were graphed (R package ‘ggplot2’ [53] to display predicted values and 95% CIs [54].

Movement Metrics. — We calculated commonly reported movement metrics, including displacement distance from hibernacula and mean daily movement distance. Daily displacement distances were calculated for each snake-year combination as the Euclidian (straight-line) distance between a snakes recorded location and its hibernacula for each day of its tracking duration. The daily movement distance for each snake-year combination was calculated as the Euclidean distance between a snake’s recorded location on each day and the previous day’s location. If multiple days had passed between successive locations, the distance moved was divided by the number of days between locations. Similar to the home range analysis, we examined the effects of sex, mass, SVL, and year on displacement distance from hibernacula and average daily movement distances using linear mixed-effects models. The candidate model structure was the same with individual snake as a random intercept and snake ID and year as random effects. The top-ranked AIC_c model(s) were graphed (R package ‘ggplot2’ [53]) to display both predicted values and 95% CIs [54]. We also examined temporal changes in displacement distances and daily distances traveled throughout the active season by plotting their daily arithmetic averages (averaged across snake ID) across the tracking period against the day of year. We averaged the data of individuals tracked for two or more years to avoid independence issues. Unlike our previous analyses in which snake-year combinations that failed to reach asymptote were removed, all snakes were included in daily movement analysis. These daily averages (± standard error) were displayed as seven-day moving averages grouped by sex.

Home Range Overlap and Site Fidelity. — We calculated intra-individual home range overlap (i.e., multi-year site fidelity) for C. horridus with multiple annual home ranges that met the previously described asymptotic criteria using several methods. Following the recommendations of Fieberg and Kochanny [55], we estimated the overlap for the 99%, 95%, and 50% BBMM-derived home ranges using the utilization distribution overlap index (UDOI) and Bhattacharyya’s affinity (BA: [56]), both of which incorporate the use-intensity aspect of the utilization distribution in overlap estimations. We also calculated simple isopleth overlaps for the BBMM isopleths and 100% MCP estimates by dividing the area of intersection between two given home range isopleths by the total combined area of both isopleths. For each overlap metric, we calculated the pairwise overlap between every intra-individual snake-year home range combination. We limited the overlap comparison to two seasons for each snake, and when there were multiple pairwise combinations we averaged all pairwise overlap values for each snake. We report the mean and standard error of the resulting values for all home range metrics.

We radio-tracked 29 individual C. horridus (13 female, 16 male) for one or more years between 2015 and 2019, accumulating a total of 51 snake-year combinations (Table 1). Nine of the 51 snake-year combinations were partial tracks (due to snake mortality or signal loss), and another six failed to reach the home range asymptote, suggesting that they had an inadequate number of radio-location fixes to estimate annual home range sizes and were subsequently removed from further analysis (Table 1). The remaining 36 snake-year combinations reached home range asymptotes between 32 and 126 locations (x̄=66.46; SE = 4.32; Table 2) and represented 21 unique individuals (11 females; 10 males) that were located on average every 1.3 days (SE = 0.06; Table 2) for a mean number of 113.81 (SE = 4.04, total = 4097) location fixes each. Radio-tracked males were generally heavier (mass x̄=1285.56 g, SE = 136.82 g) and longer (SVL x̄=111.86 cm, SE = 4.74) than both non-gravid (mass: x̄=505.63 g, SE = 43.77 g; SVL: x̄=86.91 cm, SE = 2.48) and gravid females (mass: x̄=820.05 g, SE = 29.95; SVL: x̄=102.05 cm, SE = 4.95).

Table 2

— Grand means (± SE) of snake mass (Mass), snout-vent-length (SVL), radio-tracking duration (Track length), number of GPS locations (# fixes), days between fixes (Days / Fixes), and minimum fixes required for home range asymptote (Min. Fixes) for 36 snake-year combinations from 21 individual timber rattlesnakes (*Crotalus horridus*), tracked for one or more years in Jersey County Illinois, during the active seasons of 2015–2019. Data summaries include multiple entries for individuals tracked for one or more years.
ID	Year	Sex	Den ID	Mass (g)	SVL (cm)	Track Start	Track End	Track Length (Days)	# Fixes	Days / fixes	Min. Fixes	Prop. HR	Daily Dist. (m)	Max. Disp. (km)	Total Dist. (km)	Sig1	Sig2	99% BBMM (Ha)	95% BBMM (Ha)	50% BBMM (Ha)	MCP (Ha)
7223A	2015	F	1	290	74	5/13	10/13	154	61	2.52	34	1.02	24.22	1.37	3.77	0.13	3.00	54.58	37.46	3.74	30.40
7223A	2016	F	1	290	74	5/9	10/11	156	138	1.13	78	1.01	23.82	0.72	3.93	0.16	3.00	16.44	10.52	1.56	12.00
7223A	2017	F	1	290	74	5/2	9/27	149	116	1.28	81	1.01	38.91	1.19	4.79	0.19	3.00	30.48	19.85	3.31	28.43
9624A	2015	F	1	400	90	5/5	10/13	162	97	1.67	56	1.03	36.56	1.56	5.32	0.16	3.00	42.20	27.69	4.67	40.16
9624A	2016	F	1	650	91	4/23	10/6	167	125	1.34	44	1.02	26.55	1.44	4.79	0.13	3.00	53.11	37.75	3.20	36.64
9624A	2017	F	1	700	92	4/17	9/27	164	141	1.16	79	1.01	25.67	1.44	3.89	0.14	3.00	23.16	15.00	1.82	16.11`
67201	2019	M		1870	112	6/24	7/26	33	27	1.22	--	--	--	--	--	--	--	--	--	--	--
3150A	2015	M	1	2200	126	7/7	10/13	99	46	2.15	--	--	--	--	--	--	--	--	--	--	--
6346A	2016	M		350	85	5/6	6/14	40	34	1.18	--	--	--	--	--	--	--	--	--	--	--
7346A	2017	M	3	1750	144	8/12	10/9	59	35	1.69	--	--	--	--	--	--	--	--	--	--	--
62937	2019	F		400	82	5/22	7/1	41	37	1.11	--	--	--	--	--	--	--	--	--	--	--
5126A	2015	F		190	39	5/13	10/8	149	11	13.55	--	--	--	--	--	--	--	--	--	--	--
7223A	2018	F	1	290	74	9/16	10/14	29	24	1.21	--	--	--	--	--	--	--	--	--	--	--
7223A	2019	F	1	290	74	5/22	5/31	10	9	1.11	--	--	--	--	--	--	--	--	--	--	--
9624A	2018	F	1	450	90	5/17	7/23	68	33	2.06	--	--	--	--	--	--	--	--	--	--	--
9730A	2017	M	2	1800	130	4/30	7/23	85	67	1.27	73	0.95	48.29	0.90	3.11	0.16	3.00	25.85	16.89	1.59	18.24
9623A	2015	M		200	64	5/5	10/13	162	68	2.38	74	0.93	13.78	0.72	1.81	0.11	3.00	49.83	31.07	1.00	5.83
66111	2017	M		2200	123	5/20	7/2	44	41	1.07	77	0.81	39.13	1.35	1.54	0.11	3.00	10.00	6.60	0.66	14.39
6346A	2015	M		350	85	6/10	9/5	88	66	1.33	131	0.80	33.14	1.35	3.18	0.18	3.00	34.90	23.37	1.85	30.22
63175	2019	M		1681		7/19	10/11	85	73	1.16	242	0.59	28.13	2.03	2.55	0.14	3.00	19.61	9.49	0.66	51.02
60248	2018	M		1120	110	5/16	10/14	152	54	2.81	65	0.93	33.29	1.74	3.24	0.17	3.00	110.82	71.57	6.78	12.26

Home Range Estimation. — Prior to analyzing factors that might account for variation in home range size, we removed gravid female home ranges (n = 2) from the data set due to small sample sizes and their uniquely distinct home range size and characteristics. The candidate model that included the independent variables sex and number of fixes was the top AICc-ranked model for all home range methods (Appendix Table 3). As indicated via 95% confidence intervals, sex was a significant influencer of home range size for all methods of home range estimates (Appendix Table 4), with male C. horridus exhibiting significantly larger home ranges than non-gravid females (Fig. 2, A-D). Conversely, the number of fixes was insignificant in all top models. As revealed by the top AICc models for each method, the predicted (marginal) home range sizes for male and non-gravid C. horridus when controlling for the number of fixes were 88.26 Ha (CI = 66.15-110.36) and 30.64 Ha (CI = 7.68–53.60) for 99% BBMM; 56.91 Ha (CI = 41.04–72.77) and 20.22 (CI = 3.81–36.62) for 95% BBMM; 7.56 Ha (CI = 5.47–9.65) and 2.36 Ha (CI = 0.19–4.53) for 50% BBMM; and 80.03 Ha (CI = 50.95-109.11) and 30.80 Ha (CI = 1.55–60.06) for MCP (Fig. 2, A-D, Fig. 4).

Table 3

— Average annual home range overlap indices (+/- SE) for 11 Timber Rattlesnakes (*Crotalus horridus;* n = 20 home range overlaps) tracked for two or more years in Jersey County Il. Overlap of home ranges calculated using the Brownian Bridge Movement Model (BBMM; for 99%, 95% and 50% isopleths) were quantified using Bhattacharyya's Affinity (BA), the Utilization Distribution Overlap Index (UDOI), and simple isopleth overlaps (i.e., dividing the area of intersection between two given home range isopleths by the total combined area of both isopleths). The latter metric was also used to calculate overlap of home ranges calculated via Minimum Convex Polygons (MCP).
Sex	n	# Snake Years	Grand Means (SE)
Sex	n	# Snake Years	Mass (g)	SVL (cm)	Track Length (days)	# Fixes	Days / Fixes	Min. Fixes
M	10	18	1285.56 (136.82)	111.86 (4.74)	139.11 (4.96)	111.06 (5.34)	1.29 (0.06)	68.96 (7.02)
F	11	16	505.63 (43.77)	86.91 (2.48)	144.13 (4.89)	114.44 (6.71)	1.33 (0.1)	64.03 (5.61)
F(G)	--	2	820.05 (29.95)	102.05 (4.95)	153.5 (1.5)	133.5 (5.5)	1.15 (0.06)	63.31 (18.5)
All	21	36	913.06 (94.96)	100.23 (3.3)	142.14 (3.3)	113.81 (4.04)	1.3 (0.06)	66.46 (4.32)

Sex	n	# Overlaps	BA (SE)			UDOI (SE)			% BB (SE)			% MCP (SE)
Sex	n	# Overlaps	BBMM 99%	BBMM 95%	BBMM 50%	BBMM 99%	BBMM 95%	BBMM 50%	BBMM 99%	BBMM 95%	BBMM 50%	% MCP (SE)
M	6.00	11.00	0.46 (0.03)	0.38 (0.03)	0.08 (0.02)	0.75 (0.18)	0.37 (0.09)	0.02 (0.01)	0.39 (0.02)	0.3 (0.02)	0.08 (0.02)	0.5 (0.07)
F	5.00	7.00	0.47 (0.03)	0.39 (0.03)	0.07 (0.01)	0.59 (0.05)	0.32 (0.03)	0.01 (0)	0.35 (0.03)	0.28 (0.02)	0.07 (0.01)	0.43 (0.02)
F(G)	--	2.00	0.28 (0.06)	0.24 (0.06)	0.07 (0.01)	0.56 (0.16)	0.22 (0.07)	0.01 (0)	0.19 (0.08)	0.11 (0.05)	0.03 (0.02)	0.26 (0.12)
All	11.00	20.00	0.46 (0.03)	0.39 (0.03)	0.09 (0.02)	0.78 (0.13)	0.41 (0.08)	0.02 (0.01)	0.36 (0.03)	0.28 (0.03)	0.09 (0.02)	0.43 (0.04)

Movement Metrics. — Similar to the home range analysis, the candidate model including the independent variables sex and number of fixes was the top AICc-ranked model for both daily distance traveled and maximum displacement distance from hibernacula (Appendix Table 3). Sex was a significant influencer in both movement metrics (Appendix Table 4), whereas the number of fixes was insignificant. The estimated daily distance traveled was significantly greater for males (mean = 57.25 m/day, CI = 49.06–65.43) than females (mean = 27.55 m/day, CI = 18.99–36.12) (Fig. 2E), with a marked increase in male movements from late June (~ day of year 176) to mid-August (~ day of year 231) (Fig. 3B). Similarly, maximum displacement distances from the hibernacula were significantly greater for males (mean = 2.03 km, CI = 1.57–2.48) than females (mean = 1.29 km, CI = 0.85–1.73] (Fig. 2F), and on average, males were located further from their hibernacula throughout the entirety of their active season (Fig. 3A).

Home Range Overlap. Eleven C. horridus had multiple yearly home ranges that were adequate for overlap analysis, of which 8 individuals had 2 annual home ranges, 2 had 3 annual home ranges, and 1 had 4 annual home ranges, for a total of 20 overall annual home range overlaps (Fig. 5). The mean BBMM overlap for all snakes at the 99%, 95%, and 50% isopleths was 0.46 (SE = 0.03), 0.39 (SE = 0.03), and 0.09 (SE = 0.02), respectively. The mean UDOI overlap for all snakes at the 99%, 95%, and 50% isopleths was 0.78 (SE = 0.13), 0.41 (SE = 0.03), and 0.09 (0.02), respectively.

We used VHF telemetry to record the daily locations (mean location interval of 1.3 days) of 29 free-ranging timber rattlesnakes (Crotalus horridus) during their summer activity period from 2014–2019 for a total of 51 activity periods at a site in west-central Illinois. After filtering the dataset using area-asymptote curves [47, 48], 36 of those tracking records were sufficient for home range estimation. The home ranges for males were larger than those for non-gravid females, and both were larger than the home range of gravid females (Figs. 2 and 3). However, due to the wide range of analytical methods used by studies of the home range for this species, a direct comparison of our home range size to others is difficult. Nonetheless, we can note that the home range of our males (88.26 Ha:CI = 66.15-110.36) and non-gravid females (30.64 Ha: CI = 7.68–53.60) fall close to the mean of all published home range utilization distributions (85.5 Ha and 36.2 Ha) reported for the species in the scientific literature (Appendix Table 1). Dispersal distances from hibernacula and daily travel distances were also greater for males (Figs. 2 and 3). Such results are consistent with other studies (e.g., [34–41]), where it is proposed that the search behavior of males looking for females during the mating season results in larger home ranges. Males in our study showed a distinct increase in movements between ~ day 185 (4 July) and ~ day 235 (22 August) each summer (Fig. 3), which encompasses the mating season for this location [57]. Even though sex was an important variable affecting home range size, there was still a high amount of unexplained variance (Fig. 2). We did not have enough power to explore interactions between sex and other variables. Others have found a significant interaction between sex and SVL or sex and mass [41]; however, these factors are likely correlated with the sexual size dimorphism of this species, so such analysis is problematic to interpret.

When evaluating general trends in dispersal from hibernacula, two patterns emerged (Fig. 4). Females move directly away from their hibernacula to their high-use areas, and their home ranges appear very linear. While males also move directly to foraging sites, their use areas are much broader, likely due to mate searching in mid-summer, so their overall home ranges look less linear than those of female rattlesnakes. Second, all snakes from specific hibernacula tend to move in a similar dispersal direction (Fig. 4). The snakes from hibernacula 1 (located ~ 0.74 km east of hibernacula 2) dispersed from their hibernacula toward the northwest, and snakes from hibernacula 2 moved directly north or slightly NE. The difference in dispersal direction and lack of home range overlap between snakes dispersing from different hibernacula might be due to potential barriers to dispersal, which in this case are old agricultural fields located north of hibernacula 1 and northeast of hibernacula 2. These fields are maintained under the Conservation Reserve Program of the U.S. Dept. of Agriculture and are planted in tall fescue grasses and warm-season native species. It is our experience that timber rattlesnakes typically avoid such open fields when encountered and instead skirt around their perimeters, as evidenced by the dispersal routes we document from these two hibernacula. Evidence of avoidance of these open areas can be found in the location records. Out of 4097 GPS locations from snakes with complete tracking records between 2014 and 2019, only 153 locations (3.73%) were in fields, and 863 (21.01%) were recorded along the margins (within 15 m of either side of the forest-field edge).

An alternate and possibly complimentary explanation for the dispersal pattern following fixed directions from their hibernaculua might be that dispersal direction and the subsequent high-use area are linked to timber rattlesnake tendency for natal homing. That offspring return to their natal hibernacula has been theorized as an explanation for genetic differentiation between hibernacula [38, 58–60]. If timber rattlesnakes have strong natal homing and follow their mothers into high-use areas on first departure from their birth areas, it is conceivable that they may imprint on those foraging areas, much as they seem to imprint on their natal hibernacula. Such behavior would lead snakes from a single hibernaculum to disperse to similar use areas. Evaluating the maternal origins of foraging snakes (e.g., using MtDNA) in distinct foraging areas might yield valuable insights into how this species chooses foraging locations.

Irrespective of the reason for dispersal direction, individual snakes tended to return to the same use areas each year. The average home range overlap at 99% BBMM was 0.49 and 0.78 using UDOI overlap, and both values imply relatively high overlap between years, particularly when considering all the areas a snake may choose to inhabit during the active season. These significant levels of habitat return may partially explain why relocation of timber rattlesnakes show reduced fitness [35]. Once established on a specific use area, timber rattlesnakes may lack adaptability to forage in alternate areas, and relocated snakes do not thrive.

Our study of the home range and active-season movements of the timber rattlesnake provides supporting information on the sexually dimorphic home range and movement patterns of the species in the central part of the species range. Our observations of high levels of annual home range overlap and possible segregation of high-use areas by hibernacula provide further insights into the biology of this important forest predator.

BA - Bhattacharyya's Affinity

BBMM - Brownian Bridge utilization distribution

GPS - Global Positioning System

MCP - Minimum Convex Polygon

PIT - Passive Integrated Transponder

SVL - Snout Vent Length

TL - Tail Length

UD - Utilization Distribution

UDOI - Utilization Distribution Overlap Index

VHF - Very High Frequency

Ethics approval and consent to participate.

This research was authorized under Illinois Department of Natural Resources Endangered and Threatened Species permits (15-009, 05-11s, 4956, 6573, 7304, HSCP 19-04, 14636) and approved by the University of Illinois Institutional Animal Use and Care Committee (IACUC 22167, IACUC 22168).

Consent for Publication

Authors (SAE, ACJ) consent to publication of this manuscript.

Competing interests

No, I declare that the authors have no competing interests as defined by BMC, or other interests that might be perceived to influence the results and/or discussion reported in this paper.

Funding

All funding for this research was derived from private sources, and all data used in the manuscript is the property of SAE.

Author's contribution

SAE - Funding, project conception, design and management, data collection and analysis, manuscript preparation.

ACJ - Project conception, design and management, data collection and analysis, manuscript preparation.

Acknowledgements

The authors would like to thank Hunter Benkoski , Taylor Bookout, Fletch Burbee, Rhiannon Davis, George Ghanem, Elsa Heath, Kelsie Huss, Anna Lemon, Haley Martin, Samson Myers, Tyler Nichoson, Gen Palgue, Jason Safronoff, Mallory Saylor, Chase Schneider, Kip Wadsworth and Katie Wood for the many hours spent tracking snakes each summer. Particular thanks to Ian Armesy for his long dedication to the program and coordination of the field teams. We also are extremely grateful for their generous support of veterinary and zoological staff at the St. Louis Zoo, particularly Dr. Luis Padilla, Dr. Chris Hanley, Dr. Mary Duncan, Dr. Kari Musgrave, Dr. Jeff Ettling, Mark Wanner, and Justin Elden.

Availability of data and materials statement

Due to ongoing concerns for poaching and persecution of C. horridus and at the request of permitting agencies, location data of snakes or their hibernacula are only available by direct request to the corresponding author.

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No competing interests reported.

EckertandJesperAppendix.docx

Home Range, Site Fidelity, and Movements of the Timber Rattlesnake (Crotalus horridus) in West-Central Illinois

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