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Neonate Cottonmouth Spatial Ecology and Habitat Selection: From Parturition to Hibernation
Zackary J. Delisle, Dean Ransom, and Johanna Delgado-Acevedo

Southeastern Naturalist, Volume 18, Issue 2 (2019): 334–345

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Southeastern Naturalist Z.J. Delisle, D. Ransom, and J. Delgado-Acevedo 2019 Vol. 18, No. 2 334 2019 SOUTHEASTERN NATURALIST 18(2):334–345 Neonate Cottonmouth Spatial Ecology and Habitat Selection: From Parturition to Hibernation Zackary J. Delisle1,2,*, Dean Ransom1, and Johanna Delgado-Acevedo1 Abstract - Understanding neonate ecology is imperative for effective knowledge of lifehistory stages, which have historically been based upon adult ecology. We telemetered 12 neonate Agkistrodon piscivorus (Cottonmouth) and monitored their spatial use, activity, and habitat selection. Neonates did not have large spatial requirements, nor did they disperse far before hibernation. Neonates selected habitat edges, and within edges they used areas with thick vegetative cover. We suggest that the habitat selection and limited use of space by neonate Cottonmouths are products of the migration their mothers make before parturition. Habitat edges appear to be important for both parturition and hibernation. Our study offers valuable insight into the initial life-history stages of Cottonmouths and presents a good baseline for future research on their ontogenetic ecological development. Introduction Numerous studies of the spatial ecology and habitat selection of pit vipers have been conducted over the last 40 years since the advent of radio telemetry (e.g., Reinert 1984, Roth 2005, Sutton et al. 2017, Weatherhead and Prior 1992). In earlier studies, even the smallest available radio transmitters were too large to be implanted in small-bodied snakes. Because of the inability of researchers to track smaller snakes, most of the information known on the life history of snakes is based on adults (Jellen and Kowalski 2007). This historic bias is concerning because of the ontogenetic ecological differences that are exhibited by most species (Jellen and Kowalski 2007). Technological advances have led to smaller radio transmitters, and therefore an increase in telemetric studies involving larger-bodied neonates (see Cobb et al. 2005, Figueroa et al. 2008, Gross 2017, Hileman et al. 2015, Howze et al. 2012, Jellen and Kowalski 2007, Muellman et al. 2018, Pizzatto et al. 2009, Rivas et al. 2016, Smith 2014). Although small radio transmitters are now available, most telemetric neonate studies used external radio attachment via an adhesive, but have suffered high detachment rates (Cobb et al. 2005, Howze et al. 2012, Jellen and Kowalski 2007, Muellman et al. 2018). Even so, these studies revealed interesting neonatal behavioral traits (e.g., conspecific scent trailing and arboreal behavior) and intraspecific ontogenetic ecological differences (Cobb et al. 2005, Figueroa et al. 2008, Gross 2017, Hileman et al. 2015, Howze et al. 2012, Jellen and Kowalski 2007, Muellman et al. 2018, Pizzatto et al. 2009, Rivas et al. 2016, Smith 2014). 1Department of Biological Sciences, Texas A&M-Commerce, Commerce, TX 75428. 2Current address - Department of Forestry and Natural Resources, Purdue University, West Lafayette, IN 47907. *Corresponding author - zdelisle@purdue.edu. Manuscript Editor: Brad Glorioso Southeastern Naturalist 335 Z.J. Delisle, D. Ransom, and J. Delgado-Acevedo 2019 Vol. 18, No. 2 Further telemetric studies involving the neonates of other snake species could yield additional life-history knowledge that is imperative for understanding all of the ontogenetic stages of a focal species (Jellen and Kowalski 2007). Agkistrodon piscivorus (Lacépède) (Cottonmouth) is a semiaquatic pit viper species native to southeastern United States (Gloyd and Conant 1990). Few Cottonmouth field studies have incorporated neonates, and none used a telemetric approach. For these reasons, we intraperitoneally implanted radio transmitters into neonate Cottonmouths. Our objectives were to (1) discover if intraperitoneal implantation was more efficacious than external attachment methods, and (2) monitor neonate Cottonmouth activity, spatial use, and habitat selection. We predicted that (1) intraperitoneal implantation would result in a smaller percentage of neonates experiencing radio loss when compared to previous studies using external attachment, and (2) our telemetered neonates would select areas with more vegetative ground cover, unlike past non-telemetric neonate Cottonmouth habitat selection studies that used opportunistic sighting methods (Eskew et al. 2009). We additionally recorded the habitat characteristics of parturition sites, and monitored the amount of time that gravid female Cottonmouths spent at parturition sites. Field-Site Description Our study site was a constructed wetland (TAMUC Wetlands) owned by Texas A&M University-Commerce located in Commerce, TX. This site was a hay field before being constructed into a wetland in the spring of 2007. Construction consisted of digging several levees and basins of differing size and shape that naturally filled with water. The eastern side of the wetland was immediately adjacent to Texas State Highway 24 and transitioned into a mesic bottomland forest with sporadic patches of grazed cattle pasture to the west. The northeastern section of the property was comprised of native prairie, and the southern border of the property was immediately adjacent to grazed cattle pasture (Fig. 1). Methods Surgical implantation of transmitters We obtained neonate Cottonmouths through a separate telemetric project on adult Cottonmouths within the same study site (Delisle et al. 2019). When we found a postpartum female aggregated with her neonates at a parturition site, we captured a select few of the neonates and brought them back to the lab for surgical implantation of radio transmitters. We anesthetized neonates using isoflurane (Clipper Distributing Company, LLC, St. Joseph, MO) via an anesthetic vaporizer (ASA Isoflurane, Harvard Apparatus Limited, Kent, UK), and surgery commenced immediately upon the loss of the righting reflex. We intraperitoneally implanted radio transmitters (modified model LPI-2012-COVERT, ~1g, Wildlife Materials Inc. Murphysboro, IL) into neonates, similar to Smith (2014), following the procedures described in Reinert and Cundall (1982). Radio transmitters were always less than 5% of the subject’s body weight. We sanitized surgical instruments using Southeastern Naturalist Z.J. Delisle, D. Ransom, and J. Delgado-Acevedo 2019 Vol. 18, No. 2 336 Figure 1. The Texas A&M-Commerce constructed wetlands shown with all 6 cover types and every neonate Cottonmouth parturition site (n = 4) and hibernacula site (n = 7). Southeastern Naturalist 337 Z.J. Delisle, D. Ransom, and J. Delgado-Acevedo 2019 Vol. 18, No. 2 an autoclave prior to surgery and used isopropanol to maintain sterile conditions during surgery. Upon completion of the surgery, we used a 4-0 synthetic absorbable suture (CP Medical Inc. Norcross, GA) to seal the incision, and subsequently administered 3M vetbond tissue adhesive (3M Animal Care Products, St. Paul, MN) over the suturing. Snakes were given bedding, water ad libitum, and monitored for 24 h before being released at their point of capture, which was always with their mother at the parturition site. We refrained from removing radio transmitters before hibernation because of the correlation between mortality and radio implantation surgery immediately prior to hibernation, and instead we marked hibernacula locations and attempted to find them the following spring (Rudolph e t al. 1998). Home range, dispersal, and activity We tracked neonates an average of 3 times per week from 0800 to 1800 using an R1000 radio receiver (Communications Specialist Inc. Orange, CA) and a 3-pronged yagi antenna (Wildlife Materials Inc. Murphysboro, IL). Once a telemetered snake was located, we used a GPSMAP 64 global positioning system (Garmin Ltd. Olathe, KS, error = 3.6 m) to record the snake’s location. Upon locating each snake, we recoreded their activity as either coiled, active (extended or moving), or fossorial (defined as underground in small-mammal burrow). We quantified neonate home ranges using the 95% kernel density estimate (KDE; 95% isopleths) with the least-squares cross-validation bandwidth estimation in Geospatial Modelling Environment (Beyer 2012, Seaman and Powell 1996). We also generated 100% minimum convex polygon (MCP) home ranges in order to compare our results to those of previous telemetric studies on neonate pit vipers (Howze et al. 2012), and because KDEs can overestimate the home ranges of herpetofauna (Row and Blouin-Demers 2006). Home ranges were only calculated for snakes with more than 20 unique locations. We used Arc-Map version 10.2 to measure the maximum distance dispersed from parturition sites and the distance between a neonate’s parturition site and hibernacula (defined as the small-mammal hole that the respective neonate overwintered in). We only measured dispersal for snakes that were telemetered for a minimum of 30 days, and only measured distances between parturition sites and hibernacula for snakes that were telemetered until mid-November (when all neonates remained in the direct proximity of the small-mammal burrow that they overwintered in). Home-range habitat preferences We delineated and mapped emergent herbaceous wetlands (19.05% of study site), pasture/hay (7.77% of study site), prairie (15.84% of study site), deciduous forest (21.42% of study site), open water (3.36% of study site), and edge habitat (32.55% of study site; delineated by a 15-m buffer on the edge of each cover type [Blouin-Demers and Weatherhead 2001, Homer et al. 2004]). We evaluated home-range habitat selection within the entire study site, similar to Johnson (1980) second-order selection, through a use versus availability approach. We generated Manly selection ratios (MSR; Manly et al. 2002) to compare the proportion of each available cover type within the study site to the proportion of each cover type Southeastern Naturalist Z.J. Delisle, D. Ransom, and J. Delgado-Acevedo 2019 Vol. 18, No. 2 338 within each snake’s 95% KDE. MSRs were generated using the “adehabitatHS” package in Program R (R Core Team 2017). Preferred cover types generate an MSR greater than one, and avoided cover types generate an MSR less than 1 (Manly et al. 2002). We considered the selection of a certain cover type to be significant if the 95% confidence intervals of that MSR did not overlap one. Microhabitat selection We assessed microhabitat selection, similar to Johnson (1980) fourth-order selection, through a use–availability approach. Upon locating each snake, we measured ground temperature, ambient temperature, and ground cover at the snake location and subsequently again at a corresponding random location. We identified random locations using a random-number generator to compute a bearing (0°–360°) and distance (1–50 m) from the snake’s location (Table 1). In order to not further disturb neonates, we flagged both locations and returned later to record other vegetative microhabitat variables inside a 1-m2 quadrat placed over the locations (Table 1). Coinciding snake and random microhabitat variables were always measured on the same day within 5 minutes of each other. Thus, we measured ground temperature, ambient temperature, and ground cover on the same day at both locations, and the rest of the microhabitat variables within a week later on the same day at both locations. We only measured microhabitat variables if the individual was visually observed above ground and had moved more than 1 m from the previous location. To evaluate microhabitat preferences, we generated a priori binary generalized linear models in Program R (Table 2; R Core Team 2017) where “1” represented Table 1. Microhabitat variables recorded at each unique neonate Cottonmouth location and corresponding random location. The variable “ground cover” was objectively quantified at random locations by putting a flexible rubber hose in the exact same position that the corresponding neonate was found in. All variables were measured at neonate and random locations. *variables that were measured at parturition sites. Variable Abbreviation Description Distance from water* dfw Distance (m) to the nearest water > 5 cm deep Ground temperature gt Temperature (C) of ground within 2 cm of snake Ambient temperature at Temperature (C) taken 1.3 m above the location Ground cover gcv % of snake's body concealed by vegetation < 1.3 m Max. veg. height mvh Tallest vegetation (mm) within the m2 quadrat Canopy cover* cc % of canopy cover > 1.3 m Litter* ltr % of litter cover within the m2 quadrat Coarse woody debris* cwd % of coarse woody debris within the m2 quadrat Bare ground* bg % of bare ground within the m2 quadrat Water* wtr % of water cover within the m2 quadrat Forb* frb % of forb cover within the m2 quadrat Grass* grs % of grass cover within the m2 quadrat Sedge and rush* sr % of sedge and rush cover within the m2 quadrat Woody* wdy % of woody cover within the m2 quadrat Southeastern Naturalist 339 Z.J. Delisle, D. Ransom, and J. Delgado-Acevedo 2019 Vol. 18, No. 2 snake locations and “0” represented random locations. We developed a priori models through our ecological knowledge of Cottonmouths and the study site, which allowed us to further test biological hypotheses (Burnham and Anderson 2002). For instance, each of our models were based on a cover type (e.g., edge, forest, wetland, etc.) or specific habitat-selection reasoning (e.g., thermal, cover, cryptic, etc.; Table 2). We assessed models using an information theoretic approach. Best-supported models were determined using Akaike’s information criterion adjusted for small sample sizes (AICc), the difference between the most supported AICc model and each following model (Δi), Akaike weights (ωi), and evidence ratios (ωi/ωj) (Burnham and Anderson 2002). We calculated the standardized β-coefficients of each parameter within the most supported model using the ‘reghelper’ package in Program R (R Core Team 2017) and considered a parameter significant if the probability of receiving a z-statistic greater than the absolute value of the parameter’s z-statistic was less than 0.05. Maternal activity If we found a postpartum female (females from Delisle et al. 2019) aggregated with her neonates, we immediately recorded the location and measured several different vegetative variables at the parturition site (Table 1). Following this, we recorded how many days the mother remained at the natal area with her neonates. All results are presented ± 95% CI. Results We telemetered 12 neonates from 4 different females (female 1 = 4 neonates, female 2 = 4 neonates, female 3 = 3 neonates, and female 4 = 1 neonate) from 17 August 2017 to 8 December 2017 for a total of 256 unique locations. The mean number of unique locations per individual was 21. Unknown predators depredated 2 neonates, and we lost the signal of 2 radio transmitters (due to unknown causes). One of our telemetered neonates excreted their radio (known because this individual was observed after we found the radio). Seven of our telemetered specimens were telemetered until the end of the study. At least 1 neonate from every mother survived until the end of the study. Table 2. A priori binary generalized linear models for neonate Cottonmouth microhabitat selection. Refer to Table 1 for information on microhabitat variable abbreviations. Model name Microhabitat variables included Thermal gt + at Cover ltr + bg + wtr + frb + grs + sr + wdy + cwd Cryptic gcv + cc + mvh Wetland sr + wtr + dfw + gt Forest cc + cwd + grs + wdy + at + bg + dfw Edge cc + grs + wdy + gcv + cwd + sr + at + dfw Prairie ltr + frb + grs + wdy + gt Global dfw + gt + at + gcv + cc + ltr + bg + wtr + frb + grs + sr + wdy + cwd + mvh Southeastern Naturalist Z.J. Delisle, D. Ransom, and J. Delgado-Acevedo 2019 Vol. 18, No. 2 340 Home range, dispersal, and activity We computed fall home-range sizes for 7 individuals. The mean (± 95% CI) 100% MCP was 0.93 ha (± 0.33–1.53) and the mean (± 95% CI) 95% KDE was 1.15 ha (± 0.60–1.70). The mean (± 95% CI) maximum distance dispersed from a parturition site was 159.71 m (± 106.27–213.15) and the mean (± 95% CI) distance from a neonate’s hibernacula to their parturition site was 104.47 m (± 15.16–193.78). The amount of times we telemetered neonates to locations above ground, either active or coiled, decreased from September to November. We never observed neonates above ground during the month of December. Habitat selection and maternal activity Neonates 95% KDE contained significantly more edge cover (MSR = 1.717 ± 1.2556–2.1792) and significantly less prairie cover (MSR = 0.053 ± -0.0876– 0.1946) than what was proportionally available to them within the study site. No other cover types showed a significant selection. We collected microhabitat data from 155 neonate locations and 155 corresponding random locations. The most supported model was the edge model (AICc = 187.28, Δi = 0 , ω i = 0.70, ωi/ωj = 2.34). The only other model with any support was the global model (AICc = 188.98, Δi = 1.70, ωi = 0.3). Within the edge model, neonates selected areas with significantly more canopy cover (β = 3.120, P(>|t|) = 0.004) and ground cover (β = 4.691, P(>|t|) = < 0.001; Table 3). No microhabitat variables were significantly selected against. Gravid females (n = 4) selected parturition sites that averaged 29.6 m from water and had high amounts of canopy cover, sedge and rush cover, and coarse woody debris cover. Postpartum females stayed at the natal area for an average of 18.3 days. Discussion Intraperitoneal implantation methods resulted in only 8.3% of our radios being separated from the subject (1 individual excreted radio). Previous telemetric Table 3. Standardized β-coefficients of the model parameters within the most supported model—the edge model—and their standard errors, and z-statistics. We considered parameters significant if the probability of receiving a z-statistic greater than the absolute value of the parameter’s z-statistic was less than 0.05. * = statistically significant parameters, β = standardized regression coefficient, SE = standard error, and P(>|z|) = probability of receiving a value > |z|. Parameter β SE z value P(>|z|) Canopy cover* 1.562 0.273 5.713 less than 0.001 Grass cover* 0.720 0.254 2.831 0.005 Woody cover 0.091 0.233 0.389 0.698 Ground cover* 2.349 0.316 7.443 less than 0.001 Coarse woody debris cover 0.548 0.295 1.859 0.063 Sedge and rush cover* -0.809 0.264 -3.061 0.002 Ambient temperature* 0.635 0.209 3.040 0.002 Distance from water -0.275 0.201 -1.365 0.172 Southeastern Naturalist 341 Z.J. Delisle, D. Ransom, and J. Delgado-Acevedo 2019 Vol. 18, No. 2 research on neonate snakes that used external attachment methodology reported higher detachment rates of 57.1% (8/14; Howze et al. 2012), 12.5% (2/16; Jellen and Kowalski 2007), and 73.7% (16/22; Muellman et al. 2018); supporting our first prediction. However, 2 of our neonates were depredated and 2 radio signals were lost. Even though neonates are more vulnerable to predation than adult snakes (Bonnet et al. 1999), it is possible that our methods subjected neonates to higher risks of predation. One of our individuals excreted their radio, which has been shown to occur in other species via the alimentary tract encapsulating the radio and then ridding it through defecation (Pearson and Shine 2002). Our study suggests that the spatial hierarchy of neonate Cottonmouth site selection at the TAMUC wetlands was primarily based on parturition site selection by the mother. Three of 4 gravid females made distinct migrations out of the wetland to the edge of the neighboring mesic forest where parturition occurred in mid-August to mid-September (Fig. 1). The parturition migrations of gravid females seemed to benefit neonates in several ways. Forest edges have been found to support several species of burrowing small mammals (Sekgororoane and Dilworth 1995), and many pit viper species use these burrows as hibernacula (Brown 1982, Jellen and Kowalski 2007). Such use proved especially true for neonate Cottonmouths in our study, as we frequently observed them underground in small-mammal burrows during November and December. The parturition migration incurred by gravid females relieves their neonates from having to make this migration themselves in order to find suitable hibernacula. The likely result is increased neonate survivorship, as neonates required to migrate from their natal grounds to a wintering area could be more susceptible to predation (Bonnet et al. 1999). Furthermore, neonates did not travel far from their parturition sites before entering hibernacula (mean = 104.47 m), with 2 individuals selecting hibernacula within 15 m of their mother’s parturition site (Fig. 1). Although our sample size is limited, these short distances could be evidence that gravid females are selecting parturition sites partially based upon the hibernacula needs of their young. At the micro spatial scale, gravid females selected parturition sites that had thick canopy and coarse woody debris cover, and their neonates also selected areas that had significantly more ground cover (Table 3). Forest edges readily provide these features, all of which likely help keep neonates hidden from terrestrial and aerial predators. The protective features found in forest edges could be another reason gravid females migrated there for parturition. Other viviparous snake species also select parturition sites that have dense vegetative cover because of how susceptible neonates and postpartum females are to predation (Bonnet et al. 1999, Greene 1997, Shine and Bonnet 2009). Neonate preference for thick vegetation coincides with findings from the few existing telemetric studies on other neonate pit vipers (Howze et al. 2012, Jellen and Kowalski 2007). However, our findings differ from those of previous studies of habitat selection that found neonate Cottonmouths to select areas with little vegetative cover (Eskew et al. 2009) , supporting our second prediction. Past studies could have found differing results because their observations were restricted to summer nights and their analyses were based on visual encounter Southeastern Naturalist Z.J. Delisle, D. Ransom, and J. Delgado-Acevedo 2019 Vol. 18, No. 2 342 surveys (Eskew et al. 2009), which can lead to visual biases (i.e., a surveyor may be more likely to see an unconcealed snake then a snake concealed by vegetation). Furthermore, Cottonmouths are often nocturnal hunters in the summer (Lillywhite and Brischoux 2012), and neonates could be selecting less-concealed microsites when foraging at night. However, even with minimal disturbance from data collection, it is possible that repeated telemetry could have made neonates feel more vulnerable than non-telemetered individuals, leading to telemetered individuals selecting areas with greater vegetative ground cover. The spatial use of neonates before their first winter was small, with 100% MCPs and maximum dispersal distances averaging just 0.93 ha and 159.71 m, respectively. Spatial use theory suggests that smaller home-range sizes are indicative of essential resources being in close proximity to one another because movement, most especially for neonates, is associated with increased predation and energy expenditure (Bonnet et al. 1999, Greene 1997, MacArthur and Pianka 1966). Therefore, the small home-range sizes and short dispersal distances of neonates could be further evidence that the parturition migrations made by their mothers are beneficial to them. The maternal behavior of Cottonmouths has been well documented (Greene et al. 2002, Hoss and Clark 2014, Hoss et al. 2014). Postpartum adult females usually depart from their natal areas 10–14 days after parturition when neonates complete their first shed (Hoss et al. 2014). However, we found postpartum females to stay at the parturition site an average of 18.3 days, with 1 female staying in the natal area for 28 days. We also radio tracked 1 postpartum female back to her parturition site after she had departed the area. This female used her parturition site as a hibernaculum, and at least 1 of her neonates (snake 47) also hibernated there with her. In the following spring, on 21 March 2018, we observed this same mother aggregated with snake 47 at the same site. Jellen and Kowalski (2007) also found postpartum adult female Sistrurus catenatus (Rafinesque) (Eastern Massasauga) and their neonates to hibernate at parturition sites. Although most of our gravid females migrated out of the wetland for parturition, 1 did not and instead this individual birthed on the edge of a pond within the wetland (Fig. 1). Her only telemetered neonate (snake 56) remained in her natal area for about 1 week before making a near linear eastward movement from the pond to the forest edge. Snake 56 hibernated in the forest edge near many of the other neonates, and we believe she perhaps scent-trailed conspecifics to find the area. The use of conspecific scent-trailing to locate hibernacula has also been documented in neonate Crotalus horridus L. (Timber Rattlesnake; Brown and MacLean 1983, Cobb et al. 2005). Our initial sample size (n = 12) suffered from predation and lost signals, resulting in 7 individuals from 4 different litters being telemetered until the end of our study. Small sample sizes comprised from only a few litters have been prevalent in the telemetric literature regarding neonate snakes, and are likely due to high predation and detachment rates, and the difficulty of finding multiple litters in a given year (Cobb et al. 2005, Howze et al. 2012, Jellen and Kowalski 2007, Muellman et Southeastern Naturalist 343 Z.J. Delisle, D. Ransom, and J. Delgado-Acevedo 2019 Vol. 18, No. 2 al. 2018, Pizzatto et al. 2009). Although it would have been ideal to have all of our neonates born from different litters, finding females with their litters out in the open is somewhat fortuitous (usually females and their litters resided under thick coarse woody debris). Therefore, we did not want to sacrifice sample size on the uncertainty of finding more litters. Acknowledging our low sample size from few litters, we believe our study offers valuable insight into the initial life-history stages of Cottonmouths and presents a good baseline for future research on the ontogenetic ecological development of Cottonmouths. Acknowledgments We would like to thank P. Arron and M. Crowell for allowing us to track Cottonmouths on their properties. We also thank interns E. Engelhardt and A. Savage for their help with data collection, and W.I. Lutterschmidt for providing training on radio-implantation surgery, discussions regarding random site sampling, and comments upon review of this manuscript. 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