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Long-distance Movement by American Alligators in Southwest Louisiana
Valentine A. Lance, Ruth M. Elsey, Phillip L. Trosclair III, and Leisa A. Nunez

Southeastern Naturalist, Volume 10, Issue 3 (2011): 389–398

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2011 SOUTHEASTERN NATURALIST 10(3):389–398 Long-distance Movement by American Alligators in Southwest Louisiana Valentine A. Lance1,*, Ruth M. Elsey2, Phillip L. Trosclair III2, and Leisa A. Nunez2 Abstract - As part of an ongoing study on growth and sexual maturation of Alligator mississippiensis (American Alligator) on Rockefeller Wildlife Refuge, LA, 3601 specimens, ranging in total length from 28 to 361 cm, were captured from June 2000 through August 2004. Additionally, 70 alligators were collected opportunistically as part of a teaching exercise in August 2005, and 248 more were collected in 2006 (and one in January 2007) as part of a study evaluating the effects of Hurricane Rita on alligators. Representative samples from size classes greater than 60 cm were collected in most months of the year between 2000 and 2004. Each animal was tagged, measured, sexed, and released immediately at the site of capture. A large number of these marked alligators were recaptured outside the refuge boundaries during annual alligator hunts in September. Of the 286 recaptured alligators, 214 were males, 68 were females, and four were of undetermined sex. From each recaptured alligator, total body length and date of recapture were recorded, and minimum distance from initial capture site estimated. From these preliminary data, we calculated the time interval between captures, and plotted minimum distance moved. The number of days between first capture and recapture ranged from 29 to 3336 days (9.1 years). Distance moved from initial capture site to final capture site ranged from 0.3 to 90.2 km. Eleven alligators moved between 30.0 and 39.9 km, and eight moved ≥40 km. Six of these moved between 40.0 and 49.9 km, and the others moved 87.4 and 90.2 km. These results greatly extend previous estimates of long-distance movement by alligators and demonstrate that both sub-adult and sexually mature animals move considerable distances. These data also showed that smaller alligators moved greater distances than larger alligators (P = 0.0002), and that the longer the time between captures, the greater the distance moved (P < 0.0001). Introduction Home range and dispersal (total distance moved) have been studied in a number of Alligator mississippiensis Daudin (American Alligator) populations using capture-recapture of marked animals or radiotelemetry. One of the earliest studies (Chabreck 1965) involved a total of 2024 alligators marked over a sevenyear period on Rockefeller Wildlife Refuge (RWR) and Sabine National Wildlife Refuge in coastal southwest Louisiana. Of these marked alligators, 131 were recaptured; of these, eighteen were recaptured twice, and two were recaptured three times. The conclusions of the study were that immature alligators moved greater distances than adults, and that the longer the interval between first and second capture the greater the distance moved. The longest movement recorded was approximately 16 km for an animal recaptured after four years. This study 1Graduate School of Public Health, San Diego State University, San Diego, CA 92128. 2Louisiana Department of Wildlife and Fisheries, Rockefeller Wildlife Refuge, Grand Chenier, LA 70643. *Corresponding author - lvalenti@sunstroke.sdsu.edu. 390 Southeastern Naturalist Vol. 10, No. 3 was limited to recapture attempts made within the refuge boundaries, and it is likely that any alligators that had moved greater distances to surrounding private lands would not have been recaptured. In the Florida Everglades, Hines et al. (1968) marked approximately 1000 alligators and recaptured 33. The longest reported distance moved was 11 km for an alligator recaptured after 850 days. Joanen and McNease (1970) were the first to use radio-telemetry to study the movement of alligators on Rockefeller Refuge. Five adult females were fitted with radio-collars and monitored for a period of six months. The results showed that nesting females did not move far from the nesting/den site, with home ranges of 2.6 to 16.5 ha. Taylor (1984) fitted radio-collars on nine adult female alligators in a lake habitat in north Louisiana and reported home ranges of 0.6 to 256 ha. A study by Rootes and Chabreck (1993a), in which fifteen adult female alligators (in southwest Louisiana) were fitted with radio-collars and followed for one year, indicated home ranges of 6.1 to 165 ha. In none of these studies on female alligators was the longest distance from initial site of capture reported. On the other hand, 14 adult male alligators on Rockefeller Refuge fitted with radio-collars and followed for up to one year, moved considerable distances and had estimated home ranges of 183 to 5083 ha (Joanen and McNease 1972). The greatest distance an adult male alligator moved from the initial capture site to where it was observed 146 days later was approximately 53 km; another adult male moved 22 km (Joanen and McNease 1972). Elsey (2005) reported an adult alligator photographed in the Gulf of Mexico some 63 km from the nearest land. Studies on juvenile alligators fitted with radio-collars (McNease and Joanen 1974, Taylor et al. 1976) reported no differences in home range between male and female alligators, but found that juveniles exhibited greater daily activity than adults and moved greater distances overall. Rootes (1989) suggested that radio-telemetry units attached as “collars” placed over the neck of alligators are likely stressors that could greatly limit mobility. Thus, distances moved by alligators monitored with telemetry units might be biased toward lower distances due to restricted movement. Similar observations on juvenile alligator activity (i.e., that they generally moved greater distances than adults) were made by Chabreck (1965) and Hines et al. (1968) using mark-recapture rather than telemetry. In this paper, we present data on long-distance movements of alligators based on a large sample of juveniles and adults of both sexes captured and recaptured over a period of nine years in southwestern Louisiana. Materials and Methods Rockefeller Wildlife Refuge (RWR) is a coastal marsh in southwestern Louisiana encompassing some 29,380 ha (Fig. 1) of wetlands of varying salinities (fresh: <2 ppt, intermediate: 2 to 6 ppt, brackish: 6 to15 ppt, and saline: >15 ppt; Keddy et al. 2007). As part of an ongoing study on growth and sexual maturation of American Alligators (immature: 50 to 180 cm; adult: >180 cm TL) on RWR, 3920 specimens (1960 males, 1926 females, 34 of unknown sex) ranging in total length (TL) from 28 to 361 cm were captured from June 2000 through January 2007; however, the majority of captures (3601) were made by August 2004. Representative samples from size classes ≥ 60 cm were taken in most months. 2011 V.A. Lance, R.M. Elsey, P.L. Trosclair III, and L.A. Nunez 391 Small alligators were caught by hand from an airboat, and larger animals were captured using a cable noose snare attached to a long pole (see Lance et al. 2009 for details). Each animal was marked with a monel tag in the webbing of both rear feet (Elsey et al. 2000), TL was measured, a blood sample was collected from the spinal vein (Zippel et al. 2003), a dorsal scute was removed from the tail, and sex was determined before being immediately released near the site of capture. During the annual fall alligator harvest, Louisiana Department of Wildlife and Fisheries (LDWF) employees were stationed at checkpoints where harvested alligators were brought for processing. Recaptures were identified by presence of tail notches and web tags; these specimens were measured, and distance from site of original capture was calculated by plotting the minimum distance between the property where the alligator was captured to the capture site on RWR. Alligator trappers are issued harvest tags to be used on specific wetlands they own or lease, thus enabling determination of the approximate capture area. Additional data were provided on mail-in forms for reporting the take of a marked alligator, which were supplied to all licensed trappers prior to the harvest season. Data were analyzed using SAS® statistical software. First, multiple regressions were used to determine if there was a linear relationship between total distance moved and either initial body size or number of days between release and recapture. Multiple rather than simple regression was used because the data collected on a single alligator are not independent from one another. Data were Figure 1. Satellite map of coastal southwest Louisiana showing Rockefeller Wildlife Refuge (outlined) and locations of alligators that moved the greatest distance from site of first capture (white-filled circles). The Gulf of Mexico is due south of the Refuge. 392 Southeastern Naturalist Vol. 10, No. 3 separated by sex, as plots of the raw data suggested an obvious difference between males and females. These regressions were performed without an intercept, as the distance moved should be zero when either initial body size or number of days is zero. The model for each sex was: Distance = 1 (Days) + β2 (Length) + ε, where ε represents the random variation for each subject. Both models violated the assumption of normality of residuals (P-values for both sexes were <0.0001), and also showed evidence of violating the assumption of homogeneity of variance. Consequently, the Spearman rank correlation coefficient was calculated to determine the nature of the relationship between total distance moved and either initial size or number of days between release and recapture. This statistic does not require the relationships to be linear, and the distribution need not be either normal or defined. The values of this coefficient fall between -1 and 1, with negative values indicating a decreasing relationship, and positive values indicating an increasing relationship. The P-values correspond to the hypothesis that the coefficient is zero (no relationship). Alligators recaptured on Rockefeller Refuge were not included in the analysis. Results A total of 286 alligators were recaptured outside of RWR, including 214 males, 68 females, and four of unknown sex. Eleven alligators moved between 30.0 and 39.9 km, and eight moved more than 40 km. Six of these moved between 40.0 and 49.9 km, and the other two moved 87.4 km, and 90.2 km (Table 1). Although more than half of the alligators (155) moved <10 km from point of first capture, there was a significant positive relationship between number of days since capture and distance moved (Fig. 2). There was also a significant negative relationship between size at initial capture and distance moved; i.e., smaller alligators moved greater distances than larger alligators (Fig. 3). Although the correlation coefficients for each of these relationships are low, the results were highly signifi- cant (Spearman Rank Correlation [n = 281] for distance moved and number of days since first capture, ρ = 0.26652, P < 0.0001; distance moved and length at first capture, ρ = -0.22354, P = 0.0002) when a non-parametric analysis is conducted. Thus, smaller alligators tended to move greater distances, and distances moved were greater with increasing time since first capture. Table 1. Distances moved by American Alligators marked on Rockefeller Wildlife Refuge, LA and later recaptured outside of the refuge boundaries. Of the 286 total, only data for those that moved ≥10 km are shown. Distance moved (km) Number of alligators 10.0–19.9 73 20.0–29.9 39 30.0–39.9 11 40.0–49.9 6 50.0–59.9 0 60.0–69.9 0 70.0–79.9 0 80.0–89.9 1 90.0–99.9 1 2011 V.A. Lance, R.M. Elsey, P.L. Trosclair III, and L.A. Nunez 393 The multiple regression analysis for females had an r2 value of 0.8273 (F2,65 = 161.5, P < 0.0001), and for males had an r2 = 0.5544 (F2,212 = 134.15, P < 0.0001). Larger females tended to move greater distances than smaller females (regression parameter = 0.11808, P < 0.0001); this relationship was not evident in males; i.e., smaller males tended to move greater distances. Greater distances were moved over longer time periods (number of days) for both sexes, with regression Figure 2. Plot of days between first and final capture and distance moved of American Alligators (n = 286) captured outside of the boundaries of Rockefeller Wildlife Refuge, LA. Figure 3. Plot of distance moved and total length at first capture for American Alligators (n = 286) captured outside of the boundaries of Rockefeller Wildlife Refuge, LA. 394 Southeastern Naturalist Vol. 10, No. 3 parameters of 0.00664 (r2 = 0.8325, P = 0.0035) for females and 0.00302 (r2 = 0.5586, P < 0.0001) for males. In addition to obtaining recapture data from the recovery of marked alligators outside RWR during annual hunts, 551 of previously marked alligators were captured alive during mark-recapture efforts on RWR. Of these, 423 were recaptured once, 54 were recaptured twice, and six individuals were recaptured three times. As these were all caught within the refuge boundaries, we did not expect many to have traveled a great distance (minimum distance between eastern and western boundaries of sampling sites = 22.9 km). However, seven alligators moved over 10 km (range = 12.5 to 21.3 km) within RWR, and 154 (27.9%) moved from the impoundment in which they were initially caught to adjacent areas, although distances were <10 km. Most live recaptures (390 of 551, 70.8%) within RWR remained within the same impoundment in which they were originally caught. This result was not unexpected as live recapture efforts were limited to the original area wherein the alligators were marked. Most of these impoundments are rectangular in shape, 2 to 3 km in length and width, and encompassing areas of 600 to 800 ha; thus alligators could move substantial distances, but still remain within a single impoundment. Additionally, 208 marked alligators were recaptured in the nuisance harvest on RWR. As expected, most did not move long distances, and recoveries were all within the boundaries of RWR. However, five of the nuisance alligators moved >10 km from the initial capture site (range of minimum dispersal distance = 10.7 to 16.3 km), and two others moved a minimum of 9.1 and 9.3 km from the initial capture site to the harvest site. Discussion The results of this study document unusually long distance movements by juvenile and adult alligators over a nine-year period. In this study, we were able to collect data unavailable to previous researchers, and thus greatly increase both recovery of marked individuals and confirmation of distances moved. One of the unplanned benefits of a well-regulated hunting program is the ability to collect biological data from captured alligators; thus, with a small number of trained biologists and technicians, we were able to monitor numerous sites for marked animals captured during the hunt. Hence, we were able to collect data that previous studies, carried out before legal hunting was established, were unable to do. For example, Chabreck (1965) marked 2024 alligators, but only recaptured 131 (6.4%). Likewise, Hines et al. (1968) marked approximately 1000 alligators and recaptured only 33 (3.3%). In contrast, we marked 3920 alligators on RWR and obtained data on 286 (7.3%) recaptured on lands outside of the refuge, a considerably greater number than recovered in previous studies, in part because of the long duration of our study. Long-term studies obviously have a higher likelihood of detecting longrange dispersal over time. For example, Campos et al. (2006) studied a population of Caiman crocodilus yacare Daudin (Caiman) in the Brazilian Pantanal over a 16- year period and recovered 532 of 7618 marked individuals (6.9%). We previously reported a male bias (58%) among sub-adult alligators in Louisiana (Lance et al. 2000). In the present study, 216 of 283 (76%) alligators were males, a higher percentage than previously reported for this population (Lance et 2011 V.A. Lance, R.M. Elsey, P.L. Trosclair III, and L.A. Nunez 395 al. 2000). As these alligators were collected during the annual September hunt, a male bias is not unexpected, because this harvest is timed such that most females with young will not be accessible to trappers (Elsey et al. 1994). Instead, predominantly surplus adult males are harvested, limiting the incidental take of breeding females that occupy a different habitat (Elsey et al. 1994). Despite a harvest that selects for males, when the percentage of total males is compared with the percentage of total females at the 10 to 20 km (27.6% and 19.1%) and 20 to 30 km distances (14.5% and 10.3%), these differences were not as pronounced (Table 2). In fact, the longest distance traveled (90.2 km) was by a female. This female was initially marked on 10 August 2000, and recaptured on RWR on 17 October 2000 at a site 24.9 km from the initial capture. The alligator was then harvested on private wetlands outside the refuge on 16 September 2003, some 65.2 km from where it was caught in October 2000. Clearly, a large portion of the total distance moved (90.2 km) occurred within the first two months after capture. The total number of recoveries (n = 1045) by all methods (286 harvested on private wetlands outside the refuge boundaries, 551 recaptured alive on the refuge and released, and 208 recaptured on the refuge during the controlled nuisance harvest) indicates 29.7% of marked individuals were recovered. These data support the long-held dogma that alligators have few predators, excepting other alligators (Rootes and Chabreck 1993b), after reaching a total length of about 60 cm (approximately two years of age). One factor that appears to facilitate the dispersal of alligators is flooding (Chabreck 1965). High winds and strong currents caused by Hurricane Audrey in 1957 swept alligators northward from RWR for distances “ranging from three to ten miles” (4.8–16.1 km). For alligators marked in our study, a small number (between two and seven annually) were caught from 2001–2005 on privately owned wetlands, north of RWR. Some of these had moved 10 to 20 km in the interval between being marked and recaptured. Following Hurricane Rita in late 2005, the number of marked alligators recovered in this area increased markedly to 22 in 2006. Another 15 were recovered north of RWR in 2007 (one recovered 33.9 km from the initial capture site), and 16 in 2008, one of which moved 39.8 km from the initial capture location. Some of these may have simply attained a harvestable size, having earlier moved onto privately owned wetlands when they were too small to be harvested. However, a far more likely explanation is that these alligators were pushed northward by the extensive storm surge of Hurricane Rita. Table 2. Sex ratio of American Alligators marked on Rockefeller Wildlife Refuge, LA and later recaptured ≥10 km away, outside of the refuge boundaries. Distance moved (km) Males Females 10.0–19.9 59 13 20.0–29.9 31 7 30.0–39.9 8 3 40.0–49.9 4 2 50.0–59.9 0 0 60.0–69.9 0 0 70.0–79.9 0 0 80.0–99.9 1 1 396 Southeastern Naturalist Vol. 10, No. 3 A few previous reports of long-distance movements by individual adult alligators are available. A 1.5-m female alligator first captured on Wasaw Island, GA was seen on Hilton Head Island 15 days later, a straight-line distance of >50 km (Tamarack 1989). Similarly, Joanen and McNease (1972) and Elsey (2005) documented individual alligator movements of 53 km and 63 km, respectively. In contrast to adult alligators, there is a notable paucity of published data concerning long-distance movements by juveniles. The distances moved by some juvenile alligators in our study exceed previous estimates by a wide margin. One juvenile was captured >90 km and another >80 km from site of first capture; several others moved >40 km. These findings are consistent with earlier studies by Chabreck (1965) and Hines et al. (1968) that found juvenile alligators move greater distances than adults. However, at the time of final capture, some juveniles in our study had attained adult size; therefore, it is possible that some long-distance movements occurred after these alligators reached sexual maturity. Of note, in May 2007, we incidentally recovered an immature (total length = 103 cm) nuisance alligator, which had moved >80 km from the site of its initial capture on 20 May 2002. A long-term study of C. crocodilus in the Brazilian Pantanal reported movements of up to 18 km in males and 16 km in females over a 15-year period (Campos et al. 2006). Direct comparison with our study is not possible, because this tropical habitat differs markedly from coastal south Louisiana by having distinct wet and dry seasons and widespread flooding in the wet season. During the dry season, caiman are restricted to small pools and rivers, but during the wet season they are able to disperse throughout the flooded Pantanal. However, similar to our study, smaller caiman moved greater distances than larger caiman, and there was a significant positive relationship between the number of days from first capture to distance moved (Campos et al. 2006). Long-distance movements by adult crocodiles are well documented. Allen (1974) estimated that a large adult male Crocodylus porosus Schneider (Estuarine Crocodile) observed on Ponape, Eastern Caroline Islands had traveled >1300 km across open ocean. Juvenile and sub-adult C. porosus disperse long distances up and down river systems in northern Australia (Kay 2004, Webb and Messel 1978). It is difficult to compare marsh-dwelling alligators with crocodiles inhabiting rivers, but most species of crocodilians appear to exhibit a pattern of juvenile dispersal, usually beginning one to two years after hatching (Chabreck 1965, Cintra 1989, Dietz 1979). Dispersal in juvenile Crocodylus johnstoni Krefft (Australian Freshwater Crocodile) was studied by Tucker et al. (1997, 1998). Similar to alligators, immature C. johnstoni had larger home ranges than adults, and males approaching sexual maturity entered a “nomadic” phase before establishing an adult home range (Tucker et al. 1998). Earlier studies indicated that juvenile alligators have larger home ranges than adults (McNease and Joanen 1974, Taylor et al. 1976). However, movements of 30 to 90 km are clearly too large to be considered part of the home range of a juvenile alligator. The fact that smaller alligators moved greater distances than larger alligators suggests that juveniles are less able to defend a territory from larger conspecifi cs. Aggression towards juveniles and even cannibalism by adults is known to occur in crocodilians (Cott 1961, Hunt 1977, McNease and Joanen 1977, Rootes 2011 V.A. Lance, R.M. Elsey, P.L. Trosclair III, and L.A. Nunez 397 and Chabreck 1993b). It is unlikely that juvenile crocodilians actually have large home ranges; instead, the reportedly larger home ranges may be dispersal events that occur when juveniles are driven out of an area by adults. In conclusion, our long-term study clearly documents the great distances alligators move over extended periods. Although many alligators remain within a relatively small area for long periods of time, others move unusually long distances. Additional research may provide information on why alligators select one location over another, despite similar habitat and prey availability. 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