Abstract Crocodilians have a wide distribution, often in remote areas, are cryptic, secretive and are easily disturbed by human presence. Their capacity for large scale movements is poorly known. Here, we report the first study of post-release movement patterns in translocated adult crocodiles, and the first application of satellite telemetry to a crocodilian. Three large male Crocodylus porosus (3.1–4.5 m) were captured in northern Australia and translocated by helicopter for 56, 99 and 411 km of coastline, the last across Cape York Peninsula from the west coast to the east coast. All crocodiles spent time around their release site before returning rapidly and apparently purposefully to their capture locations. The animal that circumnavigated Cape York Peninsula to return to its capture site, travelled more than 400 km in 20 days, which is the longest homeward travel yet reported for a crocodilian. Such impressive homing ability is significant because translocation has sometimes been used to manage potentially dangerous C. porosus close to human settlement. It is clear that large male estuarine crocodiles can exhibit strong site fidelity, have remarkable navigational skills, and may move long distances following a coastline. These long journeys included impressive daily movements of 10–30 km, often consecutively.

Citation: Read MA, Grigg GC, Irwin SR, Shanahan D, Franklin CE (2007) Satellite Tracking Reveals Long Distance Coastal Travel and Homing by Translocated Estuarine Crocodiles, Crocodylus porosus. PLoS ONE 2(9): e949. https://doi.org/10.1371/journal.pone.0000949 Academic Editor: David Lusseau, Dalhousie University, Canada Received: July 11, 2007; Accepted: September 5, 2007; Published: September 26, 2007 Copyright: © 2007 Read et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by the Charles Tanner Bequest, Wildlife Warriors Worldwide and an Australian Research Council Grant to CEF, MAR and SRI. Competing interests: The authors have declared that no competing interests exist.

Introduction Crocodylus porosus is the world's largest crocodilian and has the widest geographical range of any, occurring from the Solomon Islands to Papua New Guinea and across northern Australia to Indonesia, south-east Asia and the eastern coast of India [1]. It is also the most dangerous [2]. Managing problem C. porosus by translocation to a remote location is of dubious value because many of them return [3], but translocation is still suggested in the public domain as a way to ameliorate the threat posed by large crocodiles in areas close to human habitation. Data on homing by translocated crocodilians has been gathered mostly by surveillance at the original capture site for the return of tagged individuals [3]–[7]. These studies indicate that crocodilians are likely to show site fidelity. However, the drawback of mark-translocate-recapture studies is that they provide no information about the tracks taken by returning individuals, or the time profile of the journey. For this, telemetry is necessary. There have been several attempts at following translocated crocodilians by radiotelemetry, over short distances and short time frames [7]. Rodda (1984) reported radiotelemetry data from hatchling and juvenile Alligator missippiensis that implied a capacity for navigation because they homed after translocation short distances (1–7 km) to an unfamiliar area [8]. However, studying movement patterns in crocodilians by conventional radiotelemetry has proven difficult because they are mostly very cryptic, live in remote locations, have wide geographic ranges and are easily disturbed by human presence. Satellite tracking, on the other hand, allows data to be collected essentially continuously from animals in remote locations that are difficult to access, such as along the coastline and in the open ocean, and without the human interference which is hard to avoid during manual tracking. Satellite telemetry has been very successful for studying movements of various birds, mammals, fish, and marine turtles [9]–[12]. Surprisingly, there have so far been no published studies in which satellite telemetry has been used to study the movements of crocodilians. In reporting the results of his study of the movement of C. porosus in the Cambridge Gulf, northern Australia, Kay (2004) referred to the limitations of conventional VHF telemetry and recommended the use of satellite tracking in future studies in order to gather data over larger spatial and temporal scales. The aims of our study were to record and interpret the movements of translocated large male estuarine crocodiles after their release and to investigate their homing behaviour, if any, using satellite telemetry.

Discussion Satellite tracking proved very effective for studying the movements of these large crocodiles, enabling them to be monitored essentially continuously across a wide geographic range and for several months. The animals all moved extensively within river systems, undertook substantial coastal voyages, and displayed remarkable navigational abilities. All of the crocodiles returned to their sites of capture, even Crocodile C which was translocated a long distance and across a major geographic feature. These observations add to previous data showing the ability and, apparently, an inclination by translocated crocodilians to return ‘home’. Walsh and Whitehead (1993) found that 50% of 48 problem C. porosus translocated distances between 20 and 100 km from Nhulunbuy harbour in the Northern Territory, Australia, returned to their original capture locations. The fate of the remainder is unknown. Kay (2004) reported that a juvenile male C. porosus, translocated 118 km from Port Wyndham to the Ord River, also in northern Australia, returned after 12 days. On the other hand, hatchling and juvenile C. porosus translocated in the Liverpool River system in northern Australia returned equivocal results [5]. Webb et al. (1983) translocated 17 adult and sub-adult C. johnstoni within the McKinlay River (northern Australia) for 39 river km (30 km direct) and a year later recaptured eight of them, all but one at the original capture site even though there was suitable habitat between the capture and release sites. Homing tendency has also been reported over much shorter distances in Alligator mississipiensis [8], [17] and Caiman crocodilus [4]. What is different about the present study is that, for the first time in any adult crocodilian, we can report detail of the track taken and the time profile of the homeward journey. The results pose interesting questions. It is noteworthy that all three individuals spent some time at the release point before embarking on an apparently purposeful and direct travel homewards. Could the animals be making an appreciation of local cues and the direction of travel required? Also, all returned to the same place at which they were captured, and none travelled any distance in an inappropriate direction, except for a brief excursion southwards by Crocodile C, the most disrupted individual. Was this movement associated with an assessment of the correct direction home? Our observations clearly imply that crocodilians are skilful at interpreting a suite of complex cues for orientation and navigation, and this aspect of their behaviour demands additional investigation. This study confirms that the practice of translocating ‘problem’ C.porosus to a remote site is very likely to be ineffective. If a problem crocodile animal is living in an area where conflict with humans is likely, then other options need to be employed. Of particular interest were the large distances travelled in a comparatively short time. On their voyages back to their capture sites Crocodile A had an average speed of nearly 7 km day−1, Crocodile B more than 11 km day−1 and Crocodile C accomplished an average speed of more than 20 km day−1 over a period of 20 days (Figures 2, 3). No comparable journeys have been reported previously for any crocodilian. The prevailing view is that substantial energy demands in crocodilians are met anaerobically [18] but in making these journeys, C. porosus revealed a high capacity for sustaining prolonged exercise. Combined with the lingual salt glands [19], this attribute imbues this crocodilian with a great capacity for dispersal over vast distances, easily explaining its extensive geographic distribution from India to the Solomon Islands.

Acknowledgments The croc team from Australia Zoo provided outstanding support and assistance in capturing crocodiles. Cape York Helicopters provided generous support and Kevin Weldon is thanked for moving Crocodile C across Cape York Peninsula. The traditional owners of the Nesbit River country granted us access to their lands. All research work was conducted under animal experimentation ethics approval.

Author Contributions Conceived and designed the experiments: GG CF MR SI. Performed the experiments: CF MR SI. Analyzed the data: CF MR DS. Contributed reagents/materials/analysis tools: CF MR SI. Wrote the paper: GG CF MR DS.