We investigated the navigational capabilities of the world's largest land-living arthropod, the giant robber crab Birgus latro (Anomura, Coenobitidae); this crab reaches 4 kg in weight and can reach an age of up to 60 years. Populations are distributed over small Indo-Pacific islands of the tropics, including Christmas Island (Indian Ocean). Although this species has served as a crustacean model to explore anatomical, physiological, and ecological aspects of terrestrial adaptations, few behavioral analyses of it exist. We used a GPS-based telemetric system to analyze movements of freely roaming robber crabs, the first large-scale study of any arthropod using GPS technology to monitor behavior. Although female robber crabs are known to migrate to the coast for breeding, no such observations have been recorded for male animals. In total, we equipped 55 male robber crabs with GPS tags, successfully recording more than 1,500 crab days of activity, and followed some individual animals for as long as three months. Besides site fidelity with short-distance excursions, our data reveal long-distance movements (several kilometers) between the coast and the inland rainforest. These movements are likely related to mating, saltwater drinking and foraging. The tracking patterns indicate that crabs form route memories. Furthermore, translocation experiments show that robber crabs are capable of homing over large distances. We discuss if the search behavior induced in these experiments suggests path integration as another important navigation strategy.

The terrestrial life-style, size, and physical strength of the robber crabs make these long-lived animals ideal subjects for exploration via GPS tracking. We examined the behavior of freely moving crabs on Christmas Island (Indian Ocean), which holds one of the largest and mainly undisturbed remaining populations of this species ( Fig. 1D and E ). In the present paper, we provide details of the locomotory activity, routes of movement, and orientation strategies used by this arthropod, obtained over several months of monitoring. We demonstrate that male robber crabs display long periods (weeks) of exclusive site fidelity to their home but also perform directed, long-distance migrations over several kilometers. Translocation experiments further show that robber crabs are capable of homing over large distances. We discuss possible reasons for this behavior and the navigational strategies used.

Although the behavior of the robber crabs is poorly understood, certain behavioral aspects of other crustaceans have been studied in quite some detail. The marine spiny lobsters of the genus Panulirus are famous for their autumnal and storm-associated synchronous long-distance migrations, which can span over several hundred kilometers (e.g. [17] – [20] , [7] ). Clawed lobsters (Homarida) are known to seasonally migrate (e.g. [7] , [8] ). During non-migratory periods, lobsters also show a strong homing instinct to their dens. There are, however, severe difficulties associated with investigating aquatic crustacean behavior, chiefly, working underwater. Therefore, the numerous crustacean lineages that have successfully invaded land offer more feasible research possibilities (e.g. [14] , [21] ). Semi-terrestrial beach hoppers (Talitridae, Amphipoda), mangrove-dwelling grapsid crabs (Sesarma leptosoma), terrestrial hermit crabs (Coenobitidae), fiddler crabs (Uca ssp.) and Christmas Island red crabs (Gecarcoidea natalis) have all been the subjects of investigations within their habitats (reviews [17] , [18] , [22] ), and in some cases navigational mechanisms have been proposed (reviews [23] – [25] ). Although these studies have provided important insights into crustacean behavior specifically, and arthropod behavior in general, studies based on non-automated observation suffer from a poor recovery rate (mark-recapture techniques), or low levels of temporal and/or spatial resolution because direct observation or manual telemetry over longer periods (weeks or months) is arduous. Automated systems can overcome some of these shortcomings. Recently, small-scale movements of clawed lobsters (Homarida) in their habitats were monitored by automated ultrasonic fixed array telemetry, which makes use of arrays of buoys that triangulate the position of tagged lobsters under semi-natural conditions in an enclosure (mesocosm [26] ; see also [27] , [28] for details of this method). However, long-term data on what crustaceans actually do, how they move around and what places they know are scarce.

A: Photograph of the e-obs GPS-tag (2 nd tag version with single battery used in 2010 and 2011). B Tagged Birgus latro escaping into a tree (1 st tag version with twin batteries used in 2008). C Tagged B. latro (2 nd tag version) inspecting the BaseStation for wireless data download (image kindly provided by Meike Kilian). D Location of Christmas Island, Indian Ocean. E, E1 Geo-referenced 3D model of Christmas Island showing the study area at Aldrich Hill and the transect. F Topographic map displaying the filtered data set of 9,272 GPS fixes recorded in all three expeditions. The camera symbol shows the position from which G and H were taken. G, H Photographs of the coastal terrace. G shows the first author tracking radio signals near the blowholes which can be identified by the three spray plumes in the background (image kindly provided by Meike Kilian). H is taken facing north-west and the photo shows the rocky coastal terrace, the dense belt of Pandanus and in the background the vertical first inland cliff. For positions of asterisk, see F.

Here for the first time GPS tagging is used in large-scale experiments to track an invertebrate in its natural environment, namely the giant robber crab Birgus latro (Linnaeus, 1767) (Malacostraca, Anomura). These animals which are able to open and consume coconuts [10] – are the world's largest extant land-living arthropods ( Fig. 1 B, C ). Robber crabs can weigh up to 4 kg, have a leg span of almost 1 m [11] and reach an age of up to 60 years [10] . ‘Formerly common on tropical islands of the Pacific and Indian Ocean, populations of robber crabs in most island locations are now declining. Robber crabs are essentially oversized hermit crabs (Anomura), and are only dependent on water for the pelagic larval stages [12] – [14] ; in fact, adult robber crabs have lungs and drown if immersed in water for more than 24 hours [15] . Despite their terrestrial nature and large size, we know little about their biology. Of note is the lack of knowledge of their migratory behavior and its underlying orientation mechanisms. Such long-lived animals, are likely to acquire a good knowledge of their environment and the navigational skills needed to move between places of interest, such as shelters, access to conspecifics, food and water, but so far, only basic aspects of short-distance movements have been analyzed [16] .

Although it has revolutionized the field of vertebrate behavior, GPS technology has yet to reach the most abundant group of animals on earth: the arthropods, or other invertebrates. The large size of most GPS technology trackers has limited its use in understanding the behavior of small arthropods. Methods employed to follow individual arthropods through time and space apart from direct observation using grids [4] – [6] , have included mark-recapture approaches [7] , [8] and the use of radar [9] . However, none of these methods, or any other methods tried, affords the combination of constant long-term tracking and high-resolution monitoring that space-based satellite navigation systems offer.

Monitoring freely roaming animals in their natural habitat is not an easy task. Doing so for extended periods of time is even trickier. Animals are typically shy, and the very act of observation may also affect their behavior. The invention of space-based satellite navigation systems, such as the NAVSTAR Global Positioning System, or GPS for short, and the subsequent miniaturization of receiver and transmitter units, have revolutionized the field of animal tracking. This technology, which allows animals' behavior to be unobtrusively monitored, has provided valuable information on long-distance migration [1] – [3] and key insights into the daily activity patterns of many animals as they move around in their habitat.

The GPS-module calculates a horizontal error estimate by an algorithm based on the number and distance of used satellites. The GPS positioning accuracy was tested by placing one stationary tag under forest canopy to record 84 fixes over 44 h. The average horizontal error was 20.4±28.6 m (SD; range 2.3 to 244.74 m), with confidence intervals (CI) of CI50% = 19.31 m, CI95% = 56.06 m, and CI99% = 73.65 m. For comparison and for calibration of the GPS modules, the tag accuracy was also tested by placing 21 tags on a glade (open sky) near the upper end of the transect to record 669 fixes over 4 h. Here, the average horizontal error was 5.26±4.93 m (SD; range 0.02 to 27 m), CI50% = 3.32 m, CI95% = 9.65 m, and CI99% = 12.68 m. When the real horizontal error from a known position exceeds the horizontal error estimate, the values will be positive linearly correlated by a regression coefficient of determination of R 2 = 0.8 ( Fig. 3 ). The recorded accuracy of GPS fixes within the forest corresponds well with values from recent telemetric studies using GPS-tags in the rain forest of Central Panama [35] . Our entire data set was filtered by removing positions with horizontal error estimates exceeding 20.4 m, which is the average error in the forest, resulting in a total of 9,272 GPSfixes (61% of raw data) for the final analysis.

All recorded data of GPS-RF tags (e.g. GPS date, GPS daytime, horizontal error estimate and heading direction) were transferred into a Microsoft® Excel table before being imported into ArcGIS® (ESRI) as a geo-database. To minimize GPS error, recorded GPS bursts of a maximum of four positions per GPS fix were averaged. For a precise measurement of movement, a 3D model of Christmas Island was created and geo-referenced with regard to the large variations in height above sea level ( Fig. 1E ). The model is based on a topographic map of Christmas Island (Natmap; edition 1; 1∶30,000; GEOCAT 70145). The tagging data were analyzed with ArcGIS® as well as the open source tools Geospatial Modelling Environment ( http://spatialecology.com/gme ) and Home Range Tools (HRT) [32] . Individual home ranges of B. latro were analyzed by using the quadratic Kernel function [33] with the least squares cross validation (LSCV) of the mean integrated square error [34] .

In addition to tagging undisturbed animals as described above, we conducted 12 displacement experiments during 2010 (N = 4) and 2011 (N = 8) to analyze possible homing behavior. In 2010, we translocated four male robber crabs in opaque jute bags approximately 1 km from north (10°29′59.77″S, 105°35′58.53″E; elevation 180 m) to south (10°30′31.44″S, 105°35′54.83″E; elevation 38 m). We repeated the experiment in 2011 with two animals taken from north (10°30′11.18″S, 105°35′50.57″E; elevation 150 m) approx. 750 m southward (10°30′31.44″S, 105°35′54.83″E; elevation 38 m) and translocated another two crabs in the opposite direction. One animal (No. 1717) from this group was displaced twice in the same manner after first returning successfully to the capture site. In addition to these translocations towards and away from the shore covering ca. 120 m of altitude, four animals were displaced roughly parallel to the coast within the same range of altitude as their capture location. Two animals were translocated from the transect (10°29′57.34″S, 105°36′10.97″E; elevation 189 m) one km ca. eastwards (10°30′11.10″S, 105°36′29.92″E; elevation 164 m) and two from the transect (10°30′11.18″S, 105°35′50.57″E; elevation 150 m) 750 m ca. westwards (10°29′55.39″S, 105°35′18.43″E; elevation 164 m).

The BaseStation featured a flash memory, a power supply, a display, a USB interface and an RF-interface for a wireless connection to the tag via a high-sensitivity antenna (developed and custom-made by Henning Marter FUNKBAU, Rudolstadt, Germany; www.funkbau.de ). A conventional YAESU VR500 radio receiver was used to receive the “pinger” signal for each tag ranging from 868 to 867 MHz in 25 kHz steps with a directional Yagi-Uda antenna. For data acquisition, ca. every third day the data collected by the tags were downloaded along the transect until the tag batteries were discharged (maximum of 77 days).

Tags were attached to the posterior part of the dorsal carapace of B. latro with industrial two-component epoxy resin (Araldite® 2012 EP) after gridding and degreasing the surface with sandpaper and acetone ( Fig. 1B, C ). Only male crabs greater than 500 g in weight were tagged (N = 42; range 600 to 2,940 g). The tag-to-body weight ratio was 4.0±1.9% (range 1.9 to 9.5%), which is within the generally accepted range of <3 to 5% for telemetric studies [30] . Birgus latro has remarkable body strength with a reported lifting force of up to 28 kg [31] , enabling it to effectively climb trees ( Fig. 1B ). Therefore, and because Kenward [30] referred mainly to the tagging procedures of vertebrates, we assume that the proposed threshold is not applicable for land-living hermit crabs and can be easily increased up to 10 to 15% of body weight. Our observations during the tagging procedure support the claim that the weight of the tags does not impair the mobility of the animals. The tags were programmed to collect data at 30, 60, or 120 min intervals with an acquisition time of 180 s and 210 s in 2008 and 2010/11 respectively. After the first study in 2008, the tag settings and tagging procedure, and the routine of data collection, were continually optimized, resulting in an increase of maximum battery life-time from 38 days in 2008 up to 77 days in 2011 and an increase of data retrieval ratios (number of tags with data download/total tag number per year) from 71% in 2008 up to 95% in 2011 (for more details, see Table 1 and Fig. 2 ). The rate of successful GPS fixes per programmed GPS fixes was 33.34% in total.

The GPS-RF-tags (N = 61; Fig. 1A ) were manufactured by e-obs digital telemetry in Grünwald, Germany ( http://www.e-obs.de ) and consisted of a power supply (lithium polymer battery cell with 4.5 V); a flash memory SD-card; a GPS module (LEA 4S by u-blox™); a radio transmitter (“pinger”), which transmitted short and high-pitched signals at brief intervals on idiosyncratic frequencies; an on-board real time clock; an antenna; and an interface for an RF link to the BaseStation, a mobile interface between user and GPS-RF-tag (BaseStation b5, e-obs). All components were embedded into a hard, waterproof plastic housing; tags measured 6 cm in length, 1.5 cm in height and 5 cm in width and weighed ca. 57 g.

Tagging and monitoring of the crabs followed several four-wheel drive vehicle tracks currently maintained for Parks Australia access. Access to the coastal plateau from the mid-plateau was conducted on foot, as no off-road tracks were maintained to Middle Point. The northern end of the study area was accessed via the north-west baseline and was approximately 2.5 km in length (north-south). Crabs were also monitored in easterly and westerly directions following off-road tracks ( Fig. 1 ), with a total distance of 2 km. Additional incursions into the forest were often conducted to attempt to relocate crabs based on last GPS fixes. The estimated area of the study site was approximately 3 km 2 assuming an optimal receiving range for data acquisition of 300 m (as field-tested on site).

Crabs were tagged and monitored in the vicinity of Aldrich Hill (10°30′06″S, 105°36′04″E; Fig. 1 D ), within the Christmas Island National Park. The study site, which extended from the mid-plateau to the coastal plateau near Middle Point (ca. 260 m above sea level), intercepted a number of different habitats, ranging from open rainforest on deep soil to thick stands of Pandanus sp. growing on shallow soils over limestone and various small cave systems. The study area traversed a number of gentle slopes on the mid-plateau before descending a steep slope to the coastal plateau and ocean cliffs (ca. 10 m above sea level).

Christmas Island is a relatively large oceanic island located approximately 360 km south of Java, Indonesia, in the Indian Ocean ( Fig. 1D ). The island originated from an ancient reef base that has been uplifted over time [29] . The landmass is characterized by a series of stepped plateaus (the central, middle, and coastal plateau) which are separated by very rugged cliffs.

Results

Our analysis is based on a filtered data set of 9,272 GPS positions (Fig. 1F) from 55 male animals (Table 1), some observed as many as 77 days, covering a total of 1,673 crab days. The data from stationary test tags showed slight, daily fluctuations of the GPS reception with fewer fixes received in the morning hours (Fig. 4). The number of GPS fixes obtained from the attached tags during the rainy seasons in 2008 and 2010 was constant during the night but decreased dramatically during morning hours. It was lowest around noon, rising again at dusk, and reaching a peak between 20:00 and 22:00 (Fig. 4). Based on field observations, we conclude that the animals are mostly night active. Because they hide in refuges such as rock crevices and shelters between tree roots during the daytime (Fig. 5), a sky view of the tags – and hence GPS reception – is blocked. During the dry season in 2011, the rise in activity at dusk was much less pronounced than in the wet seasons 2008 and 2010, suggesting that air humidity is a major factor limiting the animals' activity [10].

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larger image TIFF original image Download: Figure 4. Circadian rhythm of locomotory behavior. Upper panel: Relative activity = percentage of mean distances between GPS-fixes per total number of GPS-fixes ± SD [percentage of m/fix]. Horizontal bars indicate times of daylight (white) and night (black). Lower panel: Daily fluctuations of GPS fixes of all attached tags of the 2008, 2010, and 2011 missions (colored solid lines) and a stationary test tag (black dashed line). Data suggest that many animals hide in refuges during daytime thus blocking GPS reception. During the 2008 and 2010 missions (wet season) migratory activity of the animals begins at dusk to reach a maximum between 20:00 and 22:00. https://doi.org/10.1371/journal.pone.0049809.g004

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larger image TIFF original image Download: Figure 5. Site fidelity. A: Overview of all animals showing site fidelity and short-distance movements only from all three years of observations. B: Higher magnification of animal No. 1520 that remained within one home range during the plotted period (19 December 2010 to 09 January 2011); open symbols: daytime fixes, solid symbols: nighttime fixes. C and C1: higher magnifications of animal 1500 that occupied three home ranges between 5 December 2010 and 17 January 2011. In C1 this period is sectioned in three episodes. D: B. latro uses hollow trees as daytime refuges. E: A tagged animal hiding in a rock crevice during the day. F, G: B. latro are strongly attracted to freshly fallen Lister's palms. Cut Arenga: site of a freshly fallen Arenga listeri palm. Scale bars in A, B, C, C1: 100 m. https://doi.org/10.1371/journal.pone.0049809.g005

Our data show that B. latro males exhibit periods of considerable site fidelity interrupted by periods of directed migration. The two main behavioral patterns observed are:

Long periods (weeks) of exclusive home site fidelity, with short-distance excursions to one or several home areas with a radius of around 25 m (N = 28; Fig. 5). Long-distance movements (N = 15; between 0.7 and 4.2 km) within a home range with interim stationary phases of home site fidelity (Fig. 6).

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larger image TIFF original image Download: Figure 6. Long-distance Y-axis migrations. A: Overview of all animals showing long-distance movements within a migratory corridor that extends from the coastal terrace ca. 2 km inland (data pooled from all three missions). Color ranges define point density estimates in points per km2 using only data of directed movements (points of stationary phases or undirected movements were removed) B: One of several animals exposed to ocean spray as observed in the upper openings of the blowholes at the coastal terrace (compare Fig. 1J). C: example of migratory animal No. 621, monitored from 5 December 2008 to 12 January 2009, and displaying typical Y-axis migration (with the X-axis being the shoreline) during this period. C1 and C2 section this migration into the outbound and inbound episodes. Note that in- and outbound paths are identical (permitting for the GPS error of max. 20 m); open symbols: daytime fixes, solid symbols: nighttime fixes. D–P: higher magnifications of all animals showing long-distance movements. For every animal, position and date (see boxed legend) of the tag application is plotted as well as dates and positions of the first and last obtained GPS fixes. Scale bars: 100 m. https://doi.org/10.1371/journal.pone.0049809.g006

Here, the term “home site” is used for a roughly circular area around a temporary residence or refuge from which the animals undertake short-distance excursions, returning every day to the refuge. A “home range” is generally defined as the “area traversed by the individual in its normal activities of food gathering, mating and caring for young” [36] or simply, “an area repeatedly traversed by an animal” [30], and we use the term “home range” in this broad meaning.