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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 effectiveness of Passive Integrated Transponders (PIT tags) was investigated in the robber or coconut crab, Birgus latro. A pilot study was conducted in Chumbe Island, Zanzibar, and a definitive study on Christmas Island, Indian Ocean. No mortality or adverse behaviour was recorded as a result of PIT tagging. On Christmas Island 1150 crabs were tagged in 2009 and 2010. In the period to April 2011, 169 crabs were recaptured, some up to four times. Of these, 67 had moulted before recapture. Approximately equal numbers of each sex were marked, and recaptured. Tag loss was estimated at ∼5%. Alternative sampling protocols using grid and transect searches were tested. They produced different results in terms of sampling efficiency and sex ratio: sampling protocol must be considered when interpreting data.PIT tagging in Birgus is a promising method for long term population studies. It promises low tag loss and good recapture rates in comparison with previous studies. ZUSAMMENFASSUNGEs wurde die Einsatzmöglichkeit von passiven integrierten Transpondern ("PIT tags") beim Palmendieb, Birgus latro, untersucht. Nach einer Pilotstudie auf der Insel Chumbe (Sansibar) wurde die abschließende Studie auf der Weihnachtsinsel im Indischen Ozean durchgeführt. Alle Tiere überstanden das Bestücken mit Transpondern unbeschadet und zeigten auch danach keine Verhaltensauffälligkeiten. In den Jahren 2009 und 2010 wurden auf der Weihnachtsinsel 1150 Palmendieben -darunter etwa gleich vielen Weibchen und Männchen -Transponder implantiert. Von diesen konnten im April 2011 insgesamt 169 Tiere erneut lokalisiert werden (einige davon bis zu viermal). Auch bei den wiedergefundenen Tieren waren etwa gleich viele Männchen wie Weibchen. Von den 169 Tieren hatten sich in der Zwischenzeit 67 gehäutet. Basierend auf unseren Daten schätzen wir, dass etwa 5% der Tiere während des Untersuchungzeitraumes ihren Transponder verloren hatten. Es wurden Vergleichsstudien durchgeführt, bei denen die Tiere entweder an -durch ein fiktives Gitter vorgegebenen -Stellen gesucht wurden, oder entlang eines linearen Transektes. )Sowohl die Auffindwahrscheinlichkeit als auch das Geschlechterverhältnis unterschieden sich für die beiden alternativen Datenerhebungen. Daher muss bei der späteren Interpretation von Wiederfangdaten, die Art der Datenerhebung berücksichtigt werden.Der Einsatz von passiven integrierten Transpondern erwies sich als ein effizientes Mittel für Langzeit-Populationsstudien. Im Vergleich zu vorherigen Studien mit alternativen Trackingmethoden gehen nur wenige Transponder verloren und man erzielt gute Wiederfangergebnisse.

Birgus latro has significant conservation and socio-economic importance throughout its range, yet very little is known about its growth in its natural habitat. An extensive markrecapture program was undertaken on Christmas Island, Australia, in which over 1440 individual crabs were uniquely identified using passive integrated transponder (PIT) tags. Over 18% of the tagged population was recaptured at least once and remeasured, providing substantial information on growth of B. latro. Our results show that in its natural setting B. latro is a very slow-growing species. Individuals commonly experience conditions that result in zero or negative growth. Sex, size, and injury all strongly influence the crab's moult increment. Males approached zero growth at around 65 mm in thoracic length (TL), while growth in females started to plateau at around 50 mm TL. Injury frequently led to negative growth, particularly in individuals that had lost a claw and/or multiple limbs -presumably due to a loss of foraging opportunities or access to other resources and a redistribution of available energy into limb regrowth. Our study indicates that previous growth curves do not adequately describe the growth of B. latro in its natural setting and that individual crabs may live over 30 yr longer than previously predicted, with large crabs likely to be over 100 years old. This may have serious implications for the long-term conservation of the species in the face of continuing exploitation. KEY WORDS: Birgus latro · Growth · Moult increment · Mark-recapture · PIT tag · ConservationResale or republication not permitted without written consent of the publisher Aquat Biol 19: 129-141, 2013 Robertson 1991) have attempted to maintain Birgus latro in captivity in order to monitor moult increment and moult frequency in a controlled environment. However, most studies on crabs > 30 mm thoracic length (TL) were deemed unsuccessful as crabs either did not moult before dying in captivity or the majority of moulted individuals had negative or zero growth. Smaller crabs held in captivity grew well, providing some good early growth information, but most studies terminated the experiments after 2.5 yr (Held 1963, Reese & Kinzie 1968, well before the crabs reached sexual maturity; believed to be at around 24.5 to 32 mm TL in females , 22.2 mm TL in males ) and at ca. 5 to 10 yr of age, as determined by the growth curve provided by Fletcher et al. (1990a).A number of mark-recapture studies have been undertaken on Birgus latro in an attempt to improve knowledge of their growth in a natural setting, but with limited success (see Drew et al. 2010 for a review). One of the primary problems with markrecapture programs has been the inability of the mark to be retained through ecdysis (Drew et al. 2012); external tags are lost during the moult process. Fletcher et al. (1989) attempted to alleviate the problem of retaining marks post-moult by using heat and freeze branding methods, enabling them to identify individual crabs for up to 3 yr. However, ...