Karela Fry

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Being crabby in the age of GPS

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The ability to manufacture GPS units with a weight of about 50 gms, which includes a lithium battery with a lifetime of over two months can clearly be a boon to surveillance. We know of its use in understanding the behaviour of a species of crabs through an article recently published in PLoS One which reads:

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). … 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.

Some of the results confirmed visual inspection, but were so much more detailed that a systematic account of the life-style of these crabs could emerge:

Robber crabs mostly range around home sites

Animals … undertook short-distance excursions mostly at night (Fig. 5B, C). This behavioral pattern was observed both inland at the northern reaches of our study site and on the coastal plateau. The average size of the areas occupied during these behavioral phases over all three years were [around 1.0 ha]. Animals were observed to stay at one home site (Fig. B) or to change between several (up to three) (Fig. C). We found that activities were centered on refuges such as rock crevices, tree roots and holes in dead wood (Fig. D and E). Other important attracting factors were fruiting trees and cut wood of the endemic Lister’s palm Arenga listeri around which crabs aggregated in large groups (Fig. F and G). The Lister’s palm is known to be a strong attractant to B. latro, and during all expeditions we encountered feeding congregations surrounding palms. For example, shortly after tagging crab No. 1500 (Fig. C1), it was noted to move toward a recently fallen A. listeri, where it remained for ca. 2 weeks (orange episode) before returning to the tagging location (blue episode).

Male robber crabs were observed performing long-distance movements within their home range, averaging 1.8±1.2 km between the inland rainforest and the coastal plateau during the wet season in 2008 and 2010. In contrast, only three long-distance movements were observed in the dry season. Long-distance movements followed a strict coast-inland pattern (north-south in our case). We observed animals moving from south to north, from north to south, and back and forth, even multiple times, during the observation period of 67 days. These primarily nocturnal migrations occurred within a confined corridor of [about] 500 m width and covering [about] 200 m of altitude. Individual crabs were recorded moving [at speeds] up to 150 m/hour. The migrations were frequently interrupted by almost stationary phases; for example, animal No. 1502 spent [almost] 39 days in the northern ranges of the transect between its two trips to the coastal plateau. The tracks indicate that within a migratory corridor between inland rainforest and coastal plateau, animals followed individual routes. Some animals were observed to use identical routes for seaward and landward migration. Animal No. 621, for example, having spent six days around the inland tagging site, walked from inland to the coastal terrace at Middle Point within five days. After a day at the coast, it reversed its path to walk back to [nearly] 160 m of altitude where it remained for three days. The animal then continued inland on the outbound path to a position some 200 m away from the original tagging site.

Other findings were new:

We displaced animals as far as 1 km within what we consider their familiar territories, and most animals thus translocated showed robust, directed homing behavior that resulted in the animals returning close to their pick-up points. Possible information sources for orientation, all of which are known or have been discussed, include celestial cues (sun, moon, anisotropic radiance distribution from skylight or reflections from the ocean or breaking surf); the earth’s magnetic field; differences of substrate features; gravitational information (slope); the breeze itself (anemotaxis), which may carry ocean odors (chemotaxis); and seismic low frequency cues from breaking surf.

Most of these features are available as orientation cues in B. latro’s habitat. By extracting navigational information from these cues and combining them with memorized familiar topographic features, animals may organize their migratory routes during the unforced migrations and translocation experiments.

“True navigation” describes the ability to navigate to a goal location even after displacement to unfamiliar locations outside the range of an animal’s experience. We interpret the search behavior that we induced in B. latro by translocation out of its familiar migratory corridor as if we displaced the animals into terra incognita. Although these animals failed to find their way back to their pick-up areas, this experiment nevertheless provided interesting insights into possible homing mechanisms to the new reference point that the release site became for these animals. During some of the outbound excursions from this central place, they simply seemed to reverse their paths for homing back. During other excursions, the animals performed wide loops as long as ca. 1 km, which nevertheless led them back to their starting points.

One wonders how many such observations are going on in humans, especially among those who willingly carry GPS devices around with them in their pockets.

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Written by Arhopala Bazaloides

November 19, 2012 at 2:02 pm

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