Ethology 094\ 756*779 "0888#Þ 0888 Blackwell Wissenschafts!Verlag\ BerlinISSN 9068Ð0502

Dipartimento di Biolo`ia Animale e Genetica {Leo Pardi| dell|Universita� de`li Studidi Firenze and Museo di Zoolo`ia {La Specola| dell|Universita� de`li Studi di Firenze\Firenze\ and Centro di Studi per la Faunistica ed Ecolo`ia Tropicali del CNR\ Firenze

Short!range Homing in Fiddler Crabs "Ocypodidae\ Genus Uca#] A

Homing Mechanism not Based on Local Visual Landmarks

Stefano Cannicci\ Sara Fratini + Marco Vannini

Cannicci\ S[\ Fratini\ S[ + Vannini\ M[ 0888] Short!range homing in _ddler crabs "Ocypodidae\ genusUca#] A homing mechanism not based on local visual landmarks[ Ethology 094\ 756*779[

Abstract

This _eld study investigated the spatial strategies and homing ability of twoEast African _ddler crabs\ Uca lactea annulipes and Uca vocans hesperiae\ usingvarious experimental procedures[ A clear!cut spatial segregation between the twospecies was observed\ with U[ l[ annulipes occupying areas even a few centimetresabove U[ v[ hesperiae[ Females of both species were extremely faithful to their holeswhile the males exhibited behavioural di}erences[ U[ l[ annulipes males were quitefaithful to their holes^ in contrast\ U[ v[ hesperiae males\ during a single low tide\visited several holes which they did not defend[ Two displacement experimentswere conducted in order to identify the stimuli that _ddler crabs use in theirhoming[ In the {dislocation experiment|\ the crabs were moved "by hand# 0 or 1maway from their refuge and released[ None of these crabs returned to its burrowand\ moreover\ the initial directions they followed were not homeward orientated[In the {translation experiment|\ the crabs were passively translated while activelyfeeding[ When the crabs attempted to go {home| after the translation\ they dartedalong a straight path that led them to points that would have corresponded to theirhomes if the translation had not taken place[ After this _rst response crabs thencommenced searching strategies that led half of them home[ Short!range homingseems to depend strongly on information actively gathered during the outwardjourney\ other than local visual landmarks[

Corresponding author] Stefano Cannicci\ Dipartimento di Biologia Animalee Genetica {Leo Pardi|\ Universita� degli Studi di Firenze\ via Romana 06\ IÐ49014Firenze\ Italy[ E!mail] cannicciÝeurorock[dbag[uni_[it

Introduction

The spatial behaviour of truly terrestrial and intertidal brachyuran decapodshas been the subject of many studies "reviews in Wehner 0881^ Vannini + Cannicci

U[ S[ Copyright Clearance Center Code Statement] 9068!0502:88:09409Ð9756,03[99:9

757 S[ Cannicci\ S[ Fratini + M[ Vannini

0884#\ most of which have focused on Ocypodidae "Herrnkind 0861\ 0872#\ a familyrich in semi!terrestrial and intertidal species[ These crabs\ which are common onthe soft!bottom shores of the tropics and subtropics "Hartnoll 0877#\ exhibit a widevariety of space!use patterns and orientation mechanisms[ These range from arandom and apparently unoriented utilization of the bidimensional space\ observedby Beer "0848# in Macrophthalmus hirtipes\ to the accurate homing performed bythe ghost crabs\ genus Ocypode\ probably based on a visual landmark memory"Hughes 0855^ Linsenmair 0856#

Among the Ocypodidae\ the _ddler crabs\ genus Uca\ cope with problems oforientation during their life history by employing a wide range of behavioural andphysiological mechanisms[ Herrnkind "0861# stated that they can undertake Y!axisastronomical orientation on their home beach\ both in air and under water\ usingthe position of the sun or polarized skylight as cues[ The directional preference canbe learned\ in relation to the shoreÐwater axis "Altevogt 0854#\ and it becomesstronger with sexual maturity "Herrnkind 0861#[ Langdon + Herrnkind "0874#demonstrated that visual cues also play a major role in orientation in Uca[ Dis!placed crabs move toward mangrove trees\ but also toward vertical shapes con!trasting with the surrounding landscape\ while they avoid curved shapes whichresemble shorebirds[ Thus\ they show quite accurate shape discrimination "Lang!don + Herrnkind 0874#[ Moreover\ in their Y!axis orientation\ _ddler crabs alsorely on nonvisual cues\ such as beach slope "Young + Ambrose 0867# and wind"Herrnkind 0872#[

Despite such a variety of mechanisms involved in the Y!axis orientation\ Ucashort!range homing seems mainly a}ected by idiothetic information alone[ Inclassical studies by Altevogt + von Hagen "0853#\ von Hagen "0856# and Hueftle"0866^ quoted in Wehner 0881# proved that foraging crabs are able to return homeby exploiting idiothetic information only[

The aim of this study was to investigate whether Uca crabs in the wild canrely on stimuli other than idiothetic cues when returning to their home refuge[

Methods

Study Area

Field observations and experiments were carried out at Dabaso "Mida Creek#\a mangrove forest 19 km south of Malindi "Kenya#\ during Nov[ 0884 and Oct[ÐNov[ 0886[

The study site was a bare muddy platform "19×09m# within the Rhizophoramucronata belt of the forest\ on the side of a small stream[ The platform was~ooded twice per day\ but because of the presence of banks and depressions\ notall of its patches had the same immersion time "Fig[ 0#[

During diurnal low waters\ the populations of Uca lactea annulipes and Ucavocans hesperiae were by far the most numerous on the platform\ although speci!mens of Uca urvillei\ Uca clorophthalmus clorophthalmus and Macrophthalmusdepressus were occasionally present[

758Short!range Homing Mechanisms in Uca

Fi`[ 0] Schematic view of the study area] the experimental site was bordered by a streamand mangroves[ Curved lines\ 4 cm isohypses^ outlined area\ Uca vocans hesperiae patch^dotted area\ Uca lactea annulipes zone^ box\ area where the dislocated animals were released

Species

U[ l[ annulipes is often present both on the landward belt of the Kenyanmangrove swamps "Icely + Jones 0867# and on the seaward mud bars\ when theseare rising up to a certain level above datum[ Morphologically\ this species belongsto the {broad front| group of the Uca genus "Crane 0864# and it exhibits thebehavioural features and mating system common to this group[ The male defendsa {breeding burrow| and a receptive female selects a mate on the basis of thisburrow\ in which she accepts the mate and then remains to incubate her eggs[

U[ v[ hesperiae is a common species in the muddy platforms situated in frontof the Kenyan mangroves\ where the immersion time is rather high[ It exhibitsmorphological and behavioural features typical of the {narrow front| group of Ucaspecies[ The male wanders in search of a female\ who accepts a mate near theentrance of her own hole[

Experimental Techniques

Activity map

On the part of the platform where the density of Uca holes was highest\ i[e[about 29 inhabited holes per square metre\ a grid was established\ made of woodpoles and nylon lines\ dividing the whole area into 02 transects\ each 0×1m[ Adetailed map of the platform was drawn\ taking care to record the relative heightof all areas above the low water spring tide "LWST# level "Fig[ 0#[ About 39 crabs

769 S[ Cannicci\ S[ Fratini + M[ Vannini

of each species were individually marked with a Dymo tape tag glued to the cardiacregion of the carapace and\ in males\ to the external face of the major claw[ At lowtide\ an observer\ about 7m away from the study site\ followed the paths of thetagged crabs with binoculars[ For every crab\ we recorded the movement withinthe grid and also which and how many holes were visited[ The refuges in which thecrabs rested during the high tide were then marked with a ~ag "a nail bearing aDymo tape tag#[

Dislocation experiments

We captured specimens of both species and sexes by hand immediately outsidetheir refuges\ taking care not to destroy the burrows[ The animals were thenpassively dislocated "by hand# 0 or 1m away\ always with full vision of the sky andthe surrounding landscape[ The release site was in an area that gently slopedupward toward the mangrove forest\ at an average incline of 3[7>[ The site was stillwithin the portion of platform where maximum hole density had been registered"Fig[ 0#\ in order to prevent strong escape reactions caused by releases carried outwithin uncomfortable microenvironments[ The specimens were released at theentrance of an arti_cial hole placed at the centre of an iron wire ring "radius 49 cm#divided into eight equal arcs[ Crabs that escaped immediately after the release werenot taken into account\ while only crabs that started their excursion after at least04 s of resting well within the arti_cial hole were considered for the analysis[ Anobserver placed at least 7m away then drew a map of the path followed immediatelyafter the crab emerged from the arti_cial hole and recorded the arc of the ring thatthe crab crossed when leaving it[

Translation displacement experiments

A 39×69 cm mobile platform of green!brown gauze was kept in the mudbefore the experiment to eliminate its odour and to make it as similar as possibleto the substratum[ This platform "Fig[ 1# was then placed near an unmarked holeof a crab and kept in place until the crabs walked over it[ If the crab\ after havingwalked onto it\ stopped and began to feed on a portion the gauze itself\ the platformwas gently pulled forward at a speed of 1Ð2 cm:s by means of a _shing line held bythe experimenter who was situated at least 4m away[ The translation distancedepended on the crab|s reaction] the experimenter stopped moving the platform assoon as the crab reacted to the disturbance:displacement\ i[e[ stopped feeding andstarted to walk away[

The following behavioural response could clearly be divided in two distinctphases[ In phase 0\ all the animals ran directly and quickly towards a certain pointand then stopped for a few seconds "Fig[ 1#[ After this _rst rapid reaction\ all thecrabs started to move again "phase 1# but never as fast as in phase 0 and never ina straight line "Fig[ 1#[ Di}erent measurements were carried out to quantify theresults of these two phases[ For phase 0\ the experimenter recorded both thedirections taken by the crabs and the success of the specimens in reaching theirhole at the end of the running\ i[e[ phase 0 homing success[ When crabs started

760Short!range Homing Mechanisms in Uca

Fi`[ 1] Schematic drawing of the technique used in the translation displacement experiment[Continuous lines represent the paths actively covered by the crabs before and after shiftingthe platform[ The dotted lines represent the passive displacement caused by the platform

shift[ H\ crab burrow

phase 1\ the observer recorded the entire path of the crab for a maximum of 2minand also recorded if the crab found its refuge "a {homer|# or entered and rested inanother hole "a {squatter|# or wandered around without entering a refuge "{home!less|#[ For homers and squatters\ the time until the occupation of a hole wasrecorded[ The 2min time limit resulted from the fact that most of the crabs stoppedwalking after such a period and crouched in pools or in the damp mud[

Data Analysis

Activity map

For each observation day and for each crab\ the length of the path\ the numberof holes visited and the area of the most similar circumscribed polygon to the map_gure were calculated[ The same crab could have been observed more than once[

Dislocation experiments

The data were analysed using the V!test with the correction for grouping"Batschelet 0870#[ The directions considered "initial directions# were actually thedirections of the arc crossed by the experimental specimens\ i[e[ the directions heldby the crabs after 49 linear cm from the arti_cial hole[ The V!test was _rst appliedto the initial directions with respect to the cumulated homeward directions[ Then\to test the presence of particular compass directions in~uenced by external stimuli\

761 S[ Cannicci\ S[ Fratini + M[ Vannini

the initial directions were computed and analysed with respect to the maximumslope direction\ which\ in this area\ was both coincident with the landÐsea axis andto the mangroveÐwater axis[

Translation displacement experiments

Results about chosen directions and homing success after phase 0 were souniform that no real data analysis was performed "see results section#[ For the datacollected after phase 1\ the crabs were considered {not returned home| if they failedto enter a refuge within a period of 2min "homeless# or if they went into a di}erenthole "squatters#[ The G!test was used to analyse the results of phase 1 data fromdi}erent distances\ between di}erent species and between males and females[ TheSpearman rank correlation test was used to investigate the relation between trans!lation distance and the time taken to return to the home burrow[ For the sake ofthe computation\ the homeless crabs were considered as returned home in in_nitetime[

Results

Activity Map

On the experimental plot\ a clear spatial segregation of the two species wasrecorded "Fig[ 0#[ The population of U[ l[ annulipes\ except for very few individuals\colonized the banks situated at least 14 cm above LWST\ while the holes of U[ v[hesperiae covered the platform between 14 and 09 cm above LWST[ Within theseseparate areas\ females of both species were strictly faithful to their holes andperformed feeding loops always shorter than 29 cm[ The males of the two speciesshowed very di}erent degrees of _delity to their holes[

Out of 19 marked U[ l[ annulipes males followed during a whole low!tideactivity period\ only four visited other refuges[ They fought with the owners and_nally rested in the newly conquered holes when high tide arrived[

Exchanges of holes were far more common among U[ v[ hesperiae males[ The19 males followed visited an average number of 3[32 1[57 holes\ spread over anaverage area of 1[292 2[44m1\ during a single low tide and only four returned tothe same holes from which they had emerged at the beginning of the low tide[ Aftera _rst phase\ in which they fed in the immediate vicinity of their holes\ the markedU[ v[ hesperiae males started long excursions "2[592 2[17m# on average\ duringwhich they approached other wandering males "Fig[ 2#[ In some cases\ the malesapproached abandoned their burrows after a brief contact\ but real _ghts were notuncommon[ The wandering phases ended a few min before the ~ooding of theplatform\ when the individuals stopped and rested in the last hole they had {stolen|[

Dislocation Experiments

After emerging from the arti_cial hole\ some of the crabs took a direction andmaintained it until they were out the experimental ring "Fig[ 3a#[ Others ran inside

762Short!range Homing Mechanisms in Uca

Fi`[ 2] Schematic representation of paths travelled by three Uca vocans hesperiae malesduring single low tides[ White dots are the starting holes\ black ones the holes visited and

crosses are the _ghts engaged in by the wandering males

Fi`[ 3] Schematic representation of di}erent kinds of paths taken by the crabs after leavingthe release points in the dislocation experiment[ Only the phases inside the experimental

ring and the direction taken when exiting the ring are indicated

the ring\ sometimes following a convoluted path\ and _nally exited the circle"Fig[ 3b#[

None of the males and females of the Uca species "U[ v[ hesperiae n�31^ U[l[ annulipes n�16# tested returned home after being moved either 0 or 1m[ Becauseno di}erences were present between males and females for both Uca species\ allthe data were pooled for analysis of the release distance[

The crabs crossed the experimental circle in a direction that was not sig!ni_cantly orientated homewards\ both when dislocated 0m "U[ v[ hesperiae] n�01\u�9[674\ p�ns\ Fig[ 4a^ U[ l[ annulipes n�03\ u�9[655\ p�ns\ Fig[ 4c# and1m away "U[ v[ hesperiae n�29\ u�9[247\ p�ns\ Fig[ 4b^ U[ l[ annulipes n�02\u�9[664\ p�ns\ Fig[ 4d#[ When the direction of maximum slope\ i[e[ the seaÐland axis direction\ was considered\ the directions taken by the U[ l[ annulipes

763 S[ Cannicci\ S[ Fratini + M[ Vannini

Fi`[ 4] Initial directions chosen by Uca vocans hesperiae "a\ b# and Uca lactea annulipes "c\d# after the release at points situated 0 and 1 m\ respectively\ from their holes[ For eachdistribution\ the cumulated home direction "H#\ the number of trials "n#\ the length "r# and

direction of the mean vector\ and the values of the V!test "u and p# are given

specimens were also randomly distributed "dislocation distance 0m] u�9[9113\p�ns\ Fig[ 5a^ 1m] u�0[348\ p�ns\ Fig[ 5b#[

The initial directions taken by the U[ v[ hesperiae specimens were not orien!

Fi`[ 5] Initial directions chosen by Uca lactea annulipes after dislocations of 0 m "a# and 1 m"b#[ The upslope direction is indicated "S#\ other details as in Fig[ 4

764Short!range Homing Mechanisms in Uca

Fi`[ 6] Initial directions chosen by Uca vocans hesperiae after dislocations of 0 m "a# and1 m "b#[ Details as in Figs 4 and 5[

tated with the maximum slope direction when the crabs were dislocated 0m away"u�0[348\ p�ns\ Fig[ 6a#\ but were signi_cantly clustered around it when thedislocation was 1m away "u�1[405\ p³ 9[90\ Fig[ 6b#[

Translation Displacement Experiments

Crab behaviour during phase 0 of the response was extremely uniform[ One!hundred per cent of both U[ v[ hesperiae "n�49# and U[ l[ annulipes "n�27# ranalong a path the same length as the path actively covered before the translationdisplacement\ but in the opposite direction "Fig[ 1#[ Thus\ none of the translatedcrabs reached home after phase 0 "homing success�9#[ They all stopped at a pointthat corresponded to the home site if the passive translation of the platform hadnot taken place[ In all cases phase 0 lasted no more than 4 s^ thereafter\ as the crabsdid not _nd their burrows\ they stopped for some seconds and then began to walkaround\ thus starting phase 1[

During phase 1\ most of the crabs walked around "never running#\ followingmore or less sinuous paths\ while two U[ v[ hesperiae happened to walk across theentrance of their own burrows without entering them[

About half of the above sinuous paths "41[9) in U[ v[ hesperiae and 40[6)in U[ l[ annulipes^ Table 0# _nally led the crabs to their own holes[ Of those whonever reached home\ U[ l[ annulipes never stopped searching for their burrows"only 1[2) squatters# while about half "i[e[ 13[9) of the total# of the U[ v[ hesperiae_nally stole another burrow "comparing frequency of the three homing categories\in both species\ gives G�09[282\ df�1\ p³ 9[90#[

The G!test indicated no signi_cant di}erence in the frequency of {homers|vs[ {nonhomers| "by pooling homeless and squatters# between the two species"G�9[990\ df�0\ p�ns#[ Moreover\ there was no di}erence in homing successbetween males and females "U[ v[ hesperiae G�9[980\ df�0\ p�ns^ U[ l[ annul!

765 S[ Cannicci\ S[ Fratini + M[ Vannini

Table 0] Homing preference data "in per cent# of Uca vocans hesperiae and Uca lacteaannulipes

Females Males Total

Uca vocans hesperiaeSample size 14 14 490[ Homers 59[9 33[9 41[91[ Squatters 17[9 19[9 13[92[ Homeless 01[9 25[9 13[91 ¦ 2[ No!homers 39[9 45[9 37[9

Uca lactea annulipesSample size 05 11 270[ Homers 51[4 39[8 40[61[ Squatters 9 3[5 1[22[ Homeless 26[4 43[4 35[91 ¦ 2[ No!homers 26[4 48[0 37[2

ipes G�0[563\ df�0\ p�ns#[ For this reason\ the data for females and maleshave been combined in the following analyses[

Searching behaviour during phase 1 was then analysed to assess whether therewere any di}erences between the two species[ The distance between the point wherephase 1 started and home "{searching distance|# was related to the time needed to_nd the home[ The homeless crabs were considered as returned in in_nite time\thus allowing the application of the Spearman rank test to the inverse of time[Only returned "homers# and homeless crabs were taken into account "thus excludingthe squatters#[ The correlation indicated a signi_cant relationship between the twovariables\ both for U[ l[ annulipes "n�25\ r�Ð2[343\ p³ 9[994# and U[ v[ hesperiae"n�16\ r�Ð2[797\ p³ 9[994# "Fig[ 7#\ i[e[ with increasing searching distances\ thehoming time increases exponentially "the inverse of time falls to zero#[ By com!paring the homing success of the two species when their searching distance is loweror greater then 14 cm\ it can be shown "G�04[71\ df�0\ p³ 9[990# that U[ l[annulipes never reached its burrow when displaced more then 14 cm from it\while U[ v[ hesperiae completely lost its homing capability only after 49 cm ofdisplacement "Fig[ 7#[

Discussion

A well!de_ned spatial segregation of U[ l[ annulipes and U[ v[ hesperiaepopulations on the East African coasts has already been reported and its morpho!physiological basis has been analysed "Hartnoll 0864^ Icely + Jones 0867#[However\ the previous studies gathered zonation data on a large scale\ representedby the seaÐland axis\ while the present observations show how marked this seg!

766Short!range Homing Mechanisms in Uca

Fi`[ 7] Translation displacement experiment] data analysis of phase 1[ Relationship betweenhoming time\ expressed as inverse homing time\ and displacement distance in Uca lactea

annulipes "white squares# and Uca vocans hesperiae "black dots#

regation can be\ even on a far smaller scale[ On a relatively small portion of themuddy platform\ these two species were still divided by clear borders\ representedby a few centimetres of di}erence in height "Fig[ 0#[ Two main spatial strategieswere recorded in the two species[ Both sexes of U[ l[ annulipes performed smallfeeding loops\ which started and ended in the home refuge[ Although hole densitywas very high\ shifting among di}erent holes was recorded only in a few males andnever in females[ Conversely\ although females of U[ v[ hesperiae showed a degreeof burrow _delity compared with U[ l[ annulipes\ the males undertook a wanderingphase that led them to change their holes frequently "Fig[ 2#[ Indeed\ it is reasonableto state that the U[ v[ hesperiae males were not faithful to any refuge\ even to theone they had cleaned and rested in during high tide[

Di}erences in hole _delity between broad front "i[e[ U[ l[ annulipes# andnarrow front "i[e[ U[ v[ hesperiae# species have already been reported "Salmon 0873\0876# and they have been explained by the di}erent mating systems[ In fact\ onlythe males of narrow front species actively search for their mates and thus mustabandon their home refuges[

Even if their burrow _delity was quite di}erent\ both faithful and wanderingindividuals became completely lost if passively displaced 0 or 1m away from theirburrow and none of the 58 used crabs were able to _nd it again[ The manipulativetype of dislocation used was surely a stressing technique for the crabs[ However\the crabs were tested during their maximum activity period\ dislocating them asrapidly as possible within an area inhabited by their cospeci_cs\ with the panoramaand sky in full view[ Moreover\ only crabs that accepted and rested for a whilewithin the arti_cial hole were considered in the analysis\ excluding those crabsshowing strong and immediate escape reactions[

Orienting cues\ such as the position of the sun "Altevogt 0854#\ polarized

767 S[ Cannicci\ S[ Fratini + M[ Vannini

skylight "Herrnkind 0861\ 0872#\ vision of surrounding landmarks "Langdon +Herrnkind 0874# and substratum slope "Young + Ambrose 0867# are known to bethe basis of Uca Y!axis orientation[ However\ our crabs presumably did not usethese cues to gather information about home direction "Figs 4\ 5#\ at least in thisexperiment[ Only U[ v[ hesperiae released 1m away from their holes tended tofollow directions clustered around the maximum slope direction\ independentlyfrom their home direction "Fig[ 6#[ Conversely\ similar experiments in the samelocality on Thalamita crenata "Portunidae# showed that crabs were able to con!stantly relocate their home burrow even after 4m of displacement "Cannicci et al[0884#[

The results of the translation displacement experiments must be discussedseparately for phase 0 "the rapid straight run\ immediately following the platformtranslation# and phase 1 "the following 2min during which crabs appear to wanderaround#[ Phase 0 results show that the crabs regularly missed their homes[ Thisshows that\ probably\ in their short!range homing they do not rely on direct visionof landmarks placed in the immediate neighbourhood of the burrow\ such as fallenleaves and discontinuities of the surface[

In this phase\ crabs appear simply to reverse the direction of the pathpreviously\ actively\ taken\ with no compensation for the passive displacement"Fig[ 1#[ Similar results were obtained in preliminary tests on Uca pu`ilator\ anAmerican broad!front species "and thus hole!faithful#\ which became completelylost after translation displacements "J[ Layne\ unpubl[ data#[

Local visual cues are crucial in the homing of other decapod species such as thepredator Eriphia smithii "Vannini + Gherardi 0877#\ the swimming crab Thalamitacrenata "Cannicci et al[ 0884#\ the tree climbing crab Sesarma leptosoma "Cannicciet al[ 0886# and ghost crabs\ Ocypode "Hughes 0855^ Linsenmair 0856#[ Moreover\vision helps the _ddler crabs in a variety of behaviours "Land + Layne 0884a\b#\but seems not to be involved in _nding their home burrow during small!scalefeeding loops[

Even if visual landmarks can be excluded\ it is still di.cult at this stage toassess which stimuli are at the base of the homing mechanism spontaneouslyadopted by these _ddler crabs[ They could rely on magnetic compass to record thechanges of orientation during their trips\ or they could use optic ~ow informationto monitor the distance travelled\ or they could not use any of the external stimulibut rely just on idiothetic information stored during the outward trip\ as has beenshown in laboratory "Altevogt 0854#[

A possible explanation for the lack of use of local visual landmarks could bethe complex visual structure of the surroundings of the Uca burrows\ coupled withthe relatively low visual acuity of these crabs "Zeil et al[ 0875^ Land + Layne0884a#[

Analysis of phase 1 suggests the lack of use of local visual landmarks also] thehigh rate of {not returned home| crabs in both species and sexes "Table 0#\ theirhoming time\ which increased exponentially with the increase of the searchingdistance\ and their homing success\ which fell almost to zero "at least in U[ l[ annulipes#even for short distances "¼ 14 cm# "Fig[ 7#\ suggest a blind!search strategy[

768Short!range Homing Mechanisms in Uca

Both the {faithful| U[ l[ annulipes and the wandering U[ v[ hesperiae malesprobably couple their homing mechanisms together with a systematic search pro!gramme\ as is known in other arthropods "Wehner 0881#[ Since in both Uca speciesthe average homing success is similar and the homing time is exponentially relatedto the searching distance\ the di}erence in homing success following distancesshorter or longer than 14 cm is probably due to di}erent searching programsperformed by the two species[ The more vagile U[ v[ hesperiae could systematicallysearch in a wider area\ because of the longer distance it commonly travels[ This isreminiscent of desert ants of the genus Cata`lyphis\ which perform wider searchdensity pro_les after having travelled for longer distances "Wehner 0881#[

The results seem to prove the coexistence of two distinct homing mechanisms\which is also known in other arthropods "Wehner 0881# and its adaptive valuemust be discussed[ A highly precise kinaesthetic short!range homing\ free from anyexternal input\ probably evolved as a quick antipredatory response[ As stated byHerrnkind "0861#\ and con_rmed by our results\ this idiothetic path integration isthe fastest way to reach the {right| burrow[ Fiddler crabs are usually active withina mass of mud pellets\ conspeci_cs and burrows whose density\ presence andposition continually varies with time[ It would thus be confusing and useless torely on these visual stimuli to dart back home in case of any attack[

Anyhow\ path integration mechanisms are not error free "Wehner 0881# andcannot cope with passive displacement\ thus it is of high adaptive value to adopt aspare mechanism such as the systematic search used by the _ddler crabs[

This searching strategy\ which is undoubtedly more time!consuming\ provedto be quite e.cient on short distances\ in helping the crabs to _nd their hole[ Inparticular\ the di}erences in homing success recorded after this phase between thetwo species can be explained by their ethological characteristics] after the release\a high proportion of U[ v[ hesperiae approached conspeci_cs and fought for theirburrows instead of actively searching for their home refuge[ This species is muchmore used to _ghting and stealing other burrows and its e}ort in looking for itsown hole cannot be as strong as the one exerted by the more faithful U[ l[ annulipes[

Acknowledgements

Thanks are due to Lorenzo Montemagno\ Luca Morino\ Samantha Serrao\ and Rocco Rorandellifor their invaluable help during the _eld work[ The authors also thank John Layne^ without his criticismand points of view\ this work would not have been completed[ Thanks for critical reading are due toFrancesca Porri\ and\ again\ to Rocco Rorandelli[ Funds were provided by Italian CNR\ University ofFlorence\ and from the Project {Macrobenthos of East African Mangroves|\ founded within the INCO!DC framework of the EU[

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Received] September 7\ 0887

Initial acceptance] January 7\ 0887

Accepted] February 04\ 0888 "W[ Wickler#