Male Uca tangeri place mudballs in the area surrounding their burrows each
low tide to delineate territory boundaries. We investigated the sequence of mudball
placement by noting the distance at which each mudball was placed and its location in
one of eight sectors around the burrow. Ten of the 13 focal males placed their mudballs
in a pattern that was significantly different from random. No relationship was found
between mudball distance from burrow and order of placement in the mudball sequence.
However, most males placed each mudball in the same or a nearby sector to that of the
previous mudball

Burford, Fiona Ruth Louise

McGregor, Peter K.

Oliveira, Rui

Copenhagen University Research Information System

The sequence of mudball placement by male fiddler crabs, <i>Uca tangeri</i>

Burford, Fiona R. L.

Oliveira, Rui Filipe

Repositório do ISPA

Etología, 8:53-55 (2000) 532000 Sociedad Española de EtologíacAbstract. Male Uca tangeri place mudballs in the area surrounding their burrows eachlow tide to delineate territory boundaries. We investigated the sequence of  mudballplacement by noting the distance at which each mudball was placed and its location inone of  eight sectors around the burrow. Ten of  the 13 focal males placed their mudballsin a pattern that was significantly different from random. No relationship was foundbetween mudball distance from burrow and order of  placement in the mudball sequence.However, most males placed each mudball in the same or a nearby sector to that of  theprevious mudball.Key words: fiddler crab, Uca tangeri, mudball.Resumen. Los machos de Uca tangeri colocan bolas de barro en el área que rodea a susmadrigueras cada vez que baja la marea a fin de establecer los bordes de sus territorios.En el presente estudio se investigó la secuencia de colocación de las bolas de barroanotando la distancia a la que cada bola se colocaba y su localización respecto a cadauno de los ocho sectores alrededor de la madriguera. Diez de los 13 machos focalescolocaron las bolas de barro en un patrón que difería significativamente del azar. No seencontró ninguna relación entre la distancia de las bolas de barro a la madriguera y elorden de secuencia en que estas bolas se colocaban. Sin embargo, la mayoría de machoscolocaron cada bola en el mismo sector en el que habían colocado la bola anterior o enuno cercano.IntroductionAt least 14 of  the 80 species of  fiddler crab (genus Uca,Ocypodidae) form sand or mud constructions on themudflat surface near the opening to the burrows thatthe crabs maintain as refuges during high tide and forprotection from predators (e.g. Christy, 1988a, b;Zucker, 1974, 1981; Salmon, 1987). Many species formmudballs from material excavated from within theburrow (P. Backwell, personal communication). Thesemudballs are then placed on the mudflat surface aroundthe burrow entrance. Until recently, the formation andplacement of  mudballs was mostly overlooked as aconstruction behaviour, being regarded as merely ameans of  excavating and repairing the burrow (Crane,1975). However, studies on U. tangeri, the only fiddlercrab to occur in Europe, have shown that mudballingis more complex than previously thought.Male U. tangeri tend to place their mudballs ina broad semi-circular pattern in front of  their burrowopening, whereas most females tend to deposit theirmudballs in a pile directly outside the burrow. Threepieces of  evidence suggest that male mudballs form aterritory boundary. First, removal of  a male’s mudballdisplay leads to a 400% increase in the number ofaggressive interactions with neighbouring males(Oliveira et al., 1998). Second, the distance at whichindividual males place the furthest mudballs each lowtide has a low level of  variation (CV of  18%) comparedwith their nearest mudballs (50%; Burford et al., inpress), suggesting that the placement of  more distantmudballs is more constrained or more critical. Finally,male U. tangeri re-position mudballs that areexperimentally moved closer to the burrow (Burfordet al., in press).One aspect of  the function of  mudballs as aterritory boundary is that we might expect the firstmudballs placed by males to be the more distant, inorder to delineate their territories early on in the lowtide period. We investigated this idea by studying thesequence of  mudball placement by male U. tangeri.Material and MethodsAll fieldwork was carried out in May and June 2000, atthe Ria Formosa Natural Park, Algarve, Portugal,where a population of  U. tangeri inhabit the mudflatsthat are exposed each low tide. The crabs spentThe sequence of mudball placement by male fiddler crabs,Uca tangeriFiona R. L. Burford1, Peter K. Mcgregor1 & Rui F. Oliveira21 Zoological Institute, University of  Copenhagen, Tagensvej 16, DK-2200, Copenhagen N, Denmark.2 Unidade de Investigação em Eco-Etologia, ISPA, Rua Jardim do Tabaco 34, 1149-041 Lisboa, Portugale-mail: PKMcGregor@zi.ku.dkReceived: 2 November 2000; accepted: 15 November 2000.Budford et al: Mudball placement by male fiddler crabs54approximately 1 to 1.5 hours mudballing each low tide.Focal males were chosen at random from the study area.The area surrounding a focal male’s burrow wasdivided into eight 45o sectors, numbered S1 to S8,clockwise from 12 o’clock (the direction of  the slopedburrow opening). Males were observed while placingmudballs. The presence of  an observer had no effecton the male subjects, as long as the observer remainedstill while the crabs were on the surface. After eachmudball was placed and the crab had re-entered hisburrow, we measured the distance from the burrowentrance to the mudball and noted which sector it wasplaced in. The observer could usually do this byreaching forward. If  the observer needed to move, wetimed the interval to re-emergence to detect disturbance,taking a longer re-emergence interval as evidence fordisturbance. Only males that we considered to beundisturbed were included in the data set. We continuedto take measures until the male ended his mudballingperiod. This was defined as occurring when enteringhis burrow and surfacing without a mudball three ti-mes consecutively, or when beginning to wavecontinuously. All subjects were measured and markedafter the observation period.To investigate whether the pattern of  mudballplacement by each male was significantly different fromrandom, we compared matrices using a Mantel test.The first matrix contained the difference between theorder in which mudballs were placed, for example thesequence difference between mudballs 1 and 10 is 9.The second matrix contained the differences in thedistances between each mudball placed, for example,the difference between mudball 1 placed 61.5 cm fromthe burrow and mudball 10 placed 53.6 cm from theburrow is 7.9 cm. The third matrix contained thedifferences between the sectors each mudball was placedin. The shortest distances were always used, thereforeif  mudball 1 was placed in S7 and mudball 10 was placedin S8, then the difference shown in the matrix is 1.However, if  mudball 20 was placed in S1, then thedifference between mudballs 1 and 20 is 2. All measureswere absolute distances (i.e. the sign of  the differenceswere ignored). We then compared matrix 1 with matri-ces 2 and 3 and the resulting t-values showed whetheror not there was a significant association between matrix1 and each of  matrices 2 and 3 (Schnell 1985).ResultsTen of  the 13 males placed their mudballs in a sequencethat was significantly different from random (Table 1).Of  these 10 males, six showed non-random placementof  mudballs in both distance and sector, while a furthertwo males showed non-random placement in mudballdistance only and the remaining two showed non-random placement according to sector only.To investigate our hypothesis that males placetheir more distant mudballs first to quickly delineatetheir territory boundaries, we correlated mudballsequence and mudball distance for the eight males thatshowed a significantly non-random pattern ofplacement. Four of  the males followed the pattern ofplacing distant mudballs early in the placementsequence (i.e. showed negative correlations betweenmudball distance and order of  placement, Table 2).However, three males showed significant positivecorrelations (i.e. placed distant mudballs last) and oneshowed no significant correlation (Table 2).The effect of  mudball placement sequence onthe sector in which mudballs were placed wasinvestigated by looking at scatterplots of  the data. Ofthe eight males that showed a significantly non-randomrelationship between mudball sequence and sector, allbut one placed each mudball either in the same sectoras the previous mudball, or up to two sectors away.For example, a male that placed his fifth mudball inS1, then placed his sixth mudball either in S1 again, orin S7, S8, S2 or S3. The exception was one male thatplaced each mudball up to four sectors away from theprevious one.Sequence v distance Sequence v sector Male t-value P t-value P 1 1.473 ns 2.089 < 0.05 2 4.486 < 0.001 -0.142 ns 3 0.958 ns -0.599 ns 4 4.546 < 0.001 2.029 < 0.05 5 4.095 < 0.001 5.572 < 0.001 6 -0.073 ns -0.034 ns 7 1.842 ns 1.574 ns 8 3.796 < 0.001 2.220 < 0.05 9 5.428 < 0.001 5.533 < 0.001 10 6.104 < 0.001 4.386 < 0.001 11 2.470 < 0.05 6.992 < 0.001 12 1.857 ns 1.973 < 0.05 13 3.815 < 0.001 -0.608 ns Table 1. Associations between the sequence of mudballplacement and mudball distance, and sector. Mantel tests wereperformed on 13 subjects. Values are the t-values from theMantel tests and their associated significance levels.rs P (1-tailed) n -0.51 0.01 51 -0.51 0.01 28 -0.43 0.01 40 -0.33 0.05 28 -0.07 ns 51 0.43 0.01 78 0.44 0.01 58 0.63 0.01 30  Table 2. The distance mudballs were placed from the burrowwas correlated with the order of their placement for the eightsubjects that showed a non-random relationship betweensequence and mudball distance. Values are the Spearman rankcorrelation coefficients (rs) between distance and placementorder, the associated significant one-tailed P values and thenumber of mudballs placed (n), ordered from strong negativecorrelations to strong positive correlations.Etología, 8:53-55 (2000) 55Males that placed their distant mudballs earlyin the placement sequence were right-handed and ten-ded to possess longer major chelae than the males (allleft-handed) that placed their distant mudballs later inthe placement sequence (Table 3). The male that showedno significant correlation between mudball placementorder and mudball distance had a larger carapace and aconsiderably longer major chela than males that showeda significant correlation (either negative or positive).DiscussionTen males showed significantly non-random sequencesof  mudball placement either in the distance at whichthe mudballs were placed, the sector they were placedin, or both. On the basis that rapid delineation ofterritory boundaries would be advantageous, weexpected the sequence of  mudball placement to beginwith males placing their more distant mudballs.However, the evidence does not support this idea: fourmales placed their more distant mudballs early on inthe sequence, but three males placed them towards theend of  the sequence. This suggests that while rapiddelineation of  territory boundaries may have someeffect on the distance at which mudballs are placed,there are probably other influencing factors. Thesefactors are likely to differ between males as aconsequence of  conditions specific to each male (e.g.neighbour density and proximity, value of  the burrowto the male or availability of  females).Eight males placed their mudballs non-randomly in the sectors around the burrow, usuallyeither in the same sector or a sector nearby the onethat the previous mudball was placed in. Such a patternof  placement could result in a semi-circular arrangementof  mudballs in front of  the burrow. The effect ofsequence on the sector in which mudballs are placed ismore apparent than its effect on the distance at whichthey are placed. There are also suggestions in our datathat the morphology of  individuals (handedness andmajor chela length) affect the sequence in whichmudballs are placed (Table 3).In summary, we have provided further evidencethat mudballing is more complicated than first thoughtand should not be dismissed simply as a process ofburrow excavation. Oliveira et al. (1998) concluded thatfemales placed mudballs as a by-product of  burrowexcavation, but recently it has been shown that this isalso an oversimplification as some females producemale-style patterns of  placement (Burford et al.,unpublished data). The non-random placement of  malemudballs and the individual differences in the sequenceof  mudballing (e.g. whether a male begins by placingdistant or closer mudballs) indicate that mudballplacement and factors influencing it are worthy offurther investigation.Acknowledgements. This research was funded by theUniversity of  Copenhagen and SNF (grant numbers 9701908and 9801928 to PKM). We thank: Ria Formosa Natural Park,Portugal for their permission to work in the park and forlogistic support; colleagues at ISPA, Lisbon, and theDepartment of  Animal Behaviour, Copenhagen, for supportthroughout the study; Joana Jordão and Ann Burford for helpin the field; Tom Peake for help with Mantel tests.ReferencesBurford, F.R.L., McGregor, P.K. & Oliveira, R.F., in press.Mudballing revisited: further investigations into theconstruction behaviour of  male Uca tangeri. Behaviour.Christy, J.H., 1988a. Pillar function in the fiddler crab Ucabeebei (I): effects on male spacing and aggression. Ethology,78(1):53-71.Christy, J.H., 1988b. Pillar function in the fiddler crab Ucabeebei (II): competitive courtship signaling. Ethology,78(2):113-128.Crane, J.H., 1975. Fiddler Crabs of  the World, Ocypodidae,genus Uca. Princeton: Princeton University Press.Oliveira, R.F., McGregor, P.K., Burford, F.R.L., Custódio,M.R. & Latruffe, C., 1998. Functions of  mudballingbehaviour in the European fiddler crab Uca tangeri. Anim.Behav., 55(5):1299-1309.Salmon, M., 1987. On the reproductive behaviour of  thefiddler crab Uca thayeri, with comparisons to U. pugilatorand U. vocans: evidence for behavioural convergence. J.Crust. Biol., 7(1):25-44.Schnell, G.D., Watt, D.J. & Douglas, M.E., 1985. Statisticalcomparison of  proximity matrices: applications in animalbehaviour. Anim. Behav., 33(1):239-253.Zucker, N., 1974. Shelter building as a means of  reducingterritory size in the fiddler crab, Uca ter psichores(Crustacea: Ocypodidae). Am. Mid. Nat., 91(1):224-236.Zucker, N., 1981. The role of  hood-building in definingterritories and limiting combat in fiddler crabs. Anim.Behav., 29(2):387-395. Distant mudballs earlier Distant mudballs later No relationship Carapace width 27 ± 1 28 ± 3 33 Carapace length 18 ± 1 19 ± 2 21 Major chela length 41 ± 3 33 ± 1 61 Major chela height 15 ± 1 13 ± 2 17 Handedness All right-handed All left-handed Left-handed  Table 3. Features of males that place their distant mudballs early in the sequence (n = 4), males that place their distant mudballslater in the sequence (n = 3) and a male that showed no significant relationship between mudball sequence and distance (n = 1). Fordetails see Table 2. Values are means ± se. All linear measures are in mm.

The sequence of mudball placement by male fiddler crabs, Uca tangeri

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The sequence of mudball placement by male fiddler crabs, <i>Uca tangeri</i>

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