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brief communications
Coalition among male fiddler crabs
Seeing off a neighbour’s intruder may be easier than negotiating with a larger usurper.
ntil now, no compelling evidence
neighbour and then returned him to his
has emerged from studies of aniterritory (n10). Once both territory
mal territoriality to indicate that a
owners were present on the surface, we
resident will strategically help a neighwatched them for another 15 minutes.
bour to defend its territory against an
We observed no threat displays or fights
intruder1,2. We show here that territorywhen the smaller neighbour stayed.
After replacement, however, the larger
owning Australian fiddler crabs will
male approached and fought the unjudiciously assist other crabs in defendfamiliar neighbour in nine out of ten
ing their neighbouring territories. This
cases (Fisher’s exact test, P0.001).
cooperation supports the prediction3
Territorial coalitions in U. mjoebergi
that it is sometimes less costly to assist a
seem to be due to by-product mutualfamiliar neighbour than to renegotiate
ism9: the ally pays to retain an estabboundaries with a new, and possibly
4
stronger, neighbour .
lished neighbour and the neighbour
keeps his territory. The circumstances
In Darwin, Australia, the sexually
under which assistance was provided
dimorphic fiddler crab Uca mjoebergi
appeared to involve judicious decisionlives in mixed-sex colonies on intertidal
making. That this occurs in an invertemudflats. Males have one greatly
brate, but has still not been reported in
enlarged claw (Fig. 1), which is used both
birds or mammals, suggests that terrifor attracting females and as a weapon5,6.
torial coalitions depend more on approEach crab defends an all-purpose terripriate circumstances than on advanced
tory containing a central burrow that is
used as a refuge during high tide. Males Figure 1 The Australian fiddler crab uses its enlarged claw to defend non- cognitive skills.
mainly fight other males and vigorously threatening neighbours from stronger, encroaching males.
Patricia R. Y. Backwell,
defend their territory against wandering
Michael. D. Jennions
‘floaters’ that are seeking a new burrow. neighbour (14.84.0 mm) to defeat the School of Botany and Zoology, The Australian
They also repel neighbours that encroach on floater. The ally was always larger than the National University, Canberra ACT 0200, Australia
assisted neighbour (binomial test,P0.0001, e-mail: [email protected]
their territory.
Fights involving three males have been 17/17). The net result was that assisted resi- 1. Morrell, L. J. & Kokko, H. Behav. Ecol. Sociobiol. 54, 385–395
(2003).
recorded, but the territorial status of the dents were evicted during the contests signifi2. Elfström, S. T. Anim. Behav. 54, 535–542 (1997).
participants was unknown5. We therefore cantly less often than unassisted residents 3. Getty, T. Am. Zool. 27, 327–336 (1987).
tracked 268 floaters until we saw them (logistic regression, 219.6, P0.002, 4. Stamps, J. A. & Krishnan, V. V. Am. Nat. 157, 154–169 (2001).
5. Crane, J. A. Fiddler Crabs of the World (Princeton Univ. Press,
fighting a territorial male. We recorded 17 n268,12% compared with 29%).
Princeton, New Jersey, 1976).
cases in which a resident that was fighting
Helping is potentially costly. Allies leave
6. Backwell, P. R. Y., Christy, J. H., Telford, S. R., Jennions, M. D. &
an intruding floater was joined by an their territory, which increases the risk that
Passmore, N. I. Proc. R. Soc. Lond. B 267, 719–724 (2000).
immediate neighbour (henceforth called their own burrow will be usurped7, and 7. Hemmi, J. M. & Zeil, J. Nature 421, 160–163 (2003).
an ‘ally’). These fights always occurred at fighting is energetically expensive and can 8. Trivers, R. L. Q. Rev. Biol. 46, 35–57 (1971).
L. A. Cooperation Among Animals (Oxford Univ.
the resident’s burrow entrance so the ally result in claw loss5,6. So why do territory 9. Dugatkin,
Press, New York, 1997).
had to vacate his own territory temporarily owners help neighbours? First, we can Competing financial interests: declared none.
to join the fight. The two neighbours never exclude direct reciprocal altruism8 because
fought each other. They pushed or grap- allies were always larger than the males they
pled only with the intruder.
assisted. Second, there is no evidence that Corrigendum
Allies helped when they were most likely floaters pose a delayed threat to allies. When Producing decaffeinated coffee plants
to have a beneficial effect. In unassisted we continued to track a floater after his first Shinjiro Ogita, Hirtaka Uefuji, Yube Yamaguchi,
fights, the smaller the resident was com- fight, we never saw him fight his previous Nozomu Koizumi, Hiroshi Sano
pared with the floater, the greater was opponent’s neighbour; floaters moved Nature 423, 823 (2003).
the likelihood that he would be evicted 73.734.9 cm between successive fights, It has been drawn to Nature’s attention that S. O., N. K.
(logistic regression, 2136.4, P0.001, compared with a mean distance between and H. S. are named as the inventors on a patent
n251). In 94% of cases when assistance neighbours of 10 cm (n30 floaters).
relevant to this work (WO2004006658, filed in July
It has been predicted that residents will 2002), which should therefore have been declared as a
was provided, the resident was smaller than
the intruding floater, but this was only true form territorial coalitions when losing competing financial interest.
in 51% of cases when no assistance was neighbours is costly3. To test this, we
given (Fisher’s exact test, P0.001, 16/17 located 20 large males with a smaller
compared with 128/251). Males therefore neighbour (size-matched to observed brief communications arising online
provided assistance when their neighbour helper–resident pairs) and watched them ➧ www.nature.com/bca
was more likely to lose his territory.
for 15 minutes. In no case did we observe
In addition, the ally was generally larger threat displays or fights between neigh- Evolutionary genetics: Ambiguous role of CCR5 in
than the floater (binomial test, P0.013, bours. To mimic neighbour eviction or Y. pestis infection
14/17; means.d. for claw length, retention, we then either replaced the S. J. Elvin, E. D. Williamson, J. C. Scott, J. N. Smith,
21.02.9mm compared with 18.54.0mm) smaller neighbour with an intermediate- G. Pérez de Lema, S. Chilla, P. Clapham, K. Pfeffer,
and therefore more likely than his smaller sized male (n10), or caught the smaller D. Schlöndorff & B. Luckow (doi:10.1038/nature02822)
U
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