The bigger you are, the weaker you are: Multispecies evidence for the weakening combatant hypothesis

As fiddler crabs increase in size, closing force weakens proportionally because claw growth is not isometric. As a result, mechanical advantage decreases at the tip with increasing claw length. Closing force at an interior tubercle is greater at the tip, and a previous study of two species claimed evidence for compensation by relative movement of tubercles toward the hinge as size increases. I reconsidered this hypothesis with a biometric analysis of eight species from five genera and all three major biogeographic regions. I tested test two null hypotheses: 1) as claw length increases, mechanical advantage did not increase at the tubercle, relative to the claw tip, and 2) closing force did not increase at the tubercle, relative to the claw tip, as claw size increased. Both null hypotheses are largely supported, which in turn supports the weakening combatant hypothesis. In all cases, log force as a function of log claw length at an interior tubercle is 1.40, which is less than 2, the slope value expected if there was complete compensation. This is only slightly greater than the slope at the dactyl tip, which is 1.21. Thus, as a fiddler crab gets larger, with an incremental increase of claw length its closing force proportionally decreases, supporting the idea of a weakening combatant. But it gains a large showy claw with relatively narrow pincers and relatively less mass required to swing around while waving. This is indeed a beautiful weapon, but not for sheer closing force.

[1]  J. Levinton,et al.  Superior performance of a trailing edge low-latitude population of an intertidal marine invertebrate , 2023, Journal of Experimental Marine Biology and Ecology.

[2]  C. E. Allen,et al.  Evolution of horn length and lifting strength in the Japanese rhinoceros beetle Trypoxylus dichotomus , 2023, Current Biology.

[3]  J. Levinton,et al.  Direct and biometrical estimates of the closing force of major claws of the sand fiddler crab Leptuca pugilator (Bosc, 1801) (Decapoda: Brachyura: Ocypodidae): Support for the weakening combatant hypothesis , 2021 .

[4]  J. Levinton.  Thermal stress: The role of body size and the giant major claw in survival and heat transfer of a fiddler crab (Leptuca pugilator) , 2020 .

[5]  L. McBrayer,et al.  Claw-Pinching Force of Sand Fiddler Crabs in Relation to Activity and the Lunar Cycle , 2015 .

[6]  M. Rosenberg.  Contextual Cross-Referencing of Species Names for Fiddler Crabs (Genus Uca): An Experiment in Cyber-Taxonomy , 2014, PloS one.

[7]  J. Christy,et al.  Evolutionary variation in the mechanics of fiddler crab claws , 2013, BMC Evolutionary Biology.

[8]  J. Christy,et al.  THE DESIGN OF A BEAUTIFUL WEAPON: COMPENSATION FOR OPPOSING SEXUAL SELECTION ON A TRAIT WITH TWO FUNCTIONS , 2013, Evolution; international journal of organic evolution.

[9]  A. Campbell,et al.  Force, function and mechanical advantage in the chelae of the American lobster Homarus americanus (Decapoda: Crustacea) , 2009 .

[10]  A. R. Jones,et al.  Leverage and muscle type in crab chelae (Crustacea: Brachyura) , 2009 .

[11]  P. Backwell,et al.  Dishonest signalling of fighting ability and multiple performance traits in the fiddler crab Uca mjoebergi , 2009 .

[12]  J. Levinton,et al.  Costs of bearing a sexually selected ornamental weapon in a fiddler crab , 2007 .

[13]  J. Levinton,et al.  The paradox of the weakening combatant: trade‐off between closing force and gripping speed in a sexually selected combat structure , 2005 .

[14]  J. Levinton,et al.  Molecular phylogeny analysis of fiddler crabs: test of the hypothesis of increasing behavioral complexity in evolution. , 1996, Proceedings of the National Academy of Sciences of the United States of America.

[15]  P. Backwell,et al.  Residency and size affect fight duration and outcome in the fiddler crab Uca annulipes , 1996 .

[16]  J. Levinton,et al.  Functional differences between the major and minor claws of fiddler crabs (Uca, family Ocypodidae, Order Decapoda, Subphylum Crustacea): A result of selection or developmental constraint? , 1995 .

[17]  J. Levinton,et al.  The relationship of closing force to body size for the major claw of Uca pugnax (Decapoda: Ocypodidae) , 1993 .

[18]  J. Christy.  Burrow structure and use in the sand fiddler crab, Uca pugilator (Bosc) , 1982, Animal Behaviour.

[19]  M. Salmon,et al.  Uca panacea, a new species of fiddler crab from the Gulf Coast of the United States , 1974 .

[20]  J. Crane.  Combat, display and ritualization in Fiddler Crabs (Ocypodidae, genus Uca) , 1966, Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences.

[21]  J. Huxley.  Constant Differential Growth-Ratios and their Significance , 1924, Nature.

[22]  R. W. Elner,et al.  The mechanics of predation by the shore crab, Carcinus maenas (L.), on the edible mussel, Mytilus edulis L. , 2004, Oecologia.

[23]  M. Murai,et al.  MATING BEHAVIOR OF UCA LACTEA PERPLEXA (DECAPODA: OCYPODIDAE) , 1998 .

[24]  M. Rosenberg.  Evolution of Shape Differences between the Major and Minor Chelipeds of Uca Pugnax (Decapoda: Ocypodidae) , 1997 .

[25]  Alick R. Jones.  Chela injuries in the fiddler crab, Uca burgersi holthuis , 1980 .

[26]  Muzammil M N Ahmed.  Development of Asymmetry in the Fiddler Crab Uca Cumulanta Crane, 1943 (Decapoda Brachyura) , 1978 .

[27]  Michael Salmon,et al.  Combat in the Fiddler Crabs Uca Pugilator and U. Pugnax: a Quantitative Analysis , 1978 .

[28]  L. Holthuis.  On a new species of Uca from the West Indian region (Crustacea, Brachyura, Ocypodidae) , 1967 .

[29]  J. Crane.  Eastern Pacific Expeditions of the New York Zoological Society. XXVI. Crabs of the Genus Uca from the West Coast of Central America , 1941 .

[30]  S. I. Smith.  Ocypodoidea. Notes on North American Crustacea, I , 1870 .