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Evolutionary variation in the mechanics of fiddler crab claws.

Swanson BO, George MN, Anderson SP, Christy JH - BMC Evol. Biol. (2013)

Bottom Line: Using phylogenetically independent contrasts, we find that the force that a claw can potentially produce is positively correlated with the strength of the cuticle on the claw where forces are delivered in a fight.There is also a negative correlation between the force that a claw can potentially produce and the size of the claw corrected for the mass of the claw.These relationships suggest that there has been correlated evolution between force production and armoring, and that there is a tradeoff between claw mechanics for signaling and claw mechanics for fighting.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biology, Gonzaga University, Spokane, WA 99258, USA. swansonb@gonzaga.edu

ABSTRACT

Background: Fiddler crabs, genus Uca, are classic examples of how intense sexual selection can produce exaggerated male traits. Throughout the genus the enlarged "major" cheliped (claw) of the male fiddler crab is used both as a signal for attracting females and as a weapon for combat with other males. However, the morphology of the major claw is highly variable across the approximately 100 species within the genus. Here we address variation, scaling, and correlated evolution in the mechanics of the major claw by analyzing the morphology and mechanical properties of the claws of 21 species of fiddler crabs from the Pacific, Gulf and Atlantic coasts of the Americas.

Results: We find that the mechanics that produce claw closing forces, the sizes of claws and the mechanical strength of the cuticle of claws are all highly variable across the genus. Most variables scale isometrically with body size across species but claw force production scales allometrically with body size. Using phylogenetically independent contrasts, we find that the force that a claw can potentially produce is positively correlated with the strength of the cuticle on the claw where forces are delivered in a fight. There is also a negative correlation between the force that a claw can potentially produce and the size of the claw corrected for the mass of the claw.

Conclusions: These relationships suggest that there has been correlated evolution between force production and armoring, and that there is a tradeoff between claw mechanics for signaling and claw mechanics for fighting.

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Related in: MedlinePlus

Scaling of morphological and mechanical variables with crab body mass. Each symbol represents a species average (± 1 S.E.M.) for both the variable in question and body mass. Scaling exponents are slopes of a regression of the logs of the mean values. (A) Mass of the major claw, (B) length of the major claw, (C) frontal area of the major claw, (D) Force required to puncture the manus cuticle, (E) Force estimated at the claw tubercle, calculated from the morphology of the claw, see text.
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Figure 4: Scaling of morphological and mechanical variables with crab body mass. Each symbol represents a species average (± 1 S.E.M.) for both the variable in question and body mass. Scaling exponents are slopes of a regression of the logs of the mean values. (A) Mass of the major claw, (B) length of the major claw, (C) frontal area of the major claw, (D) Force required to puncture the manus cuticle, (E) Force estimated at the claw tubercle, calculated from the morphology of the claw, see text.

Mentions: We found considerable morphological variation across the fiddler crab phylogeny (Figures 1, 2, 3). Most of the variables measured seem to scale isometrically with mass (Figure 4). For instance, the values of the exponents of the regressions of the mass of the major claw (1.03), the length of the claw (0.35), and the frontal area of the claw (0.68), on body mass all are exactly as expected (1, 1/3, and 2/3 respectively; Figure 4). Cuticular resistance scales at approximately 2/3 power of body mass although there is a large amount of variation in this value, with an r2 of only 0.78. The scaling exponent of the force produced at the tubercle of the claw was 0.78, higher than the expected value of 2/3 for scaling of the muscle cross sectional area (Figure 4). When the force produced by the claw is compared to the ability of the cuticle to resist puncture, we find that within a species, the claw tip should be able to produce enough force to puncture the carapace, but not the manus. Furthermore, we find that crabs should be able to produce approximately enough force at the claw tubercles to puncture the manus cuticle of a size-matched conspecific, but not enough force to cause the cuticle to structurally fail (Figure 5). When claw force and claw resistance values are adjusted for size (residuals of the OLS regression of ln transformed values on ln body mass) there is a positive correlation (r=0.615, d.f.=19, p<0.05). When values are further corrected for phylogeny (standardized independent contrasts), we find that there is still a positive correlation between claw force and the force required to puncture the manus cuticle (r = 0.57, F=8.97, d.f.=19, p<0.05; Figure 6). Finally, there is a significant negative correlation between force produced at the tubercle (mechanics that should be good for combat) and both weight specific frontal area of the claw and weight specific claw length (morphologies that should provide a large signal at low waving cost) both for size corrected values (claw area: r=-0.44, d.f.=19, p<0.05; claw length: r=-0.48, d.f.=19, p<0.05), and when using size-corrected standardized contrasts (claw area: r=-0.67, F=15.3, d.f.=19, p<0.05; claw length: r=-0.69, F=17.69, d.f.=19, p<0.05; Figure 7).


Evolutionary variation in the mechanics of fiddler crab claws.

Swanson BO, George MN, Anderson SP, Christy JH - BMC Evol. Biol. (2013)

Scaling of morphological and mechanical variables with crab body mass. Each symbol represents a species average (± 1 S.E.M.) for both the variable in question and body mass. Scaling exponents are slopes of a regression of the logs of the mean values. (A) Mass of the major claw, (B) length of the major claw, (C) frontal area of the major claw, (D) Force required to puncture the manus cuticle, (E) Force estimated at the claw tubercle, calculated from the morphology of the claw, see text.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3716949&req=5

Figure 4: Scaling of morphological and mechanical variables with crab body mass. Each symbol represents a species average (± 1 S.E.M.) for both the variable in question and body mass. Scaling exponents are slopes of a regression of the logs of the mean values. (A) Mass of the major claw, (B) length of the major claw, (C) frontal area of the major claw, (D) Force required to puncture the manus cuticle, (E) Force estimated at the claw tubercle, calculated from the morphology of the claw, see text.
Mentions: We found considerable morphological variation across the fiddler crab phylogeny (Figures 1, 2, 3). Most of the variables measured seem to scale isometrically with mass (Figure 4). For instance, the values of the exponents of the regressions of the mass of the major claw (1.03), the length of the claw (0.35), and the frontal area of the claw (0.68), on body mass all are exactly as expected (1, 1/3, and 2/3 respectively; Figure 4). Cuticular resistance scales at approximately 2/3 power of body mass although there is a large amount of variation in this value, with an r2 of only 0.78. The scaling exponent of the force produced at the tubercle of the claw was 0.78, higher than the expected value of 2/3 for scaling of the muscle cross sectional area (Figure 4). When the force produced by the claw is compared to the ability of the cuticle to resist puncture, we find that within a species, the claw tip should be able to produce enough force to puncture the carapace, but not the manus. Furthermore, we find that crabs should be able to produce approximately enough force at the claw tubercles to puncture the manus cuticle of a size-matched conspecific, but not enough force to cause the cuticle to structurally fail (Figure 5). When claw force and claw resistance values are adjusted for size (residuals of the OLS regression of ln transformed values on ln body mass) there is a positive correlation (r=0.615, d.f.=19, p<0.05). When values are further corrected for phylogeny (standardized independent contrasts), we find that there is still a positive correlation between claw force and the force required to puncture the manus cuticle (r = 0.57, F=8.97, d.f.=19, p<0.05; Figure 6). Finally, there is a significant negative correlation between force produced at the tubercle (mechanics that should be good for combat) and both weight specific frontal area of the claw and weight specific claw length (morphologies that should provide a large signal at low waving cost) both for size corrected values (claw area: r=-0.44, d.f.=19, p<0.05; claw length: r=-0.48, d.f.=19, p<0.05), and when using size-corrected standardized contrasts (claw area: r=-0.67, F=15.3, d.f.=19, p<0.05; claw length: r=-0.69, F=17.69, d.f.=19, p<0.05; Figure 7).

Bottom Line: Using phylogenetically independent contrasts, we find that the force that a claw can potentially produce is positively correlated with the strength of the cuticle on the claw where forces are delivered in a fight.There is also a negative correlation between the force that a claw can potentially produce and the size of the claw corrected for the mass of the claw.These relationships suggest that there has been correlated evolution between force production and armoring, and that there is a tradeoff between claw mechanics for signaling and claw mechanics for fighting.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biology, Gonzaga University, Spokane, WA 99258, USA. swansonb@gonzaga.edu

ABSTRACT

Background: Fiddler crabs, genus Uca, are classic examples of how intense sexual selection can produce exaggerated male traits. Throughout the genus the enlarged "major" cheliped (claw) of the male fiddler crab is used both as a signal for attracting females and as a weapon for combat with other males. However, the morphology of the major claw is highly variable across the approximately 100 species within the genus. Here we address variation, scaling, and correlated evolution in the mechanics of the major claw by analyzing the morphology and mechanical properties of the claws of 21 species of fiddler crabs from the Pacific, Gulf and Atlantic coasts of the Americas.

Results: We find that the mechanics that produce claw closing forces, the sizes of claws and the mechanical strength of the cuticle of claws are all highly variable across the genus. Most variables scale isometrically with body size across species but claw force production scales allometrically with body size. Using phylogenetically independent contrasts, we find that the force that a claw can potentially produce is positively correlated with the strength of the cuticle on the claw where forces are delivered in a fight. There is also a negative correlation between the force that a claw can potentially produce and the size of the claw corrected for the mass of the claw.

Conclusions: These relationships suggest that there has been correlated evolution between force production and armoring, and that there is a tradeoff between claw mechanics for signaling and claw mechanics for fighting.

Show MeSH
Related in: MedlinePlus