Sexual dimorphism and allometry in the giant water strider Gigantometra gigas

Canadian Journal of Zoology (Impact Factor: 1.5). 02/1999; 77(6). DOI: 10.1139/z99-071

ABSTRACT This study describes sexual dimorphism in size (total body length and lengths of leg components) and in the allometric relationships between leg-component lengths and total body length in the giant water strider Gigantometra gigas (Heteroptera: Gerridae). Gigantometra gigas is the largest known gerrid, and has been previously described as monomorphic for body size. We compare our results with analogous data collected on Gerris buenoi, a species of more moderate size, where females are larger than males. Based on 94 specimens of G. gigas, we conclude that males are larger than females in all measured traits. This dimorphism was most spectacular in the leg components, which are 10–50% longer in males than in females. Males are generally more variable in size than females, and this is especially so for leg components. Allometric analysis suggests that total leg lengths (particularly middle and rear) increase at a much greater rate with body size in males than in females, therefore there is sexual dimorphism in allometries on the scale of that in the traits themselves. The relationship between middle and hind leg lengths is strong in both sexes, and appears to differ very little between the sexes or between G. gigas, and Ge. buenoi. These data suggest a constraint on this relationship, perhaps because of the biomechanics of locomotion. We propose that sexual selection acting on middle leg lengths in males explains both the increase and variance in middle leg length, and that hind leg length follows by correlated response. Résumé : On trouvera ici la description du dimorphisme sexuel de la taille (taille du corps et taille des composantes des pattes) et des relations allométriques entre les composantes des pattes et la taille du corps chez le patineur Gigantometra gigas (Heteroptera : Gerridae). Gigantometra gigas est le patineur le plus grand que l'on connaisse et il a été décrit antérieurement comme monomorphe en fonction de sa taille. Nous comparons nos résultats à des données analogues obtenues chez une espèce de taille moyenne, Gerris buenoi, dont les femelles sont plus grosses que les mâles. D'après les mesures obtenues chez 94 G. gigas, les mâles sont plus grands que les femelles, et cela pour toutes les structures mesurées. Ce dimorphisme est particulièrement spectaculaire dans le cas des composantes des pattes, alors que les mâles ont des mesures de 10 à 50% plus grandes que celles des femelles. Les mâles sont généralement de taille plus variable que les femelles, particulièrement en ce qui concerne les mesures des pattes. L'analyse allométrique semble indiquer que la longueur des pattes (surtout la médiane et la postérieure) augmente en fonction de la taille du corps selon un taux beaucoup plus important chez les mâles que chez les femelles. Le dimorphisme sexuel dans l'allométrie suit donc la même échelle que celui que l'on observe dans les structures elles-mêmes. La relation entre la taille de la patte médiane et celle de la patte postérieure est donc forte chez les deux sexes et semble différer peu d'un sexe à l'autre ou d'une espèce à l'autre chez G. gigas et Ge. buenoi. Ces données indiquent l'existence d'une contrainte sur cette relation, peut-être reliée à la biomécanique de la locomotion. Nous croyons que la sélection sexuelle qui agit sur la longueur de la patte médiane du mâle peut expliquer à la fois l'augmentation de taille et l'augmentation de la variance de la longueur de la patte médiane et que la longueur de la patte postérieure varie de la même façon par corrélation.

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