Evol Ecol Res 5: 105-117 (2003)     Full PDF if your library subscribes.

Adaptations to an aquatic life may be responsible for the reversed sexual size dimorphism in the water spider, Argyroneta aquatica

Dolores Schütz1,2 and Michael Taborsky2*

1Konrad Lorenz-Institut für Vergleichende Verhaltensforschung (KLIVV), Austrian Academy of Sciences, Savoyenstrasse 1a, 1160 Wien, Austria and 2Department of Behavioural Ecology, Institute of Zoology, University of Bern, Wohlenstr. 50a, CH-3032 Hinterkappelen/Bern, Switzerland

Author to whom all correspondence should be addressed.
e-mail: michael.taborsky@esh.unibe.ch


Argyroneta aquatica is the only spider that spends its whole life under water, and one of very few in which males are bigger than females. We hypothesized that the unusual ecology of the water spider is responsible for its exceptional sexual size dimorphism. If ecological parameters influence optimal body size for locomotion, the size of the more mobile sex (males) is expected to vary more between populations that vary in these ecological conditions than that of the other sex (females). Indeed, sexual size dimorphism differed significantly between four populations of A. aquatica studied at Vienna, and this variation depended more on variance in male size than in female size. We found that, apart from size, body shape and the relative length of the first pair of legs also differed significantly between the sexes.

 In land spiders, among which females are usually larger than males, small male size has been attributed to the better mobility of smaller individuals, as generally males are the more mobile sex in spiders. In aquatic animals, larger individuals have mobility advantages over smaller ones. We therefore hypothesized that, in A. aquatica, large rather than small size may facilitate locomotion, and hence that males are better divers than females. This was confirmed by the results of diving experiments. It is probable that male diving superiority is mainly due to their longer first pair of legs. Female size determines fecundity to a large extent. We wished to determine which size constraints could prevent females from growing bigger. Female size may be limited by the costs of building air bells, which they use as a retreat and for brood care. In laboratory experiments, we found that females build larger air bells than males and that air bell size correlates with body size in females but not in males. Females need to collect air from the surface to refill their bells more often than males. We conclude that the need for males to move efficiently under water and the costs to females of building a retreat and breeding shelter may be important determinants of body size and morphology. Hence the reversed sexual size dimorphism in A. aquatica may be greatly influenced by mechanisms of natural selection.

Keywords: body size, diving behaviour, locomotion, natural selection, size costs and benefits.

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