Sexual size dimorphism (SSD) is a common phenomenon in both plants and animals. Further, the degree of SSD is often scales allometrically with the mean size in the studied units (populations or species). One hypothesis to explain it is the correlational selection hypothesis (Fairbairn 1997. Annu Rev Ecol Syst 28:659-687): allometry in SSD evolves as a consequence of directional selection acting primarily on one sex and correlational selection on the other sex. In most cases, female-biased SSD decreases with increasing mean size (hypoallometry) while male-biased SSD increases with increasing mean size (hyperallometry) suggesting that males are driving the SSD (and body size) evolution in most cases. This pattern was named as Rensch’s rule. However, the rule is inversed in some cases, and by studying which sex’s body size is more variable accross closely related species or populations one can evaluate which sex drives the size and SSD divergence.
We have previously reported gigantism in nine-spined sticklebacks (Pungitius pungitius) in small isolated ponds, under negligible predation and interspecific competition (Herczeg et al. 2009. Evolution 63:3190-3200) and now we aimed to study the SSD variation among nine-spined stickleback populations covering all the known size range. Our aims were to provide an intraspecific test of Rensch’s rule in a vertebrate and to see which sex is driving the evolution of gigantism in the species.
By comparing SSD from 11 populations and utilizing common garden data from three populations we found strong support for an inverse of Rensch’s rule: we found that the degree of SSD increased with mean size in a case when females were the larger sex (actually, SSD was significant only in the populations containing the largest fish). This result suggests that females drive the evolution of gigantism in nine-spined sticklebacks. This is supported by the fact that giant females from ponds had ca. three times larger reproductive output (based on mean egg number within clutch and size of freshly hatched larvae) than ‘normal’ females from marine sites.
This study adds a new piece to our puzzle about the adaptive divergence among nine-spined stickleback populations; it seems that in the small (can be as small as 0.5 ha) isolated ponds, where nine-spined stickleback is the only fish species and where sticklebacks evolved into quickly feeding, bold and aggressive giants (unlike the ‘normal’ sticklebacks in marine or lake populations), increasing female reproductive output by simply increasing female size was a main evolutionary agent. Our results also oppose the commonly held view that males drive the evolution of SSD as a result of sexual selection favouring larger males.
Herczeg G, Gonda A, Merilä J 2010. Rensch’s rule inverted – female-driven gigantism in nine-spined stickleback (Pungitius pungitius). Journal of Animal Ecology in press.