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Island and Rensch’s rules do not apply to Asellus aquaticus

Body size is a trait of fundamental ecological and evolutionary importance that is often different between males and females (sexual size dimorphism; SSD). The island rule predicts that small-bodied species tend to evolve larger following a release from interspecific competition and predation in insular environments. According to Rensch’s rule, male body size relative to female body size increases with increasing mean body size. This allometric body size – SSD scaling is explained by male-driven body size evolution. These ecogeographical rules are rarely tested within species, and has not been addressed in a cave–surface context, even though caves represent insular environments (small and isolated with simple communities).

We tested whether cave populations evolved larger and showed higher SSD than the surface populations. We found extensive between-population variation in body size and SSD, but habitat type did not explain this variation and we could not reject isometry in the male–female body size relationship. Hence, we found no support for the island or Rensch’s rules.

We conclude that local selective forces stemming from environmental factors other than island vs. mainland or the general surface vs. cave characteristics are responsible for the reported population variation.

(Photo: Charles J. Sharp via Wikimedia Commons)


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