When Rensch meets Bergmann: does sexual size dimorphism change systematically with latitude?

Area of Science:

• Ecology
• Evolutionary Biology
• Zoology

Background:

• Bergmann's rule (larger body size in colder climates) and Rensch's rule (greater male size variation) are established patterns.
• The underlying mechanisms driving these large-scale body size variations remain largely unknown.
• Investigating intraspecific variation and latitudinal clines can elucidate these evolutionary patterns.

Purpose of the Study:

• To examine sex-specific latitudinal body size clines in vertebrates and invertebrates.
• To test for patterns equivalent to Rensch's rule within species populations.
• To evaluate the relationship between Rensch's rule patterns and Bergmann's rule.

Main Methods:

• Meta-analysis of published studies on sex-specific latitudinal body size clines.
• Analysis of intraspecific variation in sexual size dimorphism across latitudes.
• Regression analysis of male versus female body size and latitudinal clines.

Main Results:

• A continuum of Bergmann (58 species) and converse Bergmann (40 species) clines were observed.
• Intraspecific sexual size dimorphism did not consistently follow Rensch's rule (55/98 species).
• A latitudinal version of Rensch's rule was significant, with steeper male clines than female clines in 66/98 species.

Conclusions:

• Latitudinal body size variation within species often aligns with a version of Rensch's rule.
• Factors systematically varying with latitude likely drive these intraspecific Rensch's rule patterns.
• Further research is needed on latitudinal variation in sex-specific selection to identify underlying mechanisms.