Latitudinal variation in sexual dimorphism in life-history traits of a freshwater fish

Sexual dimorphism is common across the animal kingdom, but the contribution of environmental factors shaping differences between the sexes remains controversial. In ectotherms, life-history traits are known to correlate with latitude, but sex-specific responses are not well understood. We analyzed life-history trait variation between the sexes of European perch (Perca fluviatilis L.), a common freshwater fish displaying larger female size, by employing a wide latitudinal gradient. We expected to find sex-dependent latitudinal variation in life-history variables: length at age, length increment, and size at maturity, with females showing consistently higher values than males at all latitudes. We further anticipated that this gender difference would progressively decrease with the increasingly harsh environmental conditions toward higher latitude. We hypothesized that growth and length increment would decrease and size/age at maturity would increase at higher latitudes. Our results confirmed female-biased sexual size dimorphism at all latitudes and the magnitude of sexual dimorphism diminished with increase in latitude. Growth of both sexes decreased with increase in latitude, and the female latitudinal clines were steeper than those of males. Hence, we challenge two predominant ecological rules (Rensch's and Bergmann's rules) that describe common large-scale patterns of body size variation. Our data demonstrate that these two rules are not universally applicable in ectotherms or female-biased species. Our study highlights the importance of sex-specific differences in life-history traits along a latitudinal gradient, with evident implications for a wide range of studies from individual to ecosystems level.