Converse Bergmann’s Rule in the red-legged grasshopper (Melanoplus femurrubrum): Body size and performance variation along a latitudinal gradient

Background/Question/Methods

Climate varies predictably along latitudinal gradients, often leading to clinal change in species adaptations.  Average annual temperatures decrease with latitude resulting in shorter season length that may limit resource availability through reduced forage time.  Clines in insect body size follow latitudinal patterns (Bergman’s Rule).  However, these patterns are more difficult to generalize, possibly because insect physiology is driven by temperature which results in different nutritional needs and energetic demands.  For insects, the challenge is to satisfy both metabolic and nutritional needs in a given environment, where temperature and food quality may be sub-optimal. We used six populations of the mixed feeding grasshopper, Melanoplus femurrubrum, collected from a broad latitudinal gradient in a common garden experiment to examine grasshopper performance in response to temperature and food quality manipulations.  We expected that grasshopper responses to food quality and temperature would vary latitudinally.  For example, we hypothesized individuals from northern populations to be darker in color, smaller in size, and experience faster growth rates as an adaptation to cooler conditions and shorter season lengths.  We also hypothesized lower whole organism standard metabolic rates (SMR) in smaller individuals, but also expected SMR to increase with decreasing food quality.

Results/Conclusions

Body color differed among populations (F_5,419=26.10, p<0.0001) and temperature treatments (F_2,419=233.68, p<0.0001) with significant interactions between population*temperature (F_10,419=3.17, p=0.0006) and temperature*diet (F_4,419=2.54, p=0.0391).  Body size was sexually dimorphic and decreased with latitude (females: F_5,179=80.07; males: F_5,187=116.91; both p<0.0001)  but was not significantly influenced by temperature or food quality in controlled experiments.  Growth rate increased with temperature (F_2,421=261.78, p<0.0001) as expected.  However, growth rate did not follow any latitudinal pattern overall and did not vary among populations or food quality at low temperatures.  Whole organism SMR increased with temperature (F_2,421=321.86, p<0.0001)  across all populations (F_5,421=13.34, p<0.0001) and varied with food quality (F_2,421=5.05, p=0.0068) as expected.  Results indicate that local adaptation in grasshopper performance varies latitudinally, but also suggest that populations likely buffer environmental variability with phenotypic plasticity to optimize survival.