Patterns in body size and melanism along a latitudinal cline in the wingless grasshopper, Phaulacridium vittatum
Harris, R and McQuillan, P and Hughes, L, Patterns in body size and melanism along a latitudinal cline in the wingless grasshopper, Phaulacridium vittatum, Journal of Biogeography, 39, (8) pp. 1450-1461. ISSN 0305-0270 (2012) [Refereed Article]
Aim: We explore geographic variation in body size within the wingless
grasshopper, Phaulacridium vittatum, along a latitudinal gradient, and ask
whether melanism can help explain the existence of clinal variation. We test the
hypotheses that both male and female grasshoppers will be larger and lighter in
colour at lower latitudes, and that reflectance and size will be positively
correlated, as predicted by biophysical theory. We then test the hypothesis that
variability in size and reflectance is thermally driven, by assessing correlations with temperature and other climatic variables.
Location Sixty-one populations were sampled along the east coast of Australia
between latitudes 27.63 S and 43.10 S, at elevations ranging from 10 to 2000 m
Methods: Average reflectance was used as a measure of melanism and femur
length as an index of body size for 198 adult grasshoppers. Climate variables were generated by BIOCLIM for each collection locality. Hierarchical partitioning was used to identify those variables with the most independent influence on grasshopper size and reflectance.
Results: Overall, there was no simple relationship between size and latitude in
P. vittatum. Female body size decreased significantly with latitude, while male
body size was largest at intermediate latitudes. Rainfall was the most important
climatic variable associated with body size of both males and females. Female
body size was also associated with radiation seasonality and male body size with
reflectance. The reflectance of females was not correlated with latitude or body
size, while male reflectance was significantly higher at intermediate latitudes and positively correlated with body size. Analyses of climate variables showed no significant association with male reflectance, while female reflectance was
significantly related to the mean temperature of the driest quarter.
Main conclusions: Geographic variation in the body size of the wingless
grasshopper is best explained in terms of rainfall and radiation seasonality, rather than temperature. However, melanism is also a significant influence on body size in male grasshoppers, suggesting that thermal fitness does play a role in determining adaptive responses to local conditions in this sex.