Using different body size measures can lead to different conclusions about the effects of climate change

Aim: Declining animal body size has been proposed as a general response to
increasing global temperatures that should be observed across a broad
biogeographical scale. However, published studies have shown large
variation in both the magnitude and direction of body size trends. We aim
to investigate how the way body size is measured (body mass, structural
size, body condition) may contribute to differences in body size trends
between studies. Location: Semi-arid Australia. Taxon: White-plumed
honeyeater (Ptilotula penicillatus). Methods: We studied two separate
populations of P. penicillatus over 30 years to investigate the
associations between weather and body size. We first investigated how body
mass has been affected by weather conditions at each site and then
determined how these same local weather conditions affected the
constituent components of body mass (structural size and body condition).
Results: The magnitude and direction of weather effects differed with the
measure of body size used. Average structural size (wing length) increased
with increasing temperatures while average body condition decreased. As
body mass is a composite of structural size and body condition, the
magnitude and direction of body mass trends was affected by trends in
these two other traits. For example, differences in temperature effects on
structural size between our two sites led to clear differences in body
mass trends. Main Conclusions: Trends in body size will be strongly
affected by the choice of body size measure used in analyses. Change in
body mass can be particularly difficult to interpret as it will be a
composite of changes in both structural size and body condition. Our
results indicate that it is difficult to compare studies using different
measures of body size. Using consistent measures of body size will be
important to better understand the general effects of climate change on
body size.