Long-standing interest in the processes that drive body size evolution has been reinvigorated by recent recognition of widespread reductions in animal body size as a correlate of contemporary climate change.
Body size affects thermoregulation and energetics, so changing size has implications for individual fitness and hence population persistence, yet the climatic factors that drive size variation remain poorly understood.
Reductions in overall body size result in increased surface area to volume ratios with improved capacity for heat dissipation, which should be advantageous in a warming climate. However despite the benefit of smaller body size, several recent reviews highlight considerable variation in the direction and magnitude of size responses to climate over the last 50 years.
In birds, one little explored explanation for this variation may be, in part, size adjustment in other morphological traits that contribute to heat dissipation. Avian bills, for example, are highly vascularized organs that have an integral role in regulating body temperature. Larger bills facilitate more effective heat loss, with recent studies reporting increases in bill size as a correlate of climate warming.
Although morphological trait adjustment may confer selective advantages, the climatic conditions in which bill versus body size changes will be favoured remains to be investigated. For example, both low water availability (arid regions) and high humidity (tropics) may constrain use of evaporative water cooling, leading to increases in bill surface area, rather than reductions in body size. This is because, unlike heat loss from body tissue, heat-loss via the bill is non-evaporative, thereby reducing pressure on water budgets in arid climates or increasing cooling efficiency in the tropics. Such subtle variation in trait morphology can influence survival in droughts or other extreme events, as elegantly demonstrated in Galapagos finches.
Our goal is to understand the climatic factors that underlie differential adjustment in avian morphological traits linked to fitness. We propose a spatially-explicit, phylogenetically controlled comparative analysis of relative changes in morphometrics (bill versus body size) over the last 50 years.
We will focus on the Meliphagoidea, a large and diverse superfamily of Australasian passerines for which we already have a large museum-based data set: body size measurements for 13,000 individuals from 93 (of ca. 145) Australian species, with each species sampled across its entire geographic range; sampling extends across multiple climatic regions.
We have also extracted climate data from 1970-2012 for the polygons that encompass each species’ distribution (1km resolution). We intend to measure bills in the same specimens, providing an unprecedented opportunity to test for associations between trait variation and climatic conditions.
Our study will test an exciting new hypothesis regarding the effects of climate change on avian morphology. It will lead to greater understanding of the abiotic mechanisms that drive morphological variation in responses to climate change, and thus the climate sensitivities of endotherms.
This study will pave the way for:
- ecological studies of other groups of Australian birds using museum specimens as only museum collections can provide the necessary temporal and spatial coverage for this task; and
- a targeted genomics program to investigate the genetic basis of climate adaptation.
In combination, such research will improve our capacity to understand the evolutionary potential of species, and the role of phenotypic plasticity, in responding to climate change.
This project aslo addresses recent calls for a more integrative approach to understanding species responses to climate change, and facilitates collaboration between CSIRO and ANU labs with complementary skills (CSIRO: Biogeography and Genomics; ANU: Ecology and Genetics).