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

Abstract

AimDeclining 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.LocationSemi‐arid Australia.TaxonWhite‐plumed honeyeater (Ptilotula penicillatus).MethodsWe 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).ResultsThe 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 ConclusionsTrends 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.

Different increase rate in body mass of two marten species due to climate warming potentially reinforces interspecific competition

“…Body mass is commonly used as an index of animal body size 41 , but this metric also includes body condition, which varies within an individual’s lifetime and may give different results than a structural size metric only 42 . However, martens have limited possibility to accumulate fat reserves and they rely little on body energy reserves 43 .…”

“…The GAMs were conducted separately for each species with latitude, longitude, year, month in which the animals were found dead or trapped, and sex as explanatory variables. The body mass includes body condition, which varies within an individual’s lifetime (lower in winter and higher in spring–summer) and may give different results than a structural size metric only 42 . To partially account for body condition variation across the seasons, we included the month in which the animals were found dead or trapped in the analyses.…”

For neither heat nor water conservation: Body size variation in Atlantic Forest frogs does not follow a general mechanism

“…Few empirical studies have compared variation in more than one morphological variable across space and climatic conditions in ectotherms (Amado, Pinto, et al, 2019;Bailey et al, 2020;Valenzuela-Sánchez et al, 2015). Our results revealed diverse environmental constraints on body size variation in SVL and stoutness in anurans exposed to different environmental temperatures and water availability.…”

Distinct body-size responses to warming climate in three rodent species

“…These different results for body length and mass in our study suggest that the two body-size metrics respond differently to environmental change. Body length is a measure of structural size, whereas mass is determined by both structural size and body condition, which can be affected by environmental change in different ways [9,38]. Moreover, body mass is an indicator of organisms' nutrient reserves, which can change repeatedly throughout life [38] and therefore might be affected by the sampling scheme.…”

“…human activities) can also affect body size [7,8]. Moreover, different body-size metrics, such as length and mass, may follow different temperature-size trends [9]. Therefore, to establish the foundation for research on climate change driven body-size change, the role of ambient temperature in causing this change needs to be clarified.…”