Abstract
Several studies have claimed that reduction in body size comprises a nearly universal response to global warming; however, doubts about the validity of this pattern for endothermic species have been raised recently. Accordingly, we assessed temporal changes in body mass for 27 bird and 17 mammal species, to evaluate if a reduction in body size during the 20th century is a widespread phenomenon among endothermic vertebrates. In addition, we tested if there are differences in the temporal change in size between birds and mammals, aquatic and terrestrial species, and the first and second half of the 20th century. Overall, six species increased their body mass, 21 species showed no significant changes in size, and 17 species decreased their body mass during the 20th century. Temporal changes in body mass were similar for birds and mammals, but strongly differ between aquatic and terrestrial species: while most of the aquatic species increased or did not change in body mass, most terrestrial species decreased in size. In addition, we found that, at least in terrestrial birds, the mean value of the correlation between body mass and year of collection differs between the first half and the second half of the 20th century, being close to zero for the former period but negative for the later one. To our knowledge, this is the first study showing that temporal changes in body mass differ between aquatic and terrestrial species in both mammals and birds.
Highlights
Several studies have claimed that a reduction in body size comprises the third universal ecological response to global warming, after species distributional shifts and phenological changes [1,2,3]
While most aquatic species increased or did not change body mass through the 20th century, most terrestrial species decreased in size
We found that before 1950 two terrestrial bird species showed a significant increase in body mass, twelve species showed no significant changes in size, and three species showed a significant decrease in body mass (Fig 3b; Table B in S1 Appendix)
Summary
Several studies have claimed that a reduction in body size comprises the third universal ecological response to global warming, after species distributional shifts and phenological changes [1,2,3]. For the particular case of endothermic species, two non-mutually exclusive ideas are usually invoked to explain this phenomenon [4,5,6,7]. A rise in ambient temperature may cause a reduction in body size due to energetic considerations –i.e., smaller size implies a larger. As for the mechanistic basis of these changes, two non-mutually exclusive explanations are usually advanced: a plastic response at the individual level [9,10] and genetic changes at the population level [11,12]. Even though very few studies have evaluated the relative weight of these two mechanisms, current evidence suggests that phenotypic plasticity usually accounts for a higher portion, if not for all, of the overall phenotypic change [13,14]
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