Guest essay by Eric Worrall
A new study claims climate stress is making some birds lose their mating ornamentation.
Climate change ecology: Hot under the collar
A 34-year study of collared flycatchers demonstrates that males are evolving to be less ornamented in response to rising temperatures.
For male collared flycatchers (Ficedula albicollis; pictured), having a large white patch on your forehead signals that you are a fearsome rival, and an attractive mate. But while large patch size was thought to give males an evolutionary advantage, it seems that climate change is turning this advantage on its head. In this issue of Nature Ecology & Evolution, Evans and Gustafsson report that the collared flycatcher’s forehead patch has declined in size because large patch males have lower fitness as the climate warms.
In many animals, males have ornamental features that are used to attract mates, and to signal to rivals during territorial interactions. While these ‘sexually selected’ ornaments can increase a male’s mating success, they can also impose a variety of costs on their bearers. Ornaments such as bright colouration can increase susceptibility to predators, or they can predispose males to competing for larger territories and more mates, at the expense of providing care for their offspring. As a result, ornamental traits are hypothesized to evolve only when the benefits of being ornamented outweigh the costs.
While the authors demonstrate a strong link between climate warming and selection on male ornamentation, they were not able to determine the mechanism behind this link.
Certainly, there are good theoretical reasons why climatic changes could influence animal ornamentation across the globe. As Evans and Gustafsson show, some ornaments will decline in response to environmental change, and it is conceivable that some ornaments could disappear altogether. However, there are also reasons to predict that climate change will drive the evolution of new, or exaggerated, ornaments in some species. Just as climate change will lead to winners and losers in terms of species’ abundance and distribution, it seems it may also lead to winners and losers in the global beauty pageant.
The abstract of the study referenced by the press release;
Climate change upends selection on ornamentation in a wild bird
Simon R. Evans & Lars Gustafsson
Secondary sexual traits have high heritabilities and are exposed to strong, environmentally sensitive selection, and so are expected to evolve rapidly in response to sustained environmental change. We examine the eco-evolutionary dynamics of ornament expression in a long-term study population of collared flycatchers, Ficedula albicollis, in which forehead patch size, which positively influences male reproductive success, declined markedly over 34 years. Annual fitness selection on forehead patch size switched from positive to negative during the study, a reversal that is accounted for by rising spring temperatures at the breeding site: highly ornamented males were selectively favoured following cold breeding seasons but selected against following warm breeding seasons. An ‘individual animal model’ describes a decline in the genetic values of breeding males during the study, which simulations showed was unlikely to result from drift alone. These results are thus consistent with adaptive evolution of a sexually selected trait in response to climate change.
Long-term population studies have provided many examples of traits undergoing substantial phenotypic change in response to climate change. While these are seemingly indicative of contemporary evolution, robust demonstrations of adaptation are conspicuously rare, particularly in light of studies showing that phenotypic change does not equate to genetic change. While direct demonstrations of shifts in allele frequencies in response to climate change have emerged, many phenotypes do not follow simple Mendelian patterns of inheritance and are instead influenced by innumerable loci, each of small effect, such that studying individual loci will provide little information about the evolutionary dynamics governing the trait. Quantitative genetics resolves this apparent impasse by providing an analytical framework that treats the summed contribution of all loci as the unit of interest, an approach that has proven highly successful in predicting the responses of domestic populations to artificial selection. In particular, the ‘individual animal model’ estimates the genetic value of each individual in the sample population, providing a robust methodology for quantifying evolutionary change in the wild. However, published demonstrations of adaptive evolution of quantitative traits in response to climate change have been conspicuously absent since the realization that earlier applications are strongly anticonservative.
Research on the evolutionary impact of climate change in vertebrates has centred on phenological traits yet selection on secondary sexual traits is highly environmentally sensitive, which, combined with their high heritabilities14, would seem to make them ideal traits for observing evolutionary responses to climate change. However, while comparative analyses support theoretical predictions that secondary sexual traits are evolutionarily labile, robust demonstrations of their contemporary evolution in the wild are scant, being limited to discrete traits18 or populations exposed to severe artificial selection. Indeed, the apparent evolutionary stasis of secondary sexual traits has been the subject of much speculation, despite the scarcity of demonstrations of contemporary evolution in the wild for quantitative traits in genera.
We studied secondary sexual trait expression in a nestbox-breeding population of male collared flycatchers, Ficedula albicollis, from 1981 to 2014. The sex-limited, white forehead patch (Fig. 1) is an established ornamental trait: males expressing a large forehead patch have a competitive advantage over rival males22,23. Forehead patch size is heritable and the possibility that evolutionary change might underlie the decline in phenotypic expression (Fig. 2a) has been raised previously. We therefore assessed selection on forehead patch size and used individual-level quantitative genetic modelling to infer the evolutionary change in ornamentation.
I can’t help thinking the inference drawn by this study is weak. The study does not identify a mechanism by which a slight change of temperature increased stress. The study authors themselves suggested that in some cases, warming could lead to more exaggerated mating ornamentation. Even if we accept that the study authors are correct about collared flycatchers, by the study author’s own words it isn’t possible to generalise this finding into a prediction about all bird species. I suspect there are many potential issues other than a slight change in temperature, which could stress an isolated population of birds.