On linking ecology to sexual selection

Together with John Waller, I have a paper that is now out in American Naturalist as an E-article, meaning that it is "Open Acess" and possible for anyone to download. Go here, if you would like to download a PDF of this paper. I am very much in favour of the OA-model of publishing, and I certainly hope that the publication fees we paid will also result in more citations.

This study, which was fun to do and write up, takes a look at the important link between ecology and sexual selection. We were interested in the functional significance and evolutionary consequences of wing pigmentation in calopterygid damselflies, and we used a mixture of comparative phylogenetic analyses and field studies using thermal imaging to adress this issue. In particular, we wanted to see if there was any obvious thermal benefit of male wing pigmentation, which also has important functions in sexual selection, male-male competition and species recognition. Turns out that the evidence for such a thermal benefit is mixed, although there is a clear biogeographic signature in the sense that pigmented clades are more common in northern regions and temperate climates.

Wing pigmentation is also significantly associated with eleved speciation and extinction rates, using so-called BiSSE-analyses ("Binary Speciation and Extinction") as implemented in Diversitree. This latter result provides comparative support to our previous experimental work demonstrating that wing pigmentation functions as a species recognition character between C. splendens and C. virgo, and suggest that wing pigmentation is generally involved across the entire group as a promoter of speciation, although most species formed by such non-ecological sexual selection tend to go extinct fairly soon after they have formed.

In general, I think there are too few studies where comparative approaches and field experiments are combined, as both have strength and weaknesses and inferences could be stronger if they are combined (Disclaimer: in case some sensitive theoretical ecologist reads this post, I do of course also think there are other interesting and useful research approaches, such as mathematical models).

Ecology and Sexual Selection: Evolution of Wing Pigmentation in Calopterygid Damselflies in Relation to Latitude, Sexual Dimorphism, and Speciation

American Naturalist (in press, November 2013)

Erik I. Svensson* and John T. Waller

Evolutionary Ecology Unit, Department of Biology, Lund University, SE-223 62 Lund, Sweden

Abstract

Our knowledge about how the environment influences sexual selection regimes and how ecology and sexual selection interact is still limited. We performed an integrative study of wing pigmentation in calopterygid damselflies, combining phylogenetic comparative analyses, field observations and experiments. We investigated the evolutionary consequences of wing pigmentation for sexual dimorphism, speciation, and extinction and addressed the possible thermoregulatory benefits of pigmentation. First, we reconstructed ancestral states of male and female phenotypes and traced the evolutionary change of wing pigmentation. Clear wings are the ancestral state and that pigmentation dimorphism is derived, suggesting that sexual selection results in sexual dimorphism. We further demonstrate that pigmentation elevates speciation and extinction rates. We also document a significant biogeographic association with pigmented species primarily occupying northern temperate regions with cooler climates. Field observations and experiments on two temperate sympatric species suggest a link between pigmentation, thermoregulation, and sexual selection, although body temperature is also affected by other phenotypic traits such as body mass, microhabitat selection, and thermoregulatory behaviors. Taken together, our results suggest an important role for wing pigmentation in sexual selection in males and in speciation. Wing pigmentation might not increase ecological adaptation and species longevity, and its primary function is in sexual signaling and species recognition.

The toucan bill as a thermal radiator: adaptation or exaptation?

Have you ever visited a rainforest in the New World, such as in Costa Rica or elsewhere in Latin America? If so, it is a great chance you have seen a toucan, those big fruit eating birds with HUGE bills (see picture above).

The function of such large bills has been quite unknown, but now a new study published in Science indicates a surprising novel function: the bill might be important as thermal radiator (bottom figure). Using a thermal image camera of similar kind as we have bought to our lab, the researchers were able to demonstrate that the many blood vessels in the bill helped to quickly cool down the bird and get rid of excess heat. The birds could actually cool down by ten degrees in only a few minutes (yellow areas are high temperature areas in the bottom picture).

This study is interesting for several reasons. First, it shows how new techniques can help to answer old questions and give new answers (sometimes quite surprising, as in this case). Second, it shows that one has to have an open mind when trying to understand the adaptive functions of traits and what traits are "for". Most people, including myself, probably thinks that the bill evolved "for" the specific purpose of efficiently picking fruit. This might very well be the case, but once the bill have reached a certain size, it can also secondarily get another function in the form of a thermoregulator.

In this context, perhaps the thermoregulatory function is simply an exaptation, i. e. not a true adaptation in the language of paleontologist Stephen Jay Gould? Or perhaps the bill was further exaggerated once it had reached its "optimum" size from the perspective of fruit picking, and only the extra length is a true adaptation in the context of thermoregulation? Perhaps future phylogenetic comparative studies involving other toucan species with different bill sizes and the thermal image camera can provide some answers to these fascinating questions?

(Thanx Maren for the tip!)