Language Instincts: The evolution of sexual signals

From October 28, 2006

This Thursday Al Uy from Syracuse University came to talk to the Ecology, Evolution and Behaviour group at Queen’s. Al spoke about his research on the ecology and evolution of visual signaling in birds.

Plumage colour in birds is known to be involved in conspecific communication, including the signaling of quality to potential mates. In many species, females can gain information about the condition or health of a certain male just by assessing the colour of his feathers. Sexual selection for honest signaling is often implicated in the evolution of brightly coloured plumage, much like the evolution of the peacock’s tail described in my last post.

Al Uy is interested in understanding how sexual selection might contribute to the speciation process, since changes in sexual signals can lead to reproductive isolation between two populations. He pointed out that this idea originated with Darwin, who noticed that often the only difference between closely related species is a sexually dimorphic trait such as male plumage colour. To understand why this might be, Al studies the plumage colour of small birds called the bearded manakins with an interesting mating system: the males gather at display sites called leks, where they clear an area of the forest floor and dance to attract females. There are several subspecies of bearded manakins, all with striking differences in male colouration (whereas the females are all plain and look alike). Here are two of the manakin subspecies Al works on (golden-collared and white-bearded males):

Golden-collared manakinWhite bearded manakin

The signal function of manakin beards

Al began his talk by discussing his investigation of the signal function of colour in the golden-collared manakin, which formed the groundwork for his research in signal diversification. Al and his students have found that the yellow colour of the male ‘beard’ plumage functions as a signal of male quality that females assess during male dancing displays, since males with brighter yellow beards tend to be larger, have higher display rates, and obtain more matings than their dull-bearded counterparts.

Al also tested whether the brightness of the male plumage translated into conspicuousness from the point of view of the female manakins. He used a model that took account of avian perception, ambient light during male display, the reflectance of male plumage and the reflectance of the visual background during display (the dancing court). Consistent with the predictions of sexual selection theory, male conspicuousness as calculated by this model was related to male mating success. In other words, more conspicuous males obtained more matings. Unexpectedly, Al found that the darkness of the visual background was actually more important in explaining the variation in mating success than plumage reflectance, suggesting an important role of the visual habitat in shaping conspicuousness and the evolution of lekking. Al thinks that the lek mating system might have arisen from males becoming more competitive for the specific areas of the forest providing the best visual background for display. In the near future Al plans to test this idea by comparing the reflectance properties of the lek habitat with other areas in the forest.

Why are there so many beard colours?

The next part of Al’s talk focused on his main research interest: understanding what factors might promote the diversification of sexual signals like manakin beard colour. The basic hypothesis he is testing is that changes in the visual habitat can drive the diversification of visual signals. In order for plumage colour to be conspicuous (and therefore most effective as a signal) it needs to match the available light, contrast the background, and be tuned to the receptivity of the target individuals (females). Changes in any of these factors could have promoted the evolution of the four different male colour types found in the bearded manakins.

Al plans to test this hypothesis in several ways. First, he will examine whether or not the evolution of retinal physiology might be driving the diversification of male colour. He plans test this idea by comparing the abundance of retinal cones between the different manakin subspecies. If females from different subspecies have retinas that are optimally sensitive to the beard colours of their mates, then changes in female perception might be driving changes in male beard colour.

Al is also testing his ideas about colour diversification by studying a hybrid zone of golden-collared and white-bearded manakin populations. This is an area of Costa Rica where a river divides populations of the two manakin subspecies, although the yellow plumage trait (found in golden-collared manakins) extends slightly into the white-beaded population. One hypothesis Al is currently testing is whether this trait introgression has occurred because yellow plumage is intrinsically more attractive to the manakin females, although at this point he can only speculate as to why this might be. Al is testing this possibility by examining mate choice in the area where both yellow and white males are competing for the same mates.

The other question Al would like to answer is: what stops yellow males from sweeping further into the white population? It seems unlikely that the river is a physical barrier isolating these populations since birds are certainly capable of crossing it. Instead, Al is looking into the possibility that a change in the visual habitat on either side of the river is maintaining the separation between these two populations. In other words, white birds in succeed in their habitat (despite some introgression of yellow males) because white is the most conspicuous and best colour for displaying in that habitat.

Although Al’s talk was a bit long, it was enjoyed by the audience at Queen’s for telling a complete story without being overly technical, and some went so far as to claim it was the “best talk ever” (Dev Aiama). You can read more about Al Uy’s research here.