Sympatry drives colour and song evolution in wood-warblers (Parulidae)

Closely related species often exhibit similarities in appearance and behaviour, yet when related species exist in sympatry, signals may diverge to enhance species recognition. Prior comparative studies provided mixed support for this hypothesis, but the relationship between sympatry and signal divergence is likely nonlinear. Constraints on signal diversity may limit signal divergence, especially when large numbers of species are sympatric. We tested the effect of sympatric overlap on plumage colour and song divergence in wood-warblers (Parulidae), a speciose group with diverse visual and vocal signals. We also tested how number of sympatric species influences signal divergence. Allopatric species pairs had overall greater plumage and song divergence compared to sympatric species pairs. However, among sympatric species pairs, plumage divergence positively related to the degree of sympatric overlap in males and females, while male song bandwidth and syllable rate divergence negatively related to sympatric overlap. In addition, as the number of species in sympatry increased, average signal divergence among sympatric species decreased, which is likely due to constraints on warbler perceptual space and signal diversity. Our findings reveal that sympatry influences signal evolution in warblers, though not always as predicted, and that number of sympatric species can limit sympatry's influence on signal evolution.

Threshold assessment, categorical perception, and the evolution of reliable signaling

Animals often use assessment signals to communicate information about their quality to a variety of receivers, including potential mates, competitors, and predators. But what maintains reliable signaling and prevents signalers from signaling a better quality than they actually have? Previous work has shown that reliable signaling can be maintained if signalers pay fitness costs for signaling at different intensities and these costs are greater for lower quality individuals than higher quality ones. Models supporting this idea typically assume that continuous variation in signal intensity is perceived as such by receivers. In many organisms, however, receivers have threshold responses to signals, in which they respond to a signal if it is above a threshold value and do not respond if the signal is below the threshold value. Here, we use both analytical and individual-based models to investigate how such threshold responses affect the reliability of assessment signals. We show that reliable signaling systems can break down when receivers have an invariant threshold response, but reliable signaling can be rescued if there is variation among receivers in the location of their threshold boundary. Our models provide an important step towards understanding signal evolution when receivers have threshold responses to continuous signal variation.

Inconsistent sexual signaling degrades optimal mating decisions in animals

Like political stump speeches and product advertisements, animal signals are highly repetitive and function to persuade receivers to adopt behaviors benefiting the signaler. And like potential constituents and consumers, receivers assess signals to inform their behavioral decisions. However, inconsistency in sexual signals is widespread and potentially injects uncertainty into mating decisions. Here, we show that females fail to make optimal mating decisions based on assessments of signal quality due to inconsistency in signal production. Natural levels of inconsistency markedly reduced female preference expression for a nonarbitrary signal of male quality. Inconsistency reshaped preferences even more profoundly than the better-known impediment of ambient noise. To our knowledge, this is the first demonstration of how inconsistent messaging degrades optimal decision-making in animals, with implications for understanding signal evolution.

Coevolution of song and egg coloration: multimodal mating signals?

The divergence of reproductive traits frequently underpins the evolution of reproductive isolation. One of the most enduring puzzles on this subject concerns the variability in egg coloration among species of tinamou (Tinamidae) —a group of birds endemic to neotropics (Cabot 1992). Specifically, some tinamous lay glossy and colorful eggs while others lay less colorful eggs. Here I tested the hypothesis that tinamou egg coloration is a mating signal and its diversification was driven by reinforcement. For most tinamou species, the male guard the nest that is sequentially visited and laid eggs in by multiple females. The colorations of the existing eggs in the nest could signal mate quality and species identities to the upcoming females to the nest, preventing costly hybridization, thus were selected to diverge among species. If so, egg colors should coevolve with the known mating signals as the tinamou lineages diverged. The tinamou songs are important mating signals and are highly divergent among species. I found that the egg luminance was significantly associated with the first principle component of the song variables among 31 tinamou species (after correcting for phylogenetic signal). Egg color and songs could be multimodal mating signals that are divergently selected as different tinamou species diverged. Mating signal evolution could be opportunistic and even exploit post-mating trait as premating signals.

Environmental and morphological constraints interact to drive the evolution of communication signals in frogs

Animals show a rich diversity of signals and displays. Among the many selective forces driving the evolution of communication between individuals, one widely recognized factor is the structure of the environment in which signals are produced, transmitted and received. In particular, animals communicating by sounds often emit acoustic signals from specific locations, such as high up in the air, from the ground or in the water. The properties of these different display sites will impose different constraints on sound production and transmission and may therefore drive signal evolution. Here, we used comparative phylogenetic analyses to assess the relationship between the display site properties and the structure of reproductive calls from 161 frog species from the frog families Ranidae, Leptodactylidae and Hylidae. Specifically, we compared the dominant frequency of species that vocalize from aquatic versus non-aquatic sites, and its relation with body size. We found that the dominant frequency of frogs calling from the water was lower than that of species calling outside of the water, a trend that was consistent across the three families studied. Furthermore, phylogenetic path analysis revealed that the call site had both direct and indirect effects on the dominant frequency. Indirect effects were mediated by call site influencing male body size, which in turn was negatively associated to call dominant frequency. Our results suggest that properties of display sites can drive signal evolution, most likely through morphological constraints, in particular the ones imposed on the sound production mechanism. Also, variation in body size between calling sites explained some of the differences we found in call frequency, highlighting the relevance of the interplay between morphological adaptation and signal evolution. Changes of display site may therefore have important evolutionary consequences, as it may influence sexual selection processes and ultimately may even promote speciation.Impact summaryTo attract or impress mates, animals have evolved a great diversity of communication signals, such as song and dance, or brightly colored body parts. Whether these sexual signals are successful depends to a large extent on the environment in which they are produced, transmitted and perceived. For acoustic signals, such as the mating calls of frogs, the environment is well known to influence both their transmission and perception. The impact of the environment on the production of sounds is however far less understood. Here we studied the relation between the environment and signal design across a wide range of frog species, specifically comparing calls of aquatic versus non-aquatic species.Frogs that called from water were found to call at lower pitch, which was partly explained by the fact that they were also larger. Our results point towards an important environmental driver of signal evolution, namely morphological constraints on signal production. We argue that the environment can impose limits on morphological traits that are either directly or indirectly involved in signal production. Such a mechanism would in particular be important when species move into new habitats, as rapid changes to display sites may lead to rapid changes in sexual signaling and sexual attractiveness.