The evolution of birdsong on islands

Signalling on islands: the case of Lilford’s wall lizard (Podarcis lilfordi gigliolii) from Dragonera

Studies of the effects of insularity on animal signals are scarce, particularly in lizards. Here, we use Lilford’s wall lizard from Dragonera (Podarcis lilfordi gigliolii) to ask how island conditions have affected its repertoire of social signals, focusing on two visual signals shared by many Podarcis species: ultraviolet (UV)–blue-reflecting ventrolateral colour patches and visual displays. We examined whether the number or spectral characteristics of the UV–blue patches are associated with traits related to individual quality. We also used visual models to assess visual conspicuousness and to measure sexual dichromatism. We did not observe foot shakes or any other visual displays usually found in continental Podarcis. We found that none of the UV–blue patch variables covaried with morphometric variables indicative of fighting ability or body condition in males, suggesting that this coloration does not signal individual quality. We also found very little sexual dichromatism. In particular, the UV–blue patches of females seem over-expressed and more similar to those of males than those of continental Podarcis. Ancestral state reconstruction reveals that the lack of sexual dimorphism in the UV–blue patches is a derived condition for P. lilfordi gigliolii and other Podarcis living on small islands. Our results thus show a pattern of reduced social signalling in P. lilfordi gigliolii relative to mainland Podarcis, with some signals being lost or under-expressed (visual displays) and others losing their signalling function (UV–blue patches). We hypothesize that these changes are attributable to the high population density of P. lilfordi gigliolii, which discourages territorial behaviour and promotes extreme social tolerance, making most social signals unnecessary. More work will be needed to determine whether this is a common pattern in lizards inhabiting small and densely populated islands.

Bioacoustic differentiation of calls in the chiffchaff complex

The chiffchaff complex is a group of common forest bird species, notorious for the number of cryptic taxa recently discovered, being a great example of speciation in action. Vocalizations have been crucial to unveil its hidden diversity. In this study we quantitatively analyze the acoustic characteristics of their calls with permutational analysis of variance, canonical variate analysis and a self-organizing map, to determine their variability and differences. We related these differences with the geographical and genetic distances between taxonomic groups, by means of Pearson correlations. We used recordings from Xeno-canto, an open database of bird vocalizations. Inter-taxa distances based on call traits were broadly consistent with geographic distances but not correlated with genetic distances. The Iberian Chiffchaff (Phylloscopus ibericus), presumably the most ancient lineage, was the most central in the variation space, while the Siberian Chiffchaff (P. collybita tristis) was the most peripheric and also very uniform, in contrast with the Canarian Chiffchaff (P. canariensis) highly variable, as expected by the "character release hypothesis" on islands. Calls proved to be an excellent tool, especially amenable for non-biased mathematical analyses which, combined with the wide availability of records in Xeno-canto, greatly facilitates the widespread use of this methodology in a wide range of species and geographical areas.

Modeling how population size drives the evolution of birdsong, a functional cultural trait

Oscine songbirds have been an important study system for social learning, particularly because their learned songs provide an analog for human languages and music. Here, we propose a different analogy: from an evolutionary perspective, could birds' songs change over time more like arrowheads than arias? Small improvements to a bird's song can lead to large fitness differences for its singer, which could make songs more analogous to human tools than languages. We modify a model of human tool evolution to accommodate cultural evolution of birdsong: each song learner chooses the most skilled available tutor to emulate, and each is more likely to produce an inferior copy than a superior one. Similar to human tool evolution, our model suggests that larger populations of birds could foster improvements in song over time, even when learners restrict their pool of tutors to a subset of individuals in their social network. We also demonstrate that song elements could be simplified instead of lost after population bottlenecks if lower quality traits are easier to imitate than higher quality ones. We show that these processes could plausibly generate empirically observed patterns of song evolution for some song traits, and we make predictions about the types of song elements most likely to be lost when populations shrink. More broadly, we aim to connect the modeling approaches used in human and nonhuman systems, moving toward a cohesive theoretical framework that accounts for both cognitive and demographic processes.

How to behave when marooned: the behavioural component of the island syndrome remains underexplored

Animals on islands typically depart from their mainland relatives in assorted aspects of their biology. Because they seem to occur in concert, and to some extent evolve convergently in disparate taxa, these changes are referred to as the 'island syndrome'. While morphological, physiological and life-history components of the island syndrome have received considerable attention, much less is known about how insularity affects behaviour. In this paper, we argue why changes in personality traits and cognitive abilities can be expected to form part of the island syndrome. We provide an overview of studies that have compared personality traits and cognitive abilities between island and mainland populations, or among islands. Overall, the pickings are remarkably slim. There is evidence that animals on islands tend to be bolder than on the mainland, but effects on other personality traits go either way. The evidence for effects of insularity on cognitive abilities or style is highly circumstantial and very mixed. Finally, we consider the ecological drivers that may induce such changes, and the mechanisms through which they might occur. We conclude that our knowledge of the behavioural and cognitive responses to island environments remains limited, and we encourage behavioural biologists to make more use of these 'natural laboratories for evolution'.

Patterns of bird song evolution on islands support the character release hypothesis in tropical but not in temperate latitudes

The character release hypothesis-which predicts that a decline in interspecific competition leads to the expansion of trait expression-remains to be tested for communication signals. Taking advantage of the fact that oceanic islands host fewer species than the mainland, we tested whether island birds show an increase in frequency bandwidth of acoustic signals compared with mainland birds. Given the higher animal diversity and more saturated acoustic space in the tropics, we expected acoustic character release, if any, to be stronger in the tropics than in the temperate zone. We field recorded 22 bird species (11 pairs consisting of an endemic island species and its closest mainland relative) breeding at similar latitudes and in similar habitats: six tropical pairs (São Tomé Island/Mount Cameroon) and five temperate pairs (Madeira Island/southern France). For each species, we measured the degree of acoustic interference experienced when vocalizing and the spectral characteristics of its song (minimum and maximum frequencies, bandwidth). As expected, island species spent more time vocalizing alone, and any overlap in vocalizations involved fewer species. The vocalizations of island species spanned broader frequency bandwidths than their mainland counterparts in the tropics (true for all six pairs), but this pattern was less evident in the temperate region (2/5 pairs with no marked differences and 1/5 with opposite pattern). Overall, the character release of communication signals only occurred where the differential in number of species was large (tropics). We discuss latitude differences and the potential factors driving the observed differences.

Independent evolution of song diversity and song motor performance in canaries, goldfinches and allies indicates clade-specific trade-offs in birdsong

The diversity and the motor performance of birdsongs can both be sexually selected. In wood warblers, most species with high motor performance sing a greater proportion of trills, presumably to advertise performance, and thus have lower syllable diversity. We tested if this trade-off between motor performance and syllable diversity extends to canaries, goldfinches and allies, a clade with much longer and more varied songs. We assembled a molecular phylogeny and inferred song motor performance based on the speed of frequency modulation either in trills or in within-song intervals. The two metrics of performance were positively, but only mildly, related across species. While performance evaluated in intervals had high phylogenetic signal, performance evaluated in trills changed independently of phylogeny and was constrained by body size. Species in densely vegetated habitats sang fewer trills, but did not differ in motor performance. Contrary to wood warblers, song motor performance did not predict the proportion of trilled syllables nor within-song syllable diversity, perhaps because large differences in the song duration of canaries, goldfinches and allies prevent trills from severely compromising syllable diversity. Opposed results in wood warblers and in these finches indicate the existence of clade-specific trade-offs in the evolution of birdsong.

Neotropical frogs and mating songs: The evolution of advertisement calls in glassfrogs

Anurans emit advertisement calls with the purpose of attracting mates and repelling conspecific competitors. The evolution of call traits is expected to be associated with the evolution of anatomical and behavioural traits due to the physics of call emission and transmission. The evolution of vocalizations might imply trade-offs with other energetically costly behaviours, such as parental care. Here, we investigated the association between body size, calling site, parental care and call properties (call duration, number of notes, peak frequency, frequency bandwidth and call structure) of the advertisement calls of glassfrogs (Centrolenidae)-a family of Neotropical, leaf-dwelling anurans-using phylogenetic comparative methods. We also explored the tempo and mode of evolution of these traits and compared them with those of three morphological traits associated with body size, locomotion and feeding. We generated and compiled acoustic data for 72 glassfrog species (46% of total species richness), including representatives of all genera. We found that almost all acoustic traits have significant, but generally modest, phylogenetic signal. Peak frequency of calls is significantly associated with body size, whereas call structure is significantly associated with calling site and paternal care. Thus, the evolution of body size, calling site and paternal care could constrain call evolution. The estimated disparity of acoustic traits was larger than that of morphological traits and the peak in disparity of acoustic traits generally occurred later in the evolution of glassfrogs, indicating a historically recent outset of the acoustic divergence in this clade.

Multi-modal signal evolution in birds: re-examining a standard proxy for sexual selection

Sexual selection is proposed to be an important driver of speciation and phenotypic diversification in animal systems. However, previous phylogenetic tests have produced conflicting results, perhaps because they have focused on a single signalling modality (visual ornaments), whereas sexual selection may act on alternative signalling modalities (e.g. acoustic ornaments). Here, we compile phenotypic data from 259 avian sister species pairs to assess the relationship between visible plumage dichromatism-a standard index of sexual selection in birds-and macroevolutionary divergence in the other major avian signalling modality: song. We find evidence for a strong negative relationship between the degree of plumage dichromatism and divergence in song traits, which remains significant even when accounting for other key factors, including habitat type, ecological divergence and interspecific interactions. This negative relationship is opposite to the pattern expected by a straightforward interpretation of the sexual selection-diversification hypothesis, whereby higher levels of dichromatism indicating strong sexual selection should be related to greater levels of mating signal divergence regardless of signalling modality. Our findings imply a 'trade-off' between the elaboration of visual ornaments and the diversification of acoustic mating signals, and suggest that the effects of sexual selection on diversification can only be determined by considering multiple alternative signalling modalities.

Insularity effects on bird immune parameters: A comparison between island and mainland populations in West Africa

Oceanic islands share several environmental characteristics that have been shown to drive convergent evolutionary changes in island organisms. One change that is often assumed but has seldom been examined is the evolution of weaker immune systems in island species. The reduction in species richness on islands is expected to lead to a reduced parasite pressure and, given that immune function is costly, island animals should show a reduced immune response. However, alternative hypotheses exist; for example, the slower pace of life on islands could favor the reorganization of the immune system components (innate vs. acquired immunity) on islands. Thus far, few island species have been studied and no general patterns have emerged. Here, we compared two immune parameters of birds from São Tomé and Príncipe islands to those of their close relatives at similar latitudes on the mainland (Gabon, West Africa). On islands, the acquired humoral component (total immunoglobulins) was lower for most species, whereas no clear pattern was detected for the innate component (haptoglobin levels). These different responses did not seem to arise from a reorganization of the two immune components, as both total immunoglobulins and haptoglobin levels were positively associated. This work adds to the few empirical studies conducted so far which suggest that changes in immune parameters in response to insularity are not as straightforward as initially thought.

The relative roles of cultural drift and acoustic adaptation in shaping syllable repertoires of island bird populations change with time since colonization

In birds, song divergence often precedes and facilitates divergence of other traits. We assessed the relative roles of cultural drift, innovation, and acoustic adaptation in divergence of island bird dialects, using silvereyes (Zosterops lateralis). In recently colonized populations, syllable diversity was not significantly lower than source populations, shared syllables between populations decreased with increasing number of founder events, and dialect variation displayed contributions from both habitat features and drift. The breadth of multivariate space occupied by recently colonized Z. l. lateralis populations was comparable to evolutionarily old forms that have diverged over thousands to hundreds of thousands of years. In evolutionarily old subspecies, syllable diversity was comparable to the mainland and the amount of variation in syllable composition explained by habitat features increased by two- to threefold compared to recently colonized populations. Together these results suggest that cultural drift influences syllable repertoires in recently colonized populations, but innovation likely counters syllable loss from colonization. In evolutionarily older populations, the influence of acoustic adaptation increases, possibly favoring a high diversity of syllables. These results suggest that the relative importance of cultural drift and acoustic adaptation changes with time since colonization in island bird populations, highlighting the value of considering multiple mechanisms and timescale of divergence when investigating island song divergence.