Patterns of morphological diversification in the Ramphastoidea reveal the dramatic divergence of toucans from a conserved morphotype

Abstract

Morphological traits offer insights into an organism s ecological niche, species interactions, and patterns of community organisation. Pantropical lineages - animals and plants distributed across the three tropical continental regions of Asia, Africa, and America - provide a way to test how different environments and communities influence morphological diversification. Here, we examined a monophyletic group of frugivorous birds, the barbets and toucans (Ramphastoidea), which diversified independently on three continents, to investigate whether clades in each region exhibit similar (phylogenetically constrained) or distinct (ecologically influenced) patterns of morphological diversification. Our results show that despite differences in community dynamics in these regions, lineage accumulation patterns through time on all three continents are broadly similar, putatively due to phylogenetic niche conservatism. We quantified morphological variation in light of phylogenetic relatedness to further reveal that all barbet lineages across continents occupy a conserved region of morphospace after correcting for variation in size. However, in the Neotropics, one lineage, the toucans, have diverged dramatically from typical barbet space and converged toward (yet are distinct from) the trait space occupied by the distantly related hornbills in Asia and Africa. Additionally, we found no link between climatic variables and morphological traits. We conclude that barbets exhibit a conserved morphotype across continents and have diversified by scaling mainly in body size. However, the absence of other large frugivorous birds may have allowed toucans to diversify into a different region of morphospace of increased bill/wing to tail/wing ratios. By examining different continental lineages of a single monophyletic bird group, we shed light on the contrasting effects of regional ecological factors and phylogenetic constraints on morphological diversification.

Climate influences bill shape diversification in cavity-excavating birds

“…The six species missing from our scan dataset are members of clades and morphological groups that were well-represented in our dataset, and are also thus unlikely to alter the patterns we observed. Drivers and functional consequences of bill shape diversification As mentioned above, previous studies have shown evidence of changes in the size of barbets without much change in body proportions (36). Here, we demonstrated that changes in size exert an influence on bill shape, with a significant effect of allometry after correcting for phylogenetic non-independence.…”

“…Barbets across the tropics broadly represent a conserved morphotype, with very similar body proportions across species, except in the Neotropics (63), where the toucans have diverged into a different morphological space (36). This may have resulted from ecological opportunity (64, 65), and future studies will aim to separately explore the question of ecological opportunity and its influence on bill shape and cavity nesting.…”

“…Here, we examine diversification of bill shape in cavity-nesting, frugivorous birds using μCT of museum skins. Asian and African barbets (two separate clades within the Ramphastoidea; Piciformes) exhibit high species diversity and conserved morphologies (36). The Asian barbets (Megalaimidae) have diversified primarily in wetter forested habitats in South and Southeast Asia, whereas the African barbets (Lybiidae) inhabit primarily drier woodland and savanna habitats in sub-Saharan Africa (37, 38).…”

“…The combination of diverse habitats, similar ecologies (including cavity excavation) and comparable species diversity render the African and Asian clades suitable to test the influence of environmental factors driving diversification of bill shape. Both clades occupy distinct climatic spaces (36), and thus putatively experience differences in the mechanical properties of the excavation substrate and the resulting impact stresses to the bill. This, in turn, may influence adaptive pressures on bill shape.…”

“…The most exclusive clade uniting Asian and African barbets, the infraorder Ramphastoidea, also contains the Neotropical barbets and the highly divergent toucans, in which dramatic morphological shifts have occurred in the latter group (36) associated with the loss of primary cavity-excavation (37). Ecomorphological divergence between barbets and toucans has been examined in a separate study (36), and we here focused on the paleotropical clades in order to compare two groups consisting entirely of primary cavity excavators, with similar bill-body proportions but occupying distinct climatic (and therefore ecological) regimes. Thus, we could disentangle the effects of phylogeny and climatic factors on bill shape in the context of cavity-excavation.…”