The repeated evolution of convergent or analogous traits is often used as evidence for adaptive evolution. Squamate reptiles show a high degree of convergence in a variety of morphological traits; however, the evolutionary mechanisms driving these patterns are not fully understood. Here we investigate the evolution of tail spines, a trait that evolved multiple times in evolutionarily independent clades of lizards. Taking a comparative phylogenetic approach, we use 2877 squamate species to demonstrate that the evolution of spiny tails is correlated with microhabitat use, with species that live in rocky habitats significantly more likely to have evolved spiny tails. In light of previous behavioural observations, our results suggest that spiny-tailed lizards have an advantage in rocky habitats through predation avoidance, where tail spines are used to prevent extraction from rocky crevices. In concordance with previous research on lizard body armour, our results suggest that the evolution of tail spines is coupled to both a rock-dwelling lifestyle and predator avoidance strategies, and highlight a complex interplay between different selective pressures on the evolution of defensive morphologies in reptiles.

Supplementary Figure 1

Distinction of the SPINY and the SPINY STRICT datasets

 

Supplementary Table 1

Data

Body size, microhabitat and spiny tail data of the 2877 squamate species used in the study

 

Supplementary Table 2

Results of the fitPagel funtion

Results of the fitPagel function, testing for correlations between the evolution of spiny tails and microhabitat use

 

Supplementary Table 3

Results of the HiSSE models

Results of the hidden state-dependent speciation and extinction models (HiSSE), testing for changes in diversification rates associated with spiny tails

 

Supplementary Table 4

Results of the D statistic

Results of the D statistic, testing for a phylogenetic signal of the spiny tail trait

 

Supplementary Table 5

Phylogenetic ANOVA results (spiny tails vs microhabitat)

Results of the phylogenetic ANOVA, testing if there are differences in the number of spiny-tailed species in different microhabitats

 

Supplementary Table 6

Phylogenetic ANOVA results (spiny tails vs. body size)

Results of the phylogenetic ANOVA, testing if there are differences in body size between spiny-tailed and non-spiny-tailed species

 

Supplementary Table 7

Phylogenetic ANOVA results (microhabitat vs body size)

Results of the phylogenetic ANOVA, testing if there are differences in body size between squamates occupying different microhabitats

 

Supplementary Table 8

Phylogenetic logistic regression results

Results of the phylogenetic logistic regression, testing for correlations between the spiny tail trait and body size

 

Supplementary references

Literature list used for coding the spiny tail and microhabitat traits

 

Tree file

Dated tree used for the analyses