Habitat determines convergent evolution of cephalic horns in vipers

Abstract

Phenotypic convergence of traits in similar environments can provide insights into the evolutionary processes shaping trait evolution. Among squamate reptiles, horn-like cephalic appendages have evolved under various selective pressures, including selection for defence, crypsis or sexual selection. Yet, among snakes, particularly vipers, the functional and evolutionary significance of horns are unknown. We used a comparative phylogenetic approach with habitat and diet data on 263 viper taxa to shed light on the selective pressures underlying horn evolution in vipers. We detected significant correlations with habitat but not diet. The relative positions of horns are ecologically divergent in that supranasal horns are positively correlated with terrestrial forest habitats while supraocular horns are negatively correlated with terrestrial forest habitats and are instead associated with arboreal or sparsely vegetated habitats. Multiple independent origins of supranasal or supraocular horns in similar habitats provide evidence of adaptive convergence. Comparisons with other snake lineages suggest that cephalic appendages may have evolved under selection for crypsis in ambush foraging snakes.

Evolution along allometric lines of least resistance: Morphological differentiation inPristurusgeckos

“…More broadly, these findings are consistent with prior discoveries in other lizards, where the differential selective pressures imposed by rocky and ground habitats have resulted in the differentiation of head and limb morphology [43,[51][52][53]. Indeed, such phenotypic differences resulting from the effects of habitat-based ecological selection have been extensively documented in reptiles as well as in other vertebrates [9,[54][55][56][57][58][59][60], and our work in Pristurus thus contributes to this growing body of literature.…”

Foraging mode constrains the evolution of cephalic horns in lizards and snakes

“…In phrynosomatid lizards, horn size appears to depend on predation pressure [10], and species with larger cephalic horns are slower and more specialized sit-and-wait foragers than are congeners with smaller horns [48]. Correlations between habitat type and horn morphology in vipers [17], and ornamentation conspicuousness (including horns) in agamid lizards [6], further support the idea that variation in habitat and, consequently in exposure to predators, may affect a species' investment into cephalic horns.…”

“…Phylogenetic associations between horns and species' morphology, ecology, and habitat have been used to infer fitness advantages of horns (e.g. [10,[16][17][18][19]). By contrast, the potential costs of possessing such structures have rarely been mentioned.…”