How aestivation evolve in turtles? a macroevolutionary and morphological approach

Abstract

Apart from hibernation, dormancy strategies in animals have been understudied. Aestivation is a functional and behavioral trait that responds to the effects of water reduction during dry and hot seasons. It has been detected in many species of terrestrial and aquatic turtles, however, several ecological and evolutionary aspects of chelonian aestivation remain to be evaluated and understood. We conducted a comparative exploration of macroevolutionary trends in turtle aestivation and tested the potential effect of shell morphology on the aestivation times. We compiled a dataset of aestivation status, aestivation times, and measurements of shell morphology of 225 turtle species. We reconstructed ancestral states along a time-calibrated phylogeny and tested different models with compared evolutionary rate changes on traits associated with aestivation. We also performed phylogenetic comparative analysis to explore shell morphological variables likely associated with maximum and mean aestivation times. We found evidence of aestivation in 44 percent of the evaluated turtle species. The longest aestivation times were found in the Chelidae, Pelomedusidae, Geoemydidae, and Kinosternidae, and shortest times were detected for Emydidae and Testudinidae. Inference of ancestral states revealed that turtle aestivation is a derived trait with multiple evolutions in the two major turtle clades. We found clade-specific evolutionary trends, with some clades showing increased aestivation (e.g., Pelomedusidae and Kinosternidae), clades exhibiting aestivation losses (e.g., Podocnemididae and Trionychidae) and contrasting patterns (loss vs. maintenance) in most clades of Testudinidae and Geoemydidae families. Otherwise, additive effects of different shell morphological variables correlated both positively and negatively with maximum aestivation times across most chelonian families. This is the first study exploring the evolution of aestivation in turtles and provides evidence that shell morphology in different chelonian families can influence aestivation time. We conclude that aestivation in turtles is a complex ecological and evolutionary process which needs to be studied in more detail.