How Aestivation Evolved in Turtles: A Macroevolutionary and Morphological Approach

Aestivation (summer dormancy) is a long-term multiday torpor in response to hot and dry periods. 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 correlation of shell morphology with the aestivation duration. 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 evolutionary models on the presence/absence of the aestivation trait. We also performed phylogenetic comparative analysis to explore several shell morphological traits likely associated with the duration time in aestivation behavior. We found evidence of aestivation in 44% of the evaluated turtle species. Aestivation times were longer in Chelidae, Pelomedusidae, Geoemydidae, and Kinosternidae, and the shortest times in Emydidae and Testudinidae. Aestivation behavior is a derived trait evolved independently and several times in the pleurodires and cryptodires turtle groups. We found some evolutionary trends in different turtle families, Pelomedusidae and Kinosternidae showed considerable increases in the presence of the aestivation trait, while families such as Podocnemididae, Trionychidae and Chelydridae showed important deductions for the same trait. Our results for the association between shell morphology and aestivation duration in turtles were contrasting among families. Overall, it was the contribution of several and different morphological traits that allow a positive and significant association with the aestivation times.