Temperature is the main correlate of the global biogeography of turtle body size

Abstract

Aims Geographical gradients in body size have been extensively studied in endotherms, and general rules exist to describe body size variation in these animals. However, the existence of broad-scale patterns in body size variation in ectotherms remains largely debated. Turtles (tortoises and freshwater turtles) are ectothermic organisms whose geographical variation in body size has not been examined widely. Here, we test a suite of hypotheses, proposed to explain body size patterns in other animals, for this group of reptiles.

Location Global.

Time period Current.

Major taxa studied Turtles.

Methods We gathered distribution, phylogenetic and body size data for 235 species of turtles, which were distributed in a global equal area grid of 200 km × 200 km. We also obtained predictor variables [mean annual temperature, actual evapotranspiration, temperature variation since the Last Glacial Maximum (LGM) and human footprint] directly associated with the main hypotheses tested in body size studies. Our analyses followed a cross-species and an assemblage-based approach and were performed for all turtles and for terrestrial and aquatic species separately.

Results Mean annual temperature was the main correlate of body size for the whole group and for terrestrial turtles in both approaches, having a positive correlation with this trait. Body sizes of aquatic turtles were not influenced by any of the tested variables. In the cross-species approach we also found that temperature variation since the LGM was an important positive correlate of body size in terrestrial turtles.

Main conclusions Our study reinforces the importance of environmental temperatures in explaining animal body size patterns. The heat balance hypothesis was not rejected by our data, whereas migration, productivity and human disturbance hypotheses were rejected. Finally, body size of terrestrial and aquatic turtles had different patterns, also suggesting that habitat is an important factor in understanding geographical variation in body size.