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.

Global patterns of body size evolution in squamate reptiles are not driven by climate

“…Riemer, Gurlanick, & White, ) and the interspecific (Blackburn & Hawkins, ; Olson et al, ; Torres‐Romero, Morales‐Castilla, & Olalla‐Tárraga, ) level. In contrast, decades of research conducted on a wide range of ectothermic organisms have uncovered mixed support for climate‐driven size clines at either the intraspecific (Adams & Church, ; Ashton & Feldman, ; Pincheira‐Donoso, ; Pincheira‐Donoso & Meiri, ; Zamora‐Camacho, Reguera, & Moreno‐Rueda, ) or the interspecific (Feldman & Meiri, ; Olalla‐Tárraga & Rodríguez, ; Olalla‐Tárraga, Rodríguez, & Hawkins, ; Pincheira‐Donoso, Hodgson, & Tregenza, ; Rodrigues, Olalla‐Tárraga, Iverson, & Diniz‐Filho, ; Slavenko & Meiri, ; Terribile, Olalla‐Tárraga, Diniz‐Filho, & Rodríguez, ; Vinarski, ) level.…”

“…Crucially, although global‐scale studies on size clines in endotherms have been conducted (birds, Olson et al, ; mammals, Riemer et al, ), to date, only a few studies have examined global size clines of an entire large clade of ectotherms (apart from turtles; Angielczyk, Burroughs, & Feldman, ; Rodrigues et al, ), making it impossible to infer a universal effect of climate on body size.…”

Geographical patterns and environmental drivers of functional diversity and trait space of amphibians of Europe

“…Macroecological analyses often describe large scale patterns of species richness (Hawkins et al, ; Jablonski, Huang, Roy, & Valentine, ; Kinlock et al, ), range size (Orme et al, ), and more recently patterns of species traits such a s body size and diet (Gainsbury, Meiri, & Lenoir, ; Olson et al, ; Rodrigues, Olalla‐Tárraga, Iverson, & Diniz‐Filho, ) and community assembly rules, like competition and environmental filtering (Karadimou, Tsiripidis, Kallimanis, Raus, & Dimopoulos, ; Ramm et al, ; Shiono et al, ). Multiple underlying mechanisms have been proposed to explain all these patterns highlighting the roles of climate and energy availability (Evans, Warren, & Gaston, ), landscape heterogeneity (Davies et al, ; Kerr, Southwood, & Cihlar, ; Yoshioka, Fukasawa, Mishima, Sasaki, & Kadoya, ), environmental stability or climate seasonality (Carrara & Vázquez, ) and human pressures (Davies et al, ).…”

Toward a general explanation for latitudinal clines in body size among chelonians

“…Our findings therefore underline that some general factor governs temperature-size responses in chelonians, regardless of reproductive cycle. To our knowledge, there is currently no working hypothesis that might explain the ubiquity of James's rule in turtles, other than the well-trodden explanations of starvation resistance and heat balance (see also Rodrigues et al, 2018), which seem unlikely for ectothermic animals (Werner et al, 2016). If a new hypothesis is to be developed, then we suggest that it should draw on the observation that close relatives of chelonians, namely squamate reptiles, tend to increase in size as temperature increases, which is the opposite of turtles (Ashton & Feldman, 2003).…”